MANAGEMENT OF Artificial Lakes and Ponds BENNETT B« REiJslHQLD BOOKS IN THE BIOLOGICAL SCIENGEd, 3S^^^B^^Q^^^^^^E3{ [ Marine Biological Laboratory Library | ID Woods Hole, Mass. D] II (D ID IE [Q [I Presented by [D (D ID Reinhold Publishing Corp. []] ^ July 2, 1962 [Q ID ID ID ID ID 01 ID (D Q £3 Management of Artificial Lakes and Ponds REINHOLD BOOKS IN THE BIOLOGICAL SCIENCES Consulting Editor Professor Peter Gray Department of Biological Sciences University of Pittsburgh Pittsburgh, Pennsylvania MANAGEMENT OF Artificial Lakes and Ponds GEORGE W. BENNETT Head, Aquatic Biology Section Illinois Natural History Survey Urbana, Illinois New York REINHOLD PUBLISHING CORPORATION Chapman ir Hall, Ltd., London Copyright © 1962 by Reinhold Publishing Corporation All rights reserved Library of Congress Catalog Card Number: 62-15249 Printed in the United States of America Foreword In the struggle for existence that occupies the time of all successful living tilings, there is constantly the pressure on the part of an organism to increase its numbers, and the opposite pressure of competing organisms to reduce its numbers. This is done unconsciously, as far as we know, by all living things but man. Man, though, is so peculiarly endowed by nature with reason, and tlie ability to think of the future and to plan for it, that diis matter of changing the numbers of living things becomes an obsession with him. Much of man's activity, either indirectly or directly, is aimed at manipu- lating populations. He increases the numbers of wheat plants, and de- creases the numbers of Hessian flies. He increases the numbers of sheep and decreases the numbers of those organisms which parasitize sheep. He increases the numbers of grouse or rabbits and then concerns himself with the diseases which attack these game species. He wishes to increase the numbers of lake trout and at the same time he searches for methods to destroy the sea lamprey. From the beginning of time man has used other organisms in his ascendency to his present state. Early expressions of culture which have come down to us include arrow points and hammer heads for the capture of game, and spears and hooks for the capture of fishes. In man's attempt to manipulate populations, the art of fish and game management had its roots. But a new phenomenon has appeared within the present century, and much of it after the first two decades had passed. This has been the changing of the art of management to the science of management. As we have learned more about fish and game, we have found that these organisms often react in an empirical, predictable way, and the old wives' tales and rules of thumb of just a little while ago have been found to be erroneous unless they happened to be based on what we now know as scientific fact. The knowledge which we have of a field such as fish management be- comes so hidden in the literature, and sometimes so abstruse to the non- speciahst, that it must be brought together and interpreted by someone who is conversant with the field. This is what Dr. Bennett has done in his book. vi Foreword There is certainly a need for organized information on fish management. According to a survey recently published by the United States Fish and Wildlife Service, more than twenty-five and one-quarter million Americans fished in 1960. They spent about 2.7 billions of dollars in this activity. That the need is growing is indicated by a similar survey conducted in 1955, which showed that at that time there were about twenty-one million fishermen who spent more than 1.9 billions of dollars. During the period 1955-1960 the number of fishermen in America increased by an average of almost one million per year. Therefore the present book should have a great audience, and the facts which it contains should be of importance and of value to a great many people. The author has found it necessary to simplify many terms, and define many things in this book, for there is no profession of sport fisher- men. People with this delightful addiction may vary from those handy with a shovel to those skilled with an electron microscope. They start fishing when they can hardly hold a pole, and, at the other end of the age span, stop only when they can again hardly hold a pole. To most of these people this book should be in part or in the whole a valuable tool. And there will be more interested people as time goes on, for the already large number of Americans who fished in 1960 is bound to increase. Harlow B. Mills Urbana, Illinois May, 1962 Preface Any book that is written and published probably stems from the belief of the author that there is a need for such a book. Therefore it seems only fair that the preface should let the reader know for whom the book is intended. This one was planned as a general reference for the professional fishery biologist, and for the recreation expert assigned to the task of producing hook-and-line fishing in the artificial impoundments of parks and forest preserves. It will interest fishermen who wish to be informed on lake-management matters, and should serve as a baseline from which re- search biologists in warm-water fisheries launch further investigations. Students of aquatic biology and fishery management will find both theory and practice here, as well as references for further reading on many subjects. A sincere effort has been made to recognize and acknowledge those researchers whose activities have contributed to a well-balanced theory of management and to avoid the pitfalls of oversimplification in manage- ment. Out of respect for biological variability, I usually have avoided specific directions, such as, for example, how to stock a lake. Rather, I have tried to show the range of reasonable stocking and its relationship to the range of potential results. Our purposes will have been satisfied if the reader gains enough insight into what might happen and why, to appreci- ate the danger of fish management by cookbook methods, and hence seeks to make use of the general principles of management as well as the sources of information available in the realistic solution of management matters that may come his way as a professional biologist and citizen. The organization of the present book was devised to achieve its pur- poses. First a brief, concise view of fish culture is presented to place the modern approach to the management of artificial lakes and ponds in a proper perspective. Next, artificial aquatic habitats are distinguished from natural bodies of water, are described, and, as much as is feasible, cate- gorized. Then, the ecological interrelationships of fishes and lake habitats are investigated and the implications for the professional manager are discussed. After a reasonably thorough treatment of such large concepts as carrying capacity, productivity, growth, reproduction, competition, and Vll viii Preface predation, the book comes to grips with the theory and techniques of management per se. Now, the complex problems of fishing mortality and natural mortality are handled before concluding with chapters on sensory perception and behavior in sport fishing and the commercial aspects of this most popular of all outdoor sports. No attempt has been made to avoid technical matter, although technical terminology has been reduced to a minimum and the mathematical ap- proach to population dynamics has been relegated to a list of papers, many of which will be available in the nearest university library. For the convenience of the student and individual pond owner who will be using the book, important terms and concepts are defined when they are first presented. No scientific names of fishes appear in the nine chapters of this book; however, the scientific names as well as the common names of all fishes mentioned may be found in the Appendix. A few scientific names other than those of fishes appear in the text, most of which are the names of aquatic plants as given by Fassett.* Any author who completes a book is indebted to many people. In this respect, I have been very fortunate in receiving the counsel of all of the members of the Aquatic Biology Section of the Illinois Natural History Survey. I am particularly indebted to Drs. William C. Starrett, R. Weldon Larimore, and Donald F. Hansen of our Section editorial committee for giving of their own time to read and criticize this manuscript. Dr. Harlow B. Mills, Chief of the Illinois Natural History Survey, has contributed greatly through his encouragement, his suggestions for im- proving certain areas in each of the nine chapters, and through his special ability to recognize and point out the author's bias regarding several con- troversial subjects. Mr. Royal B. McClelland, Executive Secretary of the Illinois Federation of Sportsmen's Clubs and Editor of Illinois Wildlife magazine, has read the manuscript through the eyes of a fisherman and lake owner, and has suggested changes to make the book more understandable and readable. Much of the original planning for subject matter included in Chapter 6 came from discussions with Mr. Sam A. Parr, Executive Assistant for the Illinois Department of Conservation, and Mr. W. W. Fleming, Director of Fish and Game, Indiana Department of Conservation. This chapter on theories and techniques of management was later presented to Mr. Wil- liam J. Harth, Superintendent of Fisheries, Mr. Al Lopinot, Chief Fishery Biologist of the Illinois Department of Conservation and to other profes- sional fishery biologists with the Department for general discussion at the 1961 meeting of Illinois aquatic and fishery biologists. I am grateful for the suggestions offered at this meeting. * Fassett, N. C, "A Manual of Aquatic Plants," with revised Appendix by A. C. Ogden, University of Wisconsin Press, Madison, Wisconsin ( 1957). Preface ix Many people furnished invaluable cooperation as lake and pond owners willing to allow the collection of fish and fishing in their waters. These cooperators would number more than a hundred. Of these, I have space to mention a few: Mr. WilHam Utterback and Mr. David Malcomson, each of whom owns a number of gravel-pit ponds and who not only allowed us to use these ponds for research but gave of their own time to assist in research activities; Mr. Faye H. Root, Assistant Professor of Camp and Park Management at the University of Illinois Robert Allerton Estate near Monticello, who has arranged for our use of 4-H Club ponds and has given cooperation in many ways; and Mr. Max McGraw, owner of the Fin 'n Feather Club, who has built special ponds for research and has fur- nished creel and management data from ponds and lakes on the club property for our use. I especially wish to acknowledge the valuable assistance of Miss Mary Frances Martin, Technical Assistant and Secretary for the Section of Aquatic Biology, who was always willing to help in various phases of this work, and of others who have typed material, a page at a time. My wife, Mary Ellyn, read the manuscript at each stage of progress for spelling, punctuation, and meaning. I do want to express my appreciation to Dr. Peter Gray and Messrs. Ross, Chastain, and Hart of the Reinhold editorial staff for their constant encouragement and assistance in improving my manuscript. George W. Bennett XJrhana, Illinois April, 1962 Contents Foreword Preface Chapter 1. History of Fish Management Earlv Pond-Fish Culture The Development of Hatcheries Early Attempts at Management The Importance of Studying Total Populations Pond Management Reservoir Management Early Investigations Fish Cycles in Reservoirs Recent Advances in Management Phases of Operation Reducing Undesirable Populations What is in the Future? Literature Vll 2 3 5 7 8 9 9 10 11 11 12 12 13 Chapter 2. Artificial Aquatic Habitats The Farm Pond Purposes of Farm Ponds Engineering Considerations for Farm Ponds When is a Pond a Lake? Artificial Lakes for Domestic Uses Water-Supply Reservoirs Navigation Pools Lateral Levee Reservoirs Multi-Purpose Reservoirs Ponds and Lakes with Excavated Basins Gravel-Pit Lakes Strip Mine Lakes Quarry Lakes XI 15 17 17 18 20 20 21 22 22 22 24 24 26 27 79909 xii Contents Lakes Built for Recreation 28 Planning an Artificial Lake or Pond 29 Watershed, Run-off, and Water Manipulation 30 Thermal Stratification and Loss of Oxygen 31 Seasonal Thermal Stratification 31 Variations in Thermal Stratification 33 Fall Overturn 33 Attempts to Upset Stratification 34 Thermal Stratification and Reservoir Outlets 34 Other Factors Affecting Thermal Stratification 36 Biological Productivity of Water 38 Lake Size and Productivity 39 Literature 40 Chapter 3. Interrelationships of Fishes and Lake Habitats 42 Sewage Pollution and Fertility 43 pH and Chemistry of Water 44 Effects of Water Temperature on Fish 45 Effects of Turbidity 46 Oxygen and Carbon Dioxide 48 Winterkill and Summerkill 49 Winterkill 49 1. Aeration of Water Under Ice 51 2. Aeration of Water Above Ice 51 3. Pumping of Well Water 51 4. Snow Removal 52 5. Lamp Black 52 6. Circulation of Bottom Water 52 Results of Partial Winterkill 53 Summerkill 54 -Other Dangers of Impoundments 56 Loss of Ponds Because of Burrowing Animals 56 Wind Action 56 Dangers from Insecticides 57 Literature 57 Chapter 4. Carrying Capacity, Productivity, and Grow^th 59 Carrying Capacity and Standing Crop 60 Surface Area 61 Experimental Testing 61 Factors Affecting Poundage 62 Fertilization 63 Contents xiii Chemical Basis for Fertility 63 Kinds of Fishes 63 Growing Season 69 Other Factors Related to Standing Crop Size 69 Fish of Useful Sizes 70 Fish Production 71 Definition of Production 71 Food Conversion 72 Relation to Standing Crop 73 Estimating Production 74 Relative Plumpness of Fish 75 Methods of Measuring Condition 76 Coefficient of Condition 76 Index of Condition 76 Condition Factor of E. M. Corbett 77 Condition Factor of Cooper and Benson 78 Condition Cycles 78 Condition and Growth Rate 79 Use of Condition in Management 79 Growth 79 EflFects of Starvation 81 Growth of Fish in New Waters 81 Factors AflFecting Rate of Growth 82 Interpretation of Growtli from Fish Scales 85 Literature 89 Chapter 5. Reproduction, Competition, and Predation 91 Reproduction 92 Reproductive Potential of Fishes 92 Spawn Production and Number of Spawners 94 Age and Sexual Maturity 96 Sex Ratios 96 External Sex Characteristics 97 Spawning 97 Hybridization 99 Selective Breeding 101 Unsuccessful Reproduction 103 Stocking 103 Fishes Used 104 The Bass-Bluegill Combination 104 Other Combinations of Fishes 107 Changes in Fishing 110 xiv Contents Stocking for Improvement of a Population 110 Stocking to Improve a Food Chain 112 Failures in Stocking Fish 113 Predation 115 The Role of Predation in Fish Management 115 Competition 118 Competition for Food 118 Competition for Space 119 Competition for Specific Habitats 123 Starvation 123 Inter- and Intraspecific Competition 123 Reproduction, Competition, and Predation 124 Balance 125 Literature 127 Chapter 6. Theories and Techniques of Management 130 Fish Sampling 130 Reasons for Sampling Fish Populations 131 Sampling Methods 132 Gill Nets 133 Trammel Nets 133 Seines 133 Hoop Nets, Wing Nets, and Trap Nets 134 Minnow Seine Sampling 135 Minnow Seine Method of Pond Analysis 136 Spot Poisoning 137 Boat Shocking 137 Angling 139 Management Techniques 140 Complete Fish Population Removal 140 Population Removal by Draining 140 Population Removal by Rotenone Treatment 142 Selective Poisoning 147 DDT and Other Insecticides 147 Toxaphene 148 Sodium Cyanide 149 Sodium Sulfite 149 Fish Population Adjustment 150 Use of Nets and Seines 150 Partial Poisoning 150 Timing in Partial Poisoning 151 Shoreline vs. Sectional Treatment 152 Contents xv Artificial Fluctuation of Water Levels 155 EflFects Upon the Exposed Lake Bottom 158 Effects Upon Rooted Aquatic Vegetation 158 Effects Upon Invertebrates 159 Effects Upon Fishes and Other Vertebrates 159 Types of Drawdowns 161 Lake Fertilization 162 Manganese 164 Lime 164 Potassium 164 Phosphorus 165 Nitrogen 165 Other Functions of Fertilizers 165 Dangers From the Use of Fertilizers 166 Aquatic Vegetation and Control Measures 168 Types of Aquatic Plants 169 Algae as a Basic Food 170 Dangerous Algae 171 Nuisance Algae 171 Control of Algae 172 Loss of Fish Production Through Rooted Vegetation 172 Sudden Plant Die-Offs 173 Role of Aquatic Vegetation in Management 176 Control of Higher Aquatic Vegetation 177 Literature 177 Chapter 7. Fishing and Natural Mortality 181 Population Estimation 181 Forces Acting Upon a Fish Population 182 Fishing Mortality 183 Angling Compared to Natural Predation 183 Yields and Standing Crop 184 Maximum Yields and Length of Growing Season 185 Underfishing 187 Overfishing 189 Fishing Pressure vs. Yield 190 Types of Fishing Pressure 193 Factors Related to Rate of Catch 195 Role of Commercial Fishing in Sport-Fish Management 196 Natural Mortality 199 Causes of Natural Death 199 When Do Fish Die? 200 xvi Contents Scavengers 201 Length of Life of Fishes 201 Problems of Measuring Natural Mortality 202 Fishing Mortality, Natural Mortality and Recruitment 207 Restrictions and Mortality 208 Size Limits 208 Creel Limits 209 Closed Seasons 209 Literature 210 Chapter 8. Fish Behavior and Angling 212 Vision 213 Color Vision 213 Underwater Vision 214 Changing Pigmentation 216 Direct Observation and Sun Orientation 217 Light Sensitivity 217 Hearing 218 Sound Location 218 Sound Production 219 Odor Perception and Taste 219 Location of Taste Organs 220 Some Uses of Odor Perception ' 221 Schoohng 221 Fright Reaction 221 Identification of Common and Uncommon Odors 222 Odor as an Aid in Migration 222 Odor as an Aid in Homing 224 Temperature Perception and Responses 224 Sensitivity to Temperature Change 225 Mortalities Caused by High Temperatures 225 Temperature Acclimation 226 Preferred Temperatures 227 Behavior Patterns 229 Some Types of Behavior Patterns 229 Social Groupings 229 Seasonal Rhythm 230 Daily Activity 231 Responses to Specific Stimuli 232 Hyperactivity as a Lethal Factor 233 "Stay at Home" Fish 233 Homing and Home Range 234 Contents xvii Theories of Migration in Fishes 236 Responses of Fishes to AngUng 237 Factors that Influence Biting 238 Water Temperatures 238 Water Transparency 240 Diurnal Effects 241 Rising and FaUing Water Levels 241 Barometric Pressures and Fishing Tables 242 Resistance of Fish to Being Caught 243 Fisherman "Know-how" 245 Literature 246 Chapter 9. Commercial Aspects of Sport Fishing 249 Interest in Angling 251 Supplying the Needs of the Sport Fisherman 251 Costs Assigned to an End Product, Usually Fish 252 Supplying Fishermen's Baits 253 Earthworms and Other Invertebrates 253 Minnows 254 Fishing for Sale 254 "Executives" Fishing and Hunting Clubs 255 Fishing-Lake Investments 255 Trespass-Rights Fishing 256 Catch-Out Ponds 257 Ponds in Which Fish Are Artificially Fed 259 Floating Fishing Docks 261 Fish for Sale 263 Fish Management Service 265 Literature 266 Appendix 267 Index 271 1 History of FisnManagement Fish management can be defined as the art and science of producing sustained annual crops of wild fish for recreational and commercial uses.^^ But, this activity is not synonymous with fish farming, or the production either of hatchery fish for put-and-take fishing or fish fry and fingerlings for the purpose of stocking. Nor does it consist merely of regulations to control the take of kinds, numbers, and sizes of fish (as when limits are placed upon fishing seasons) any more than it is restricted to "habitat improvement" per se. Nonetheless, fish management makes use of knowledge gleaned from fish farming, the products of the hatcherv, legal assistance to regulate fishing, and methods of habitat modification. However, these facets of fishery husbandry and knowledge can be employed only when integrated into a master plan that eliminates physical and physiological barriers to the well-being of the fishes selected for management. Thus, as stated above, fish management is defined as the art and science of producing sustained annual crops of wild fish for recreational and commercial uses. The reasons that we have stressed the words "art" and "science" should soon be apparent. Dr. T. H. Langlois -^ in his studies of fish production in Lake Erie has illustrated the complexity of factors influencing the fish in an aquatic habitat by demonstrating that turbidity and plankton abundance control the size of surviving year classes of important commercial fishes. We can see from Dr. Langlois' study that the crop of fish available for capture in any given year was not related to fishing intensity in past seasons, but instead to th e amount of topso il carried into the lake during some pre- ceding year when the fish being studied were hatching. Although naturally no^ amount of legal regulatforTof the fishery can be expected to change such a cause-and-effect relationship, the results themselves might be changed by intensive soil conservation practices on the land in the water- sheds tributaryloXaFe~Erie. This points~"up the importance and com- 1 2 History of Fish Management plexity of fish management; however, its history is bound up with the broader term "fish culture." We might somewhat arbitrarily divide fish culture and management into three time periods— a division that has its greatest relevance to the student of fish management. The first, which stretches from its earliest development in the pre-Christian era to about 1900 a.d., is characterized by classical fish culture. The second, which roughly covers the period from 1900 to the 1930's, represents the first gropings (often blind and erroneous ones ) toward the manipulation of wild populations. The third, which began in the 1930's and extends to the present time, is identified with tlie development of modern ideas and methods related to managing "wild" fish in natural and artificial waters. Thus, fish management as an integrated science is rather recent, and it may he said to have had its beginnings when fishery biologists began to study fish populations as composite units. Nevertheless, to recognize the importance of what went before, one must consider the historical records of pond-fish culture that began with the earliest historical writings. EARLY POND-FISH CULTURE In almost all written history there are references to fish ponds or fish culture. A study of these records reveals that the Chinese were well versed in raising fish many years before the time of Christ. Also, it can be seen that the Romans copied the techniques of this art at a very early time, although they probably added nothing to the knowledge of fish culture that existed in the ancient Chinese civilization. Pond-fish culture spread through Europe during the Middle Ages. The first carp ponds were built in Wittingau ( Czechoslovakia) in 13 58, and for the next 400 years in Europe this was the center forraising^ond fisL^^ During this period, fish culture became quite^compTex. Forexample, carp culture very early demanded specialized holding areas, such as spawning and hatching ponds (where fry were allowed to grow during the first summer) and growing and fattening ponds. Further, in the fall, carp had to be moved to deep wintering ponds. Special strains of carp were developed, much as various strains of domestic animals have been produced. Thus, from the 15th to the 18th ce nturies, with this growth in complexity, men, such as North,^'' Baccius,^ and others, presented detailed te chniques for raising pedigreed carp and other common fish useful for food. These investigations have been continued up to the present, and much progress has been made in an understanding of the many facets composing the pond habitat for fish. P Since pond-fish culture in Europe represented food production, it sup- plied a cash crop that was harvested much as any other farm crop. How- The Development of Hatcheries 3 ever, this type of "farming" has not been profitable in North America until very recently - " because of an early and extensive development of the commercial fishing of wild fresh-water and marine populations which provided a more than adequate amount to supply domestic demands. For example, the commercial yield of fresh-water fishes from the Illinois Ri ver ( I llinois ) alone was 24 million pounds in 1908; '^^ the catch was composed mostly of carp, buflFalo fish, and catfish and was largely shipped by rail to eastern markets. In fact, it is interesting to note that special strains of carp imported to this country from Europe in the 1880's soon reverted to tlie original wild type. However, in the period since 1908, the fisheries of the Great Lakes and coastal marine waters have largely supplanted those of inland rivers and smaller lakes, so that now the commercial operations in inland rivers are much reduced, except those for catfish which always have a ready market. THE DEVELOPMENT OF HATCHERIES A normal outgrowth of European fish-culture practices brought to this country by immigrants was the development in the U.S. of hatcheries to supplant tlie natural production of young wild fish. The earliest hatcheries were privately operated, usually for the production of trout. Dr. Theo- datus Garlick, the Rev. Dr. John Bachman, and Seth Green were all operating private hatcheries prior to 1865."^ In 1872, at the urging of the American Fish-Culturists' Association, the Congress of the United States enlarged the duties of the newly formed Fish Commission to include the propagation of fish.* In 1875 both federal and state governments were operating hatcheries for the artificial production of fish. The late nine- teenth and the early years of the twentieth centuries were marked by attempts at hatchery production and stocking of the kinds of fish useful for sport and food in the more important waters. However, many of these attempted introductions resulted in failure for tlie following reasons: a lack of understanding of physical and biological limitations, the release of the fish into habitats unsuitable for them, and their inability to survive predation and/or to compete with other organisms already present in the waters. These failures were due largely to tlie fact that at that time the science of fish ecology was practically unknown, while the art and science of fish culture was well advanced. This was the heyday of the men engaged in the artificial propagation of fish. States vied with one another in the race to put out larger and larger numbers. "Paper fish" flourished in the reports of hatchery super- intendents: numbers were important; little else mattered. Moreover, in the late nineteenth century only a few trained professional * U.S. Comm. of Fish Kept. 1872-73 (1874). 4 History of Fish Management biologists were employed by State Conservation Departments; most of the limnologists and ichthyologists were attached to universities and were given little or nothing to say in formulating the programs of Conservation Departments. The scientists and fish culturists came together at the annual meetings of the American Fisheries Society. The latter probably looked upon the scientists as men having little practical use for anything except for the identification of some strange fish or aquatic bug; during this period, the university biologists were primarily engaged in taxonomy and distributional studies. Undoubtedly, some of the things the "practical men" said at the American Fisheries Society meetings mildly irritated the biologists, but not sufficiently to cause them to become crusaders. After all, at that time, they had little real information on the ecology of fishes, except in a broad, general way. This divergence of beliefs was perhaps nowhere more clearly illustrated than in Illinois in the 1880's and '90's when Professor Stephen A. Forbes and his group of scientists were studying the biology of the Illinois River. Concurrently, the Illinois Fish Commission was working in this area, but with the primary objective of rescuing the fish stranded by the receding waters of early summer along the Illinois and Mississippi Rivers. Forbes' emphasis is clear: he recognized the loss of fish as a natural phenomenon: ^^ "As the waters retire, the lakes [of the Illinois River bottoms] are again defined; the teeming life which they contain is re- stricted within daily narrower bounds, and a fearful slaughter follows; the lower and more defenseless animals are penned up more and more closely with their predaceous enemies, and these thrive for a time to an extraordinary degree.' Fish stranded in land-locked pools were either preyed upon by other, larger fish or by amphibians, reptiles, fish-eating birds or mammals; or if the pools dried completely, the fish died and decayed where they lay exposed. Since these victimized fish were mostly small ones, the products of the current reproductive season, Forbes and his colleagues recognized them as being in excess of the small number required to maintain the population at a constant level. They realized that this apparent waste was normal and had been occurring on the over- flow lands of large rivers for many thousands of years. The Illinois State Fish Commission, on the other hand, engaged crews of men with seines and wagons to rescue these fish for stocking in other lakes or for release in open water. These crews usually operated each year from the time the first fish became stranded in the spring, until the waters had receded to their usual summer low-water levels.^ Professor Forbes must have recognized that this program was entirely useless, not only because the fish were "expendable," but also because their survival was in doubt when they were seined up and transported Early Attempts at Management 5 during hot wcatlier. Yet, judging from \\ hat was pubhshed at that time, there was no animosity between Professor Forbes and the State Fish Commissioners. Perhaps the former reahzed that the Commission was responding to the desires of the pul)hc. It is against this background tliat we can visuahze the important place hatcheries held in the minds of fish culturists at that time. The products of inland hatcheries were trout, salmon, whitefish, walleye, largemouth bass, smallmouth bass, northern pike, muskellunge, and several other species propagated for stocking in special locations. Since hatchery operators were thoroughly convinced of the worth of their product, they scarcely gave any thought to how fish managed to survive before the hatchery was developed. Hatchery superintendents and fish- and game-department officials on both state and federal levels exerted considerable ejffort to convince the public that hatchery fish were needed to maintain populations of fish in the face of advancing civilization— a drive which gave great impetus to the hatchery movement. However, almost no eflFort was made to de- termine the final disposition of hatchery fish or to estimate the importa nce of the hatche ry produce on the basis of fish stocked per acre of wat er. Since such questions were dangerous to the hatchery movement they were simply avoided. Even so, it is interesting to note that the hatchery movement was so successful that even today the otherwise uninformed layman inquires about recent stocking of the waters in which he plans to fish. In later sections of this book, we will see some of the important ways the hatching of fish is useful in fish management. EARLY ATTEMPTS AT MANAGEMENT In spite of the fact that the artificial propagation of fish in hatcheries continued to hold the center of the fisheries stage, some of the early investigations were not directly related to artificial fish propagation. These studies helped to pave the way for the modern concept of fish manage- ment which is the subject of this book. Before 1759, Hederstrom -^ rec- ognized rings of growth on the vertebrae of fishes as representing annuli; later other biologists discovered the growth rings on the scales of fish, but Borodin^ and Barney- were responsible for bringing a method of aging fish from scales to the attention of fisheries workers. Wiebe ^^ and C. Juday, et al.~^ w^ere among the first to do comprehensive experiments with fertilizer materials in water and to measure the increase in plankton resulting therefrom. Surber ^^ tested sodium arsenite as a chemical means for the control of aquatic vegetation. The first electric fish shocker for research purposes was developed by Burr.^^ Markus ^^ investigated the 6 History of Fish Management relationship between water temperatures and the rate of food digestion in largemouth bass. Thompson *^ tagged fish and studied their migration in rivers. Several early pond studies gave promise of things to come, such as that of Dyche ^^ who observed that interspecific predation between large- mouth bass and bluegills might favor bluegills rather than bass. Barney and Canfield^ studied the fish production of an 0.22-acre pond over a period of 5 years and gathered some evidence that production and total standing crop were related to the lengths of the food chains of the fishes introduced. The first record of the use of tlie bass-bluegill combination was published in 1902.^^ Barney and Canfield^ used largemouth bass, crappies, and bluegills, or bluegills alone prior to 1922. A comprehensive inventory of current thinking on fishery management in 1938 was given by Carl L. Hubbs and R. W. Eschmeyer in their book, "The Improvement of Lakes for Fishing." -- Dr. Hubbs, then head of the Michigan Institute for Fishery Research, and Dr. Eschmeyer, one of his students recognized for his independent thinking, pooled their experiences and hypotheses and built, with a strong assist from Leopold's concepts of game management, a thesis that populations of fish in lakes could be managed also. Actually, they had very little to go on, but they held with the assumption that if game habitat could be improved by the addition of cover on land, fish habitat must be deficient of cover under water and could be improved in the same manner. So the book begins with a section on improving cover, based on the theory that such cover was one of the larger needs. Other subjects discussed were Managing Plant Growths, Bettering Spawning Conditions, Regulating the Fluctuations of Water Levels, Pre- venting Erosion and Silting, and many more. Under the topic Handling Populations with Stunted Growth, the authors suggest that one might do the following: (1) increase food (however, without suggestions as to how to go about it ) , ( 2 ) avoid overstocking, and ( 3 ) reduce the popula- tion by liberalizing size limits, season limits, and bag limits, as well as by destroying nests, by planting fish-eating game fishes and, as a last resort, by killing the entire population as Dr. Eschmeyer had already done at the time the book was published. In all, there were 20 types of fish-management practices described, none of which had yet been care- fully tested. Evidence that the authors were still affected by the earlier beliefs and operations is to be found under the subject Place of Lake Improvement in Fish Management: "At least for the near future, lake improvement cannot be foreseen as a substitute for the long-recognized practices in fish management [re- strictive and protective legislation, law enforcement, and the introduction Importance of Studying Total Populations 7 of fish and the stocking of artificially propagated and reared fish.^^] Methods of lake improvement would need to be enormously perfected, before this new practice, if ever, could be expected to replace the older means of maintaining the fish supply." (Bracketed matter mine.) Hence, the book, "The Improvement of Lakes for Fishing" has its greatest importance in that it initiates a rather bold break with the past, points toward tilings to come, and presents a precise picture of current thinking in 1938; whereas, the actual lake-management data which it contains is of less significance. IMPORTANCE OF STUDYING TOTAL POPULATIONS An important step in the understanding of fish-management problems was the censusing of populations of fish by the poisoning or draining of lakes so that a population could be observed as a unit.^' ^^ Such a census was particularly enlightening when conducted on a lake population with past stocking and fishing records available, because, under such cir- cumstances, the effects of stocking could be evaluated and good or poor fishing associated with a specific population. Almost immediately it became evident that there was never a shortage of fish; in fact, usually there appeared to be an overabundance of individuals, particularly of the fish of smaller sizes. When many complete fish censuses became available, some of the causes for poor fishing were obvious: (1) an excess of undesirable fish, that is, the domination of these populations by species of no interest to anglers, whereas, proportionally, the number of acceptable fish was so small that the chance of catching them was remote; (2) an excess of desirable fish, that is, in populations containing only hook-and-line species, overpopulation led to stunting, so that few of them were large enough to interest fishermen. Thus, the causes for poor fishing were domination by undesirable fish and the overpopulation of desirable ones with consequent stunting. This type of information gave direction to attempts at fish management, something that had been lacking before. At this time, several studies of entire populations helped to give us an understanding of certain aspects of population dynamics. Thompson,^^ who took periodic samples of the fish population of Lake Senachwine (Illinois) with wing nets and used the mark-and-recovery method of population estimation, came to the conclusion that the "fine" fish com- ponent of this population ( consisting of largemouth bass, crappies, blue- gills, and other centrarchids ) totaled between 50 and 55 pounds per acre, regardless of whether the lake level was high and the area was 6000 acres in extent or whether it was reduced by drought to 3000 acres or less. Moreover, the poundage was constant from year to year, in spite of a 8 History of Fish Management cycle of size and numbers. In certain years there were ten times as many fish as in other years, but the average weight was only one tenth as great. This was a crappie-dominated cycle, wherein a dominant brood of crappies curtailed the survival of its own young and those of other species until this brood was decimated by natural mortality associated with senile degeneration. Then another dominant brood was produced and the cycle was repeated. The cycle shifted between high numbers of black crappies with low numbers of white crappies and bluegills, and moderate numbers of white crappies and bluegills with low numbers of black crappies. Hence this investigation demonstrated that, in spite of constancy of poundage, continual changes might be taking place among the fishes of certain populations. One of the most significant studies, in that it helps to show the true position of hatcheries in the fish management picture, was that of Car- bine,^- who followed the spawning and hatching of nest-building cen- trarchids in Deep Lake (Michigan). Fry were sucked through glass tubing from nests guarded by males (to give identification of kind) and counted. On the basis of these counts and the number of occupied nests observed, Carbine estimated that the hatch of fish per acre in Deep Lake exceeded one-half million during a single spawning season. Thus, he was able to demonstrate that the hatching success of fishes in natural lakes was as high or higher than that observed in fish hatcheries. POND MANAGEMENT Several investigators working with fish in ponds demonstrated the capacity of fish populations to expand and contract in relation to tlie capacity of the habitat to support them ^- and the relationships between length of the food chains and poundages of fish supported.^^ Probably the most extensive pond research unit in North America was developed between 1934 and 1944 by H. S. Swingle and E. V. Smith at the Alabama Polytechnic Institute ( now known as Auburn University ) , Auburn, Alabama. This unit consisted of more than 100 ponds which could be drained and refilled; the ponds ranged in size from one tenth of an acre to more than one acre. These ponds were used for developing simple pond-management techniques which could be used by laymen for increasing the fish yield of farm ponds. Recommendations that worked well in the region of Auburn, Alabama, caught the interest of sports writers throughout the country, and many magazines of national circula- tion published articles on the Alabama methods of pond management. The U.S. Soil Conservation Service, foremost pond-sponsoring agency in the United States, and many State Conservation Departments, also put to use the recommendations of the Alabama biologists. The national publicity on pond management stimulated such wide- Reservoir Management 9 spread interest in ponds that many states developed programs of pond researeh on their own. However, it soon beeame evident that the same kinds of fishes that produeed satisfactory hook-and-hne yields in Alabama ponds, when stocked in the "correct" numbers in ponds in other parts of the country, behaved in an entirely difiFerent manner. This was not only because die habitats and fish food complexes were different, but also because the behavior and physiology of the fishes varied within the limits of their natural range. Thus, the program of stocking fingerling bass and bluegills in a ratio of 1 to 10 or 15, and fertilizing the ponds with inorganic fertilizer— a program w^hich produced a harvestable fish crop in the southeast— created overpopulation problems in the central states and was still less useful in more distant parts of the United States. Fisherv biologists began to study life histories of many common warm- water fishes and to test combinations other than that composed of large- mouth bass and bluegills, with the objective of finding new combinations that would work as well or better than the bass-bluegill combination. Soon nearly every state developed its own stocking recommendations for largemouth bass and bluegills, and many states recommended the stock- ing of bass or some other piscivorous species with one or several other omnivorous species, often not including the bluegill at all."^- ^^ At the present time, there is still considerable variation in recommenda- tions for many aspects of pond management throughout the United States. Most biologists believe that no combination of fishes is entirely satisfactory for producing sport fishing in a selected impoundment, yet any of several combinations may be reasonably useful for this purpose. RESERVOIR MANAGEMENT Early Investigations The progress of fish management in large reservoirs has been detailed by Jenkins.-^ In the early 1920's, many large ones for flood control and hydro-electric power were constructed, and among the first studied was Lake Keokuk— a low-dam impoundment on the Mississippi River near Keokuk, lowa.^^' -^ As reservoir construction gained momentum in the 1930's, studies of these new reservoirs consisted primarily of inventories of plankton, bot- tom fauna, and fish,'^-^' ^' but provided, in addition to these inventories, opinions on how to improve the fish-producing capacities of the reser- voirs.^^ How^ever, during the latter part of the 1930's, a number of reservoirs, previously superb for fishing, became poor. Consequently, some biologists concluded of these large reservoirs that, after an initial high productivity brought about by the decay and utilization of the organic matter present at impoundment,^!' ^^ we could expect them to become aquatic deserts. 10 History of Fish Management However, a team of fishery biologists, employed by the Tennessee Valley Authority to investigate the fish populations of TVA reservoirs,^^' ^^ con- cluded that there was insufiicient evidence to substantiate this belief. Fish Cycles in Reservoirs The work of the TVA in this direction was strengthened by the studies of others, and, in time, biologists, who had seen fishing in a number of new water-supply reservoirs change from excellent to very poor in a matter of a few years, were ready to predict a reservoir fishing cycle; ^ "At the first spawning season (May-June) after the reservoir is filled and stocked with fish, the young of largemouth bass will be very abundant. These will grow rapidly to legal size and produce excellent bass fishing for about three years. Moderate numbers of young crappies, bluegills, and other sunfish and bullheads may be produced during the first spawning period. These will grow rapidly to large average sizes and add to the excellence of fishing. "Carp, buflFalo, and suckers, as well as some other fish present in the stream flowing into the impoundment will move into the lake in small numbers and produce some young the first season. "During the first few years the reservoir will be clear except immediately following heavy rains, and the recreational attractions other than fishing, such as swimming and boating, will be at their maximum. "Reduced recreational values will be apparent in about the length of time that the original spawn of bass survives ( usually four to six years ) . By this time the bass fishing will be largely gone. Crappies and bluegills will be present in such large populations that they will have become small, stunted and unattractive to fishermen; carp and other rough fish will have multiplied so successfully that their bottom-feeding activities will continually stir up the bottom mud. The lake will remain turbid throughout the year, regardless of periods of dry weather, and will have lost much of its attractiveness to fishermen and bathers. Many of the aesthetic values of boating will be gone. The conditions will be entirely the result of changes in the relative abundance of certain fishes, and as the primary function of the reservoir is to supply water, and not recreation, almost nothing can be done to bring back conditions that were obtained in the early years of impoundment." As a result of investigations of small impoundments not used for water supply, many examples were available by 1946 to show that the chemical treatment or the draining of such small impoundments to remove un- desirable populations (such as those that developed in water-supply reservoirs) entirely eliminated "aquatic desert" conditions, and that once tliese impoundments were restocked with desirable fish they did become very productive. Thus, the theory of Ellis ^^ that high fish produc- Reservoir Management 11 tion in the early years of impoundment resulted from organic decay in the new lake basin was largely disproved, since this cycle of production could be repeated as often as the reservoir was completely drained (or poisoned) and restocked with small numbers of fish J In addition, the hypothesis of progressive loss of fertility that had been advanced could be attacked in some situations on the basis of the fact that the amount of flooded vegetation in a lake basin was too meager, in relation to the huge volume of water impounded, to produce a "hay infusion" that would result in an initial high fish production. Recent Advances in Management In 1945 the Federal Office of River Basin Studies was formed. Within the framework of this organization, biologists could appraise the fishery resources of a river before a federal impoundment was built and thereby estimate the eflFect of the impoundment on that resource. Although these benefits or losses were incorporated in the cost-benefit ratios prepared by the U.S. Corps of Engineers, the predicted gains or losses of a fishery seldom influenced the decision to build a reservoir. In 1944, Norris Reservoir (Tennessee) was opened to year-round fish- ing; the results were so encouraging that Tennessee dispensed with a closed season on all of its remaining reservoirs in 1945, as did Ohio that year and some additional states shortly thereafter. After World War II there was a marked expansion of studies of fish populations in reservoirs, particularly in the states of Tennessee, Kentucky, Missouri, Oklahoma, Texas, and California. In many instances there seemed little to be done that would have any effect upon the fish popula- tion of a large reservoir, and the fishermen and biologists were forced to go along with fish population changes resulting from natural biological phenomena.^^ Phases of Operation. By the late 1950's, reservoir management had been reduced to about five phases of operation: (1) The manipulation of water levels to favor ceHain species and depress others.^' '^'^'^^'-^'^^'^^ (2) The addition to reservoirs of certain species not originally present, in order to contribute directly to the creel or to "fill in" indirectly the trophic levels in the food chain of some important fish already present. Good examples of the first type of addition are to be found in the introduction of the white bass and striped bass (the striped bass was isolated in the Santee- Cooper Reservoir and then added to Kentucky Lake ) . On the other hand, the introduction of the threadfin shad is an example of a "fill in" fish to improve food conditions for game fish.--^ (3) The control of overabundant rough fish and/ or forage fish to reduce their competition with game fish for food and space. There was a need for more efficient methods of con- trolling these undesirable fish, of which the rough fish species were 12 History of Fish Management catostomids (bu£Falo fishes, carpsuckers, etc.), carp and other large cyprinids, and drum, and the forage fish, usually gizzard shad. (4) An increase in the harvest of hook-and-line fish. This was done through year-around open seasons, by heated fishing docks, the creation of tem- perature gradients, installation of brush piles, and by other means of attracting and holding fish in concentrations where fishermen could harvest them in the most efficient manner possible. (5) Piihlicitij so that the public would know ''where to go and when" to catch fish. Newspaper stories of catches stimulated interest and induced fishermen to go fishing, but newspapers should also furnish information on where fishes are being caught. Reducing Undesirable Populations. In the management of large res- ervoirs, our greatest progress will probably come with the development of more efficient methods for the reduction of undesirable populations. Recently, fishery management in water-supply reservoirs received a great stimulus through the granting by the Public Health Departments of some states, of permission to use rotenone for the control of unde- sirable fish in these reservoirs. The Public Health Department of the State of Oklahoma prior to 1954 permitted the use of rotenone to kill gizzard shad in a water-supply reservoir. Other states soon followed this lead (Illinois in 1956) until many states now sanction the use of rotenone for the removal of excessive populations of gizzard shad by partial poisoning, or for the poisoning of all fish in a reservoir with subsequent restocking of new populations of selected species. In no case has the use of rotenone or the presence of dead fish for the short period before they are picked up and hauled away from the lake had any noticeable effect upon the water supply after it had been filtered and chlorinated. In many water-supply reservoirs where the rooting of rough fish had kept the water continuously muddy, the com- plete poisoning of these fish with rotenone stopped all mud-stirring activity and greatly reduced the task of water treatment because of the much-reduced problem of filtering-out suspended silt. Water-supply reser- voirs, renovated and restocked with desirable fish, regained all of their former recreational values— fishing, swimming, boating, and aesthetics. WHAT IS IN THE FUTURE? The farm pond, originally used chiefly for livestock, has demonstrated its potential for recreation. Anglers have been quick to appreciate the excellent fishing from properly managed ponds, and some farmers have channeled this interest to supplement the farm income. Furthermore, local sportsmen or sportsmen's organizations are ready to pay well for outdoor recreation directly associated with these ponds. Thus, the pond-building What Is in the Future? 13 movement continues to l:)oom witli recreational uses sharing equally with those of water supply. Many communities still have inadequate water supplies for their resi- dents and for commercial developments. Others that have adequate water today will find they need supplemental water sources in the future. Thus within the near future, most of the available small sites for artificial impoundments in the more thickly populated sections of the country will be in use. In most instances these water-supply reservoirs will be available for public recreation. The U.S. Corps of Army Engineers has created reservoirs or has plans for impoundments on nearly every stream in the United States with any record of primary or secondary flood damage. Many of these planned reser\'oirs may not be built within the immediate future, but enough will be authorized to spread a network of reservoirs throughout the drainage basins of all major rivers. These reservoirs will be created primarily for flood control, but nearly all will have conservation pools that will be used extensively for aquatic recreation. As hunting becomes progressively more restricted and localized, op- portunities for fishing will continue to increase and become more widely distributed. The development of management methods for impoundments will be intensified, but along lines of greater sport production, rather than for the production of fish for food. In the following chapters I have detailed the known characteristics and dynamic processes of warm-water fish populations. These must be thor- oughly understood and appreciated before one can apply management. While the principles of warm-water populations are broad, any applica- tion to a selected population is specific, and hence requires understanding not only of these general principles but also of all the ramifications of a current situation. LITERATURE 1. Baccius, G., "A Treatise on the Management of Fresh Water Fish," London, J. Van Voorst, 1841. 2. Barney, R. L., Am. Fish. Soc. Trans., 54, 168-177 (1924). 3. Barney, R. L., and Canfield, H. L., Fins, Feathers and Fur, 30, 3-7 (1922). 4. Bartlett, S. P., "Rept. Bd. 111. Sta. Comm., Oct. 1, 1890 to Sept. 30, 1892," Springfield, 1893. 5. Bennett, G. W., III. Nat. Hist. Surv. Bull., 22, 357-376 (1943). 6. Bennett, G. W., ///. Wildl, 1(2), 8-10 (1946). 7. Bennett, G. W., N. A. Wildl. Conf. Trans., 12, 276-285 (1947). 8. Bennett, G. W., N. A. Wildl. Conf. Trans., 19, 259-270 (1954). 9. Borodin, N., Am. Fish. Soc. Trans., 54, 178-184 (1924). 10. Burr, J. G., Am. Fish. Soc. Trans., 61, 174-182 (1931). 11. Cahn, A. R., Am. Fish. Soc. Trans., 66, 398-405 (1937). 12. Carbine, W. F., N. A. Wildl. Conf. Trans., 4, 275-287 (1939). 14 History of Fish Management 13. Coker, R. E., Bull. U.S. Bur. Fish., 45, 141-225 (1930). 14. Dyche, L. L., Kan. St. Dept. Fish and Game Bull, 1, 1-208 (1914). 15. ElHs, M. M., Am. Fish. Soc. Trans., 66, 63-75 (1937). 16. Eschmeyer, R. W., N. A. Wildl. Conf. Trans., 3, 458-468 (1938). 17. Eschmeyer, R. W., and Jones, A. M., N. A. Wildl. Conf. Trans., 6, 222-239 (1941). 18. Eschmeyer, R. W., and Tarzwell, C. M., Jour. Wildl. Mgt., 5(1), 15-41 (1941). 19. Forbes, S. A., Sta. Lab. Nat. Hist. Bull, 15(9), 537-550 (1925). 20. GaltsoflF, P. S., Bidl U.S. Bur. Fish., 39, 347-438 (1924). 21. Hederstrom, H., Ron Fiskars Alder. Handl Kunal Vetenskapsakademin (Stockholm) 20, 222-229 (1759). Rep. in Inst. Freshwater Res. (Drott- ningholm) 40, 161-164 (1959). 22. Hubbs, C. L., and Eschmeyer, R. W., Inst, for Fish. Res. Bull, 2, 1-233 (1938). 23. Hulsey, A. H., Proc. Ann. Conf. SE Assoc. Game b- Fish Comm., 10, 285-289 (1957). ^24. Jenkins, R. M., "Reservoir Fish Management— Progress and Challenge,'* Sport Fishing Inst., Washington, D.G., 1-22, 1961. 25. Jenkins, R. M., and Elkin, R. E., Okla. Fish Res. Lab. Rept, 60, 1-21 (1957). 26. Jeppson, P., Frog. Fish-Cult., 19(4), 168-171 (1957). 27. Johnson, M. C, Frog. Fish-Cult., 21(4), 154-160 (1959). 28. Juday, C, Schloemer, C. L., and Livingston, C, Trog. Fish-Cult., 40, 24-27 (1938). 29. Langlois, T. H., Bingham Oceanographic Collection, IX(4), 33-54 (1948). 30. Larimore, R. W., Ill Nat. Hist. Surv. Bull, 27(1), 1-83 (1957). 31. Leopold, A., "Game Management," 1-481, N.Y., Chas. Scribner's Sons, 1933. 32. Lucas, G. R., Am. Fish. Soc. Trans., 68, 67-75 (1939). 33. Markus, H. G., Am. Fish. Soc. Trans., 62, 202-210 (1932). 34. Meehean, O. L., Jour. Wildlife Mngt., 16, 233-238 (1952). 35. Moore, E., Am. Fish. Soc. Trans., 61, 139-142 (1931). 36. Neess, J., Am. Fish. Soc. Trans., 76, 335-358 (1949). 37. North, R., "Discourse of Fish and Fish Ponds," printed for E. Gurll, London, 1713. 38. Shields, J. T., Am. Fish. Soc. Trans., 87, 356-364 (1958). 39. Stranahan, J. J., Am. Fish. Soc. Trans., 31, 130-137 (1902). 40. Stroud, R. H., Jour. Tenn. Acad. Sci., 23(1), 31-99 (1948). 41. Surber, E. W., Am. Fish. Soc. Trans., 61, 143-148 (1931). 42. Swingle, H. S., and Smith, E. V., N. A. Wildl Conf. Trans., 4, 332-338 (1939). 43. Tarzwell, G. M., Am. Fish. Soc. Trans., 71, 201-214 (1942). 44. Thompson, D. H., Ill Nat. Hist. Surv. Biol Notes, 1, 1-25 (1933). 45. Thompson, D. H., "A Symposium on Hydrobiology," p. 206-217, Madison, Wis., Univ. Wis. Press, 1941. 46. Thompson, D. H., and Bennett, G. W., N. A. Wildl Conf. Trans., 4, 311- 317 (1939). 47. Wickliff, E. L., Am. Fish. Soc. Trans., 62, 275-277 (1933). 48. Wickliff, E. L., and Roach, L. S., Am. Fish. Soc. Trans., 66, 78-86 (1937). 49. Wiebe, A. H., Am. Fish. Soc. Trans., 59, 94-106 (1929). 50. Wood, R., Jour. Tenn. Acad. Sci., 26(3), 214-235 (1951). 2 Artificial Aquatic Habitats Artificial lakes are of two kinds: (1) water impoundments for some definite purpose, such as flood control, water supply, or recreation, and (2) water-filled depressions with surface materials or mineral deposits removed. It is interesting to note that these bodies of water show a considerable diversity of aquatic habitat. Thus, artificial lakes (such as lateral-levee lakes along large rivers, navigation pools created by low dams across river channels, or deep main-stream storage reservoirs) are biologically quite similar to natural lakes, whereas, impoundments across small-drainage channels may contain limited biota, species-wise. In fact, when these latter lakes are in densely populated regions, the aquatic animals (primarily birds and mammals) that customarily inliabit remote regions and shun close association witli man, are almost entirely absent. The heavy use of a lake by recreation seekers may even drive away animals only moderately man-shy. Neither coots, nor migrating ducks (where protected) are wary of man, but tliey will leave a lake if boat traffic becomes heavy. When some of the animals that ideally constitute the natural complex of remote standing waters are absent from artificial lakes, interrela- tionships of the living organisms that are present will differ from those found in a more primitive environment. Thus when man is disparaging of the kinds and sizes of fish that an artificial lake produces in contrast to a natural one in some remote region, he is not making a proper com- parison. The differences in animal populations inhabiting natural and artificial lakes will be considered further in Chapter 5. Beyond the area of direct human interference is the natural migration of plants and animals. Since most artificial lakes have not been in existence as long as natural ones, certain organisms have not as yet had the time or the opportunity to populate these newly-created waters. Some organisms get about much more readily dian others, many of the smaller forms being carried by the wind as spores, seeds, eggs, or resting stages that are protected from desiccation by waterproof coverings. 15 16 Artificial Aquatic Habitats Aquatic animals and plants might be arranged in a scale of decreasing ability to traverse the gap between one body of water and another, with some moving in almost as soon as a new lake has been created but with others arriving less rapidly. In fact, opportunities for the movement of some aquatic organisms may be so infrequent as to require many years for their arrival, and others, lacking their usual mode of transportation, may never breach the gap. Certain organisms may gain entry as a result of accident or stocking by man. Because of these variations in migration time and the relatively short existence of artificial lakes, populations of their organisms are usually simpler than those of natural ones. Motivation for the construction of artificial lakes varies with our need for water. Today in the U.S. we inhabit all of our arable lands, and must devise ways to supply water for diverse uses. Although sometimes water problems are related to too much water, usually the amount is inadequate or availability is not synchronized with need. At the turn of the century, engineers envisioned multiple uses for impoundments. Reservoirs were built on community, state, and federal levels to supply water for cities and industries, to irrigate dry lands, and to generate power. More recently, impoundments have been constructed to supply water for navigation during dry seasons and to control floods during abnormally wet ones. However, it was not until the 1930's that many reservoirs were built for recreational purposes, because aquatic recreation had little or no recognized monetary value prior to this time. After the drought and depression years of the 1930's, considerable in- terest centered on farm ponds, largely as a result of the activities of the U.S. Soil Conservation Service which began during that period. Not only were ponds promoted as sources of water for stock but also for their use- fulness as a general farm-water supply for orchard spraying, fire protec- tion, limited irrigation in dry years, and for recreation in the form of fishing, swimming and boating. Furthermore, the damming of eroded gullies stopped the movement of soil down hill so that impoundments created by these dams could be combined with contour plowing and strip cropping as an integral part of the soil and water conservation plan. Other types of artificial water resulted from man's activities in digging at the earth's surface for sand and gravel, limestone and other rocks, coal and other minerals. The empty holes became filled with water and formed ponds and lakes. These, then, are the artificial waters available for fish management. They are often more manageable than natural lakes because they are man-created and are so engineered that they can be better manipulated: In some, construction was originally planned to give maximum recrea- tional values; in others, recreational uses were planned to follow an original but transitory value (such as the removal of gravel). The Farm Pond 17 THE FARM POND The most common type of artificial impoundment and the least ex- pensive to create is probably the woods or pasture pond made by building an earthen dam across a small intermittent watercourse (Figure 2.1). Superficially, these ponds seem to be die simplest type of aquatic habitat, and perhaps thev are; however, intensive investigation of the physical, chemical, and biological characteristics of ponds indicate that even this type of habitat is so far from simple that an exact duplication of any pond is nearly impossible. Figure 2.1. Pasture pond formed by damming a small intermittent water course. No one knows the exact number of farm ponds in the United States, but in a recent report of the U.S. Bureau of Sport Fisheries and Wild- life,^^ Dr. Willis King estimated that since World War II the Bureau has stocked from 30,000 to 40,000 farm ponds annually, which would approxi- mate a total of 450,000 to 600,000 in this 15-year period. Because many ponds had been built prior to World War II, it is quite possible that tlie total number approaches a million or more. Purposes of Farm Ponds When farmers who had built ponds were asked to list their reasons for doing so, 80 per cent gave water for livestock as a reason; 70 per cent 18 Artificial Aquatic Habitats wanted to provide fishing; 13 per cent, to provide irrigation water; 9 per cent, for swimming; 5 per cent, for wildlife; and 4 per cent for all other uses.^^ Many farm families are interested in various forms of out- door recreation, and the farm pond may be the center of these activities : fishing, swimming, picnicking, hunting, boating, and in the north, ice skating. Engineering Considerations for Farm Ponds Engineering specifications for ponds must vary for parts of the country in accordance with differences in rainfall, runoff, tightness of soils, and types of vegetative cover. Ponds must be deeper in the north than in the south in order that the cold winters and thick ice do not result in loss of fish. Increased depth is also a boon to regions characterized by long periods of dry weather. In planning the shore line of the pond, water areas less than 2 or 3 feet in depth should be eliminated, because shallow waters may become choked with aquatic vegetation such as cattail and bulrush, which may form a continuous band around a pond edge. A satisfactory pond must have an adequate water supply that is silt-free. This water supply may be runoff from lands managed under a soil con- servation program, or from springs, flowing wells, or very small streams.''^ The pond should be impounded behind a well-built dam with a spillway adequate to carry off flood waters. Trees and brush should not be allowed to grow on the dam, or continuously around the shore of the pond. The pond should be supplied with a drain pipe and valve large enough to allow fish to pass through the pipe out of the pond with the outflowing water (Figure 2.2). Much attention has been given to the regional engineering aspects of pond construction by the United States Soil Conservation Service and the agricultural colleges of many State universities. Information for most localities of the United States is available for those wishing to construct ponds, and no attempt will be made here to consider more than a few of the simpler aspects of farm-pond construction. Engineering methods for ponds in the southeastern U.S. are much different from those of northeastern, central, southwestern, or north- western regions. In the West, certain ponds ( for example, in Colorado and Arizona ) are used as sources of water for irrigation. ^^ These ponds are pumped full and then are partly drained to irrigate crops during a 24-hour period. These irrigation ponds fluctuate as much as 10 feet, and the water is usually cold. Such ponds no not provide satisfactory fish habitats. However, most ponds are built for uses other than irrigation and are more suitable for fish production, even though their primary function may be supplying water for stock or fire protection. Even if all ponds were The Farm Pond 19 built primarily for sport fishing, the engineering considerations for each section of the country would be highly variable. United States Geological Survey quadrangle maps are very helpful for locating sites suitable for ponds. These maps show 5- or 10-foot contour lines on relatively small areas of land, so that it is possible to select on these maps locations for dams on intermittent water courses, to outline die pond shore line above each selected dam site, and to estimate the approximate acreage of land sloping toward the pond site from which surface water will drain into the pond. Once the prospective builder ■% j^ ■-" „»^:;-*' <«*"■ ::i^ Ji^is**- ■■^V*^*'^ i Figure 2.2. A partially drained pond. The deep area near the center of the picture was specially constructed to permit fish to congregate there in winter. has located all possible sites for his dam, then a local engineer experienced in pond construction can look over the actual sites, select the best ones, make test borings to determine soil strata under the dam sites, and plan the dams and spillway structures necessary to handle the estimated runoff. Many ponds have been built without engineering assistance, and some of them have been successful. However, do-it-yourself pond building is not recommended beyond the preliminary steps described above, because of the close tolerances between the runoff water handled and the type and size of spillway structure required for it. Thus, if the spillway is of the wrong type or is too small, the first flood may wash away the dam. On the other hand, if the watershed is not large enough in relation to the storage capacity of the pond, the pond may be full or nearly full only in wet 20 Artificial Aquatic Habitats weather. In dry years such a pond could become useless both for water supply and recreation. Farm ponds usually range in size from /i acre to several acres. Those smaller than about 1 acre are unsatisfactory for fishing. Often when a pond or lake is planned to exceed 10 acres, the builder has in mind some commercial use, rather than farm water supply and recreation. WHEN IS A POND A LAKE? a The question of when a body of water is a lake and when it is a pond has never been settled to everyone's satisfaction. According to some limnologists,^^ "A lake is thermally stratified, through most of the year, into an epi-, meta-, and hypolimnion.* Only a body of water conforming to this specification will be considered when using the term lake." [asterisk mine] To many this definition is unsatisfactory because most small ponds built by damming steep-sided ravines are thermally stratified "through most of the year," although the stratification is such that the upper edge of the hypolimnion may be indefinite. In contrast, it seems illogical to call a large, shallow body of water a pond; for example, Chautauqua Lake, a natural basin in the floodplain of the Illinois River valley near Havana, Illinois, almost never shows thermal stratification. By definition this lake should be considered a pond, although it has a surface area of nearly 3500 acres. In Oklahoma, 10 acres represents the point of separation between lakes and ponds; thus bodies of water with less than this surface area are ponds and those above it, lakes. ^^ However, Dr. W. C. Starrett suggests that the point of separation be set at 4 acres, and Humphrys and Veatch ^^ con- sider Michigan waters of less than 5 acres as "lakelets and ponds." In any case, if we are not to use the terms interchangeably, the separation should be based on an arbitrary upper size-limit for ponds, and any body of standing water above this limit should automatically be con- sidered a lake, regardless of its limnological characteristics. ARTIFICIAL LAKES FOR DOMESTIC USES A great many artificial lakes have been constructed throughout tlie United States for urban and/or industrial water supplies. These artificial impoundments may vary in size from a few hundred acres to many thousands of acres. On these lakes, recreation is of secondary importance to water-supply uses, although an effort often is made to sell the reservoir to the public on the basis of its recreational attractions. * Epilimnion ( upper lake ) , metalimnion ( middle lake or thermocline ) , liypolimnion (lower lake). Artificial Lakes for Domestic Uses 21 Water-supply Reservoirs Water-supply needs for towns and small cities frequently are met tlirough the construction of artificial impoundments of intermediate sizes. In the East, an attempt has been made to restrict trespass on these water- supply reservoirs and thus to prevent their use for public recreation. Only recently has this policy been reversed. In the Midwest and West, almost no attempt has been made to restrict the recreational uses of water-supply reservoirs, and fishing, swimming, and boating are common. Where recreation is not restricted, a great deal of use may be made of water- supplv reser\^oirs; however, water consumption holds priority, and where or when other uses conflict with the primary use, they must be sacrificed. Recently, the health departments of some states have allowed the use of rotenone in water-supply reservoirs to control the overabundance of small stunted fish, or dominant populations of bottom-feeding fishes responsible for strirring up the bottom mud. Rotenone in dosages great enough to kill fishes is nontoxic to warm-blooded vertebrates. A detailed discussion of the use of rotenone will be given later. Reservoirs built for city and town water supplies are often created by damming permanent streams or small rivers. The smallest stream capable of filling the reservoir basin and also of supplying the annual needs of the community is the most practical choice. This is true because the silt load carried by a stream is roughly proportional to its size, and the useful life of a reservoir depends upon the rate of silt deposition in its basin. All permanent streams contain fishes: some species can not maintain their populations in impounded waters, others multiply excessively in reservoirs and create a constant turbidity through bottom-rooting activ- ities. These latter species spoil the fishing by reducing the visibility for fish that feed by sight, and also reduce aesthetic values for swimmers and boaters. New water-supply reservoirs stocked with bass and pan fish usually go through a regular fishing cycle which requires about 6 or 7 years from the time water is first impounded.-^ At the end of that time active measures must be taken if recreational and aesthetic values are to be maintained. These techniques will be discussed in Chapter 6. Most water-supply reservoirs for cities in agricultural regions are relatively shallow because of the moderate slope of the land. These reser- voirs are almost always thermally stratified in summer and lose their supply of oxygen in the deeper water (eutrophic lakes). Usually in rough or mountainous regions, water-supply reservoirs are comparatively deep and infertile. Because the deeper sterile waters do not lose their summer oxygen supply, these lakes ( oligotrophic ) support cold-water fishes such as lake trout and whitefish. 22 Artificial Aquatic Habitats NAVIGATION POOLS In some of our larger rivers, locks and dams have been installed to maintain water depths for navigation. Examples of such rivers are the Mississippi, Ohio, and Illinois. Although these relatively shallow impound- ments retain some current, the river rapids (important in the successful spawning of some fishes such as the blue sucker) have been largely eliminated. Navigation locks and dams are under the jurisdiction of the U.S. Corps of Army Engineers, which builds new dams, maintaining the present installations and also a river channel of a specified depth for the movement of towboats and barges. Studies of the fishes living in the navigation pools of the upper Mississippi indicate that they support both an extensive sport, and a commercial fishery.^' -^ Low dams across a large river will result in the permanent flooding of backwater areas adjacent to the river, except when the level of an up- stream pool may be lowered to furnish water for navigation downstream. When this occurs, backwater lakes may be drained quite rapidly, some- times to the detriment of their fish, particularly in winter when these backwaters are covered by thick ice. LATERAL-LEVEE RESERVOIRS Low lands in the flood plains of rivers are sometimes protected from the river by levees; these low lands are pumped dry for agricultural uses. However, when these areas are abandoned or reconverted into lakes with the levees still intact, they become lateral-levee reservoirs. These shallow reservoirs are very productive. Some of them are supplied with stone or concrete spillways to allow the entrance of water from the adjacent river when it rises above the spillway crest. Then, as the river level recedes, water flows out of the lake until the spillway crest level is again reached. The water in such a lateral-levee reservoir may fluctuate moderately to follow changing levels in the river when the level of the reservoir is below the spillway crest due to slow seepage through the levee. These lakes are quite turbid, due primarily to the action of wind.^^ They are productive of hook-and-line fish,'^' -' and at the same time may support a large commercial fishery for such river species as carp, buffalo, freshwater drum, and channel catfish. MULTI-PURPOSE RESERVOIRS Large impoundments constructed by the federal government in many parts of the United States have been justified on the basis of a combina- Multi-Pur pose Reservoirs 23 tion of two or more uses, such as flood control, navigation, the generation of electric power, irrigation, and recreation. Not all of these values are assigned to one reservoir; usually irrigation is a western assignment, navigation may be a localized or general assignment, and the generation of power requires a dependable and constant source of water. Some of these uses appear to conflict with one another. For example, flood conti-ol demands an empty reservoir to give maximum flood storage, whereas navigation, irrigation, and the generation of power require a full basin, since they depend upon the release of water from the reservoir. Furthermore, although recreation may ride along with a changing water level, it is gone when the reservoir basin is dry ( as in some flood control projects ) . However, generally speaking, needs are seasonal, so there may not be intensive competition for water at any one time. These apparent conflicts of purpose are resolved by assigning a range of levels to specific uses. First, the basic purpose is taken care of by setting a conservation-pool level or elevation near the bottom of the reservoir. Until this "absolute minimum" water level has been exceeded, no large amount of water will be released. In a reservoir of 24,000 to 30,000 acres, water at the conservation-pool level might create a lake of 3000 to 6000 acres. Then, other fractions of the reservoir's storage capacity may be assigned to power, navigation, or irrigation. Usually the flood control function belongs to the upper layer of reservoir capacity which is drained off after each flood as rapidly as the river channel below the dam will permit (bank full), so that the upper lake in the reservoir will be available for storing water once again should another flood occur. In the operation of a multi-purpose reservoir, water in the river channel below the dam is never allowed to exceed the top of the river banks ( as long as any storage capacity exists in the reservoir), or to fall below a certain minimum flow in drought periods, even if it means using some water assigned to the conservation pool. This minimum flow is so small in relation to the capacity of the conservation pool that there is little danger of ever draining the latter. The controlled release of water into the river channel below the dam insures a constant supply for agrarian users and towns situated on the river and maintains the fish population that inhabits the river below the dam. Experience has shown that fish congregate in waters below tlie outlets of these large reservoirs through upstream migration, and extensive sport fisheries have developed in these tailwaters. The type of fishery that results is dependent upon ( 1 ) the temperature of the water released from the dam during summer, ( 2 ) what fish are available in the stream below, and ( 3 ) which ones may be stocked in it. The temperature of the water, in turn, depends upon the vertical location of the outlet gates on the face of the dam. 24 Artificial Aquatic Habitats PONDS AND LAKES WITH EXCAVATED BASINS Although lake and pond basins can be completely excavated with earth-moving equipment, this is infrequently done because of the high cost. However, sometimes the clay needed for a pond dam may be re- moved from the sides of the pond basin, so that the basin can be enlarged and deepened in the process of building the dam. Also, ponds are some- times dug or enlarged and deepened in real-estate developments or in other special locations where cost is of secondary importance. On the other hand, there are many kinds of excavations made by man that become filled with ground water and are eventually stocked with fish. These "holes" are excavations for the removal of gravel, limstone, stone, coal, or other near-surface mineral deposits. Some of these water-filled pits are among the most attractive waters to be found south of the lake states, because they are clear and quite infertile. Recent construction of super-highways has resulted in many ponds where clay has been removed to build grades for road overpasses. Gravel-pit Lakes Gravel deposits have been left by rivers from melting glaciers. Most of these deposits, although covered with soils, are readily relocated by test borings in regions where excavations ( such as well drillings ) or other evidence have shown the presence of gravel. Since considerable gravel is needed for road beds and as a component of concrete, this product is in constant local demand. The sale of gravel, therefore, while not so remunerative as that of most other minerals, furnishes more than enough to pay for the cost of pond excavation. With a little planning, the excava- tions left when operations are over, become attractive recreational waters. When digging is done primarily to develop recreational ponds (often the case when gravel deposits are located under high-priced farm lands ) , plans may be made for the arrangement of ponds, the leveling of the spoil banks, and the respreading of the top soil over leveled areas in order to greatly improve the pit area ( Figure 2.3 ) . In regions where lakes and ponds are scarce, a well-planned recreational area superimposed on an abandoned gravel works may bring a better price than the original farm land, or, if strategically located and properly managed, produce annual income equal to, or exceeding, the income from farm crops. A flooded gravel works may consist of a number of small ponds separated by levees of sand or clay. Often when the pit owner decides to develop the area for recreation, his first thought is to connect all of these ponds. Yet, experience shows that several isolated ponds are more easily managed for fishing than a single large one. Moreover, separated ponds allow a greater variety of fishing because species can be stocked in some, Ponds and Lakes with Excavated Basins 25 that would not survive if placed in one large pond containing other more aggressive fishes. However, no pond should have an area of much less than one acre; otherwise, it \\ ill be too small for satisfactory fishing. The standing crop of fish supported by gravel pits may be lower than that of artificial ponds receiving direct surface runoff from farm lands, Figure 2.3. Gravel-pit ponds planned for recreational uses after gravel removal. Current gravel-removal operations were confined to the long narrow ponds in the pit area at the upper left. Bodies of water were kept separated and stocked with various combinations of fish, so that each pond could be easily renovated if fishing became poor. The square pond at the upper left is about 4 acres; a bathing beach and diving pier were built at its lower left corner. but the fish-population size (weight) is related to the fertility of the surrounding land, in spite of the fact that there may be no direct surface runoff into the gravel-pit ponds. The water level in a gravel-pit pond is that of the water-table level because the sand-bottomed basin will not hold water. This may be 26 Artificial Aquatic Habitats demonstrated by transferring water from one pond into another by a large pump. The pond from which water is being pumped may drop several feet and the other pond rise in proportion, but if the pump is stopped, water will seep out of the high pond and into the low one, so that within a very short time both will again be at ground water level. Since levels in gravel pits fluctuate from one to three feet during most years, this annual fluctuation should be taken into consideration when bottom contours are being planned. The exposure of large areas of the bottom, as a result of normal annual fluctuation, is unsightly and may be avoided if the more shallow areas are deepened. Where gravel beds are extensive, gravel digging operations can be planned to create areas of open water up to four or five acres, or to create relatively narrow meandering channels. Once the spoil banks have become vegetated with trees, shrubs and herbaceous plants, the channel arrange- ment is more attractive to anglers than is a large area of open water. Gravel-pit ponds are thermally stratified in summer because they are surrounded by high lands that reduce the wind action on the water. Stripmine Lakes Stripmine lakes result from the flooding of surface excavations after the removal of coal deposits. Layers of coal may vary in depth below the surface, and stripmining equipment can operate at a profit on deposits as deep as 50 to 90 feet. Before the coal can be stripped, the top soil and clay overburden must be removed. This operation is done with giant cranes supplied with digging buckets which pile the waste material in long ridges running parallel to the cuts they are making. As the coal in each cut is exliausted, the crane moves over to dig a new cut parallel to the other one, and the material from the new strip is piled into or along the previously exhausted excavation. Thus, strip mining actually turns land upside down (because the topsoil is buried under the clay over- burden ) and leaves an area of land as a series of long parallel ridges many feet high. In some stripped areas, water collects in the valleys between these ridges and forms long narrow lakes usually connected to similar lakes between other ridges by channels. The last "cut" or strip that the crane makes before abandoning an area is not filled in and may form a lake several hundred feet wide, as deep as 60 feet, and a mile or more long, depending upon the size of the stripped area, the depth of the coal, and the lengths of the booms of the stripping cranes. Coal deposits are associated with deposits of sulfur, iron, and other minerals. When the abandoned mines are flooded, these dissolved minerals often make the mine ponds too acid to provide a habitat for aquatic organisms.-- However, ponds become less acid with age; the sulfuric acid buffered with calcium from limestone deposits and other carbonates, Ponds and Lakes ivith Excavated Basins 27 or the runoff water from higher lands may flush out the stripmine lakes, removing some of the acidic material. Occasionally, stripped lands ad- jacent to small rivers are flooded by the latter, resulting in a great improve- ment in the sulfate composition of the stripmine water after the flood recedes. In many respects, stripmine lakes are similar to gravel-pit lakes, but they usually differ markedly from them in the acid and mineral content of the water. Stripmine waters usually contain several hundred parts per million of sulfate; in one instance, fish were found living and reproducing in a mine pond that contained 1500 ppm of sulfate. Stripmine ponds vary considerably with one another in the weight of fish supported because of their great range of dissolved salts. It is reason- able that fish production might be low in new stripmine waters, since many invertebrate animals and algae in the food chain are more sensitive than fishes to abnormal mineral content. But aging, weathering, flooding, and the annual accumulation of dust and leaves gradually build up the basic fertility and reduce the chemical imbalance of these waters until they become very fertile. ^^ The waters of the South Polly wog Association, an 80-year-old stripmine area in Vermilion County, Illinois, support populations of miscellaneous fishes as high as 750 pounds per acre. Quarry Lakes A great deal of limestone is used in agriculture as limestone dust to neutralize acid soils and as crushed rock in road building and other construction. Other deposits of rock of value in building may be quarried from near-surface deposits. Quarrying and strip mining operations are somewhat similar in that often the top soil and overburden are first removed, leaving the strata of limestone or other rock exposed. In quarry- ing, limestone and other rocks are subsequently loosened by blasting, and loaded in large trucks by cranes. Then, the blasted rock (the lime- stone portion) is taken to a rock crusher capable of producing particle sizes, from limestone dust to rocks as large as hens' eggs. Since deposits of limestone are often below the level of tlie water table, quarrying operations depend upon pumps to remove the water as it seeps into the quarry pits. When all of the valuable rock has been extracted, the pumps are stopped and the pits fill up to the level of ground water (Figure 2.4), forming one or several bodies of water. New limestone quarry waters are usually quite infertile because they contain almost no phosphates, nitrates or organic material. However, minerals causing hard water are present in abundance and organic nutrient materials may be carried into quarries through surface runoff from surrounding lands, so that the production of fish may increase quite rapidly as the water in the abandoned quarry ages. 28 Artificial Aquatic Habitats Quarry ponds are similar to gravel-pit ponds and stripmine ponds in that they are usually dependent on subsurface waters rather than super- ficial drainage. Also, they are thermally stratified in summer, due to their relatively great depths and to the limited action o£ winds on the surfaces of these ponds. In depth, quarry ponds may exceed gravel pits and strip- mines, depending upon the depths of the deposits, and whether it is economically feasible to quarry them. Figure 2.4. Ponds resulting from the quarrying of limestone are quite sterile and their waters are usually very clear. They make a satisfactory habitat for smallmouth bass. LAKES BUILT FOR RECREATION Within the past decade, many artificial lakes have been built entirely for the purpose of furnishing aquatic recreation (Figure 2.5). There is scarcely any way in which recreation funds can be spent to produce such a large return over so long a period. At first, artificial lakes for recreation appear to be expensive. However, costs of such public lakes can be amortized over the life of the impoundment,'' providing a long period of usefulness and, consequently, an intangible return great enough to make them highly practical. Furthermore, where lakes are not built primarily for recreation, they still should be planned and constructed to allow easy management of their fish populations, and should have an outlet large enough for quick drainage and a sloping basin that will empty completely. From the standpoint of ease in management, it is more practical to build five 100-acre lakes than one lake of 500 acres. A satisfactory site for a recreational lake requires a basin with a Planning^ an Artificial Lake or Pond 29 minimum area of shallow water (less than 4 feet deep). Shallows become problem areas because thev fre([uently become choked with aquatic vegetation. These areas, when filled with dense, rooted acjuatic plants, are useless for fishing, boating, and swimming, and may become a breeding location for mosquitoes because the fish are unable to reach the mosquito "wigglers." Extensive shallow areas are unnecessary for the successful reproduction of nest-building fishes as may be demonstrated in stripmine Figure 2.5. Many lakes are built wholly for recreation. This one on the Fin 'N Feather Club property near Dundee, Illinois is used primarily for large- mouth bass fishing. All trees on the immediate shore line were planted from nursery stock, with even the logs and rocks brought in from outside sources. and quarry ponds where shallows are very limited. In planning the height of a dam it is sometimes possible to raise or lower the proposed water level a few feet to give a minimum of shallow water. Lakes built for recreation may be developed to any degree, that is, the grounds may be left in a relatively natural state with only access roads or there may be surfaced roads, boat docks (and boats for rental), bathing beaches, bath house facilities, picnic areas, pavilions, and cabins. PLANNING AN ARTIFICIAL LAKE OR POND As was mentioned in connection \\ith the planning of farm ponds, a layman may handle certain preliminary details of impoundment provided 30 Artificial Aquatic Habitats he can read a contour map and estimate land areas. In looking for suitable sites, the planner will find the quadrangle maps of the United States Geological Survey very useful. These maps, which are usually available through the federal or state geological surveys, show land elevations through the use of contour lines. This makes it possible to locate on them sites for lake basins and dams, or if a potential site has been found through field observations, to determine several suitable locations for a dam, and to estimate the amount of land that will drain into the pond or lake once the location for the dam has been decided upon. Watershed, Runoff, and Water Manipulation Rainfall, slope of land, and vegetative cover vary within certain limits, but a definite relationship exists between the volume capacity of a pond basin and the area of the watershed needed to keep that basin filled. Although usually it is impossible for the layman to calculate the volume capacity of a selected pond basin, he may, by the use of a quadrangle map, arrive at the approximate surface area of the basin and tlien consider this in relation to the area of the watershed. Where soils are relatively tight ( for example, in Illinois, Iowa, and Missouri ) , the approximate limits of range for the watershed are a minimum of 10 acres and a maximum of 50 acres to 1 acre of pond surface assuming a basin of average depth and contour. However, if this index is used and the drainage area is less than 10 acres, insufficient runoff water will be available during dry periods. On the other hand, if the watershed is 50 or more acres, so much water in excess of pond capacity must be passed through the basin that a large and expensive spillway is necessary. Probably the optimum ratio between watershed and pond surface area is in the neighborhood of 20 or 25 to 1. Optimum relationships between the drainage area and the pond size and volume vary greatly among parts of the country with differences in rainfall, slope, soil types, vegetative cover, and evaporation rates. Small ponds in the north central states having ratios of watershed to pond surface of less than 15 to 1 may be safely constructed with grass waterways to carry off excess water. However, where the watersheds and ponds are larger, spillways are usually constructed of concrete or stone. It is unnecessary to screen spillways to prevent the departure of fish from a pond or lake, as only a small fraction of the fish population will leave. Screens across spillways have a way of becoming clogged during floods and sometimes are responsible for washouts of dams. It is more im- portant to prevent fish from moving up over a dam from below, and spill- ways should be planned to provide insurmountable barriers to fish moving upstream. There are many problems involved in handling water flowing in and Thermal Stratification and Loss of Oxygen 31 out of a pond basin. Some of these are associated with difiFerences in rainfall and evaporation rates. Others involve local situations such as variations in land slope and cover, control of water from a constant source (such as a spring or a flowing well), or the by-passing of excess water w^here the onlv suitable site for a pond is adjacent to a water course too large to be impounded. Solutions to most of these special problems will require the services of a competent hydraulic engineer. THERMAL STRATIFICATION AND LOSS OF OXYGEN Most artificial ponds and lakes in the United States are thermally stratified during the warmer months. This stratification may develop in earlv March and extend well into November in the South. In the extreme North, summer thermal stratification may begin in late May or early June and end in late August or early September. Most artificial impound- ments, with the exception of the deeper power and water-supply reser- voirs, are eutrophic in character, that is, they contain no oxygen in the colder, deeper waters during the greater part of the period of summer stratification. It is true that early in the season, once the lake has become thermally stratified, there may be oxygen in the lower waters. Gradually, however, the oxygen demand from decay and from respiration of bacteria, plankton, and fishes uses up all of the available oxygen in the lower lake level (hypolimnion) so that this water may be completely devoid of oxygen. Sometimes dewatering structures ( spillways ) have their lake-side open- ings near the bottom of the lake, in order to expel oxygen-deficient water from far below the surface. For example, at Ridge Lake, Coles County, Illinois,^ a tower spillway on the inner face of the dam was designed to release water from the bottom of the full lake each time runoff from rain- fall raised its level. Studies of dissolved gases and bottom fauna in this lake indicated that the beneficial effect on it of this disposal of oxygen- deficient water was, at best, very temporary. Even though oxygen was added to the lower levels of the lake each time a substantial rain fell on the watershed, this new oxygen was so rapidly used up that no aerobic bottom organisms had an opportunity to develop. Seasonal Thermal Stratification A lake or pond is stratified when layers of water at various depths do not and will not mix with one another. For a detailed explanation of all of the ramifications of thermal stratification see Welch, Ruttner, or Hutchison.14' 26, 31 Rriefly, stratification has its basis in the fact that water shows maximum density (weight) at 4°C (39.2°F), becoming less dense (lighter in 32 Artificial Aquatic Habitats weight) both above and below this temperature. Let us consider the fact that soon after the ice melts in the spring, the temperature of the water in a lake rises to 4°C, bringing the entire lake to a uniform density. Then, winds blowing across the lake surface pile the water up on the windward side, and in order to compensate for this, water passes downward across the bottom of the lake to the upwind side. The entire lake begins to circulate from top to bottom. As spring advances, however, there are days when the wind blows lightly or not at all, and the sun beating down on the lake begins warming the water on the surface, causing it to become less dense ( lighter ) than the colder water below. After the surface water has warmed a few degrees above the water in the lake depths, thermal stratification has begun and no ordinary wind will cause the two to mix. The surface water will mix with itself down to a depth of several feet or yards (meters), the depth depending upon the wind velocity and the area of lake surface acted upon. Thus the warm surface layer (epi- limnion) tends to be thicker on large lakes than on small ones. The temperature of the epilimnion is about the same from top to bottom, but this may vary a few degrees during days when the surface is warming rapidly and winds are light. Below the epilimnion is a layer of water (the thermocline or meta- limnion) where the temperature of the stratum decreases rapidly as one progresses downward, that is, one degree centigrade per meter (about 1.7°F per yard). The thermocline may vary in thickness in different lakes and at different times during the period of stratification. Although in large lakes the thermocline usually is a thinner layer than the epilimnion, in small ponds it may continue from the lower edge of the epilimnion to the pond bottom. In large deep lakes the volume of water below the thermocline ( hypo- limnion) tends to show a fairly uniform temperature. Where there is a significant temperature decrease as one moves downward, that change is less than 1°C per meter. As mentioned previously, lack of dissolved oxygen makes the hypolimnions of some lakes uninhabitable for most aquatic organisms. The rapidity and extent of eutrophication is dependent upon the volume of oxygenated water and the amount of organic decay and respiration placing demands upon the oxygen. For example, after the spring period of complete circulation, very deep and relatively in- fertile lakes have a very great volume of oxygenated water in the hypolimnion, and the oxygen-consuming organisms and processes are proportionately small. In these lakes, the oxygen is not used up in the hypolimnion during summer stratification, and they are inhabited by all kinds of oxygen-requiring organisms, including such fishes as lake trout, white fish, and walleye. This lake type is called oligotrophic. Thermal Stratification and Tajss of O.xf/^en 33 Variations in Thermal Stratification Bardach - describes die progress of suPiiiner stratification in Lake West Okoboji, Iowa. In diis lake, tliermal stratification normally begins between May 15 and June 1 after die water below 30 meters has already reached 10°C (50°F. ) Then, during the summer the hypolimnion warms up further, to 12° or 13°C (53.6° or 55.4°F). However, in 1925, 1926, and 1950, when unusually heavy winds were recorded in late spring, West Okoboji did not form a diermocline until very late in the season or not at all, and, if it did form one, it was situated at a greater depth than usual. In some years this abnormal sti*atification consisted of an upper warm layer, below which the temperature gradually dropped as one progressed toward the lake bottom, until at 22 meters (72.2 ft.) the temperature was 6 degrees centigrade lower than in the epilimnion [ep)ilimnion, 20.6°C (69.1°F), and bottom, 14.3°C (57.7°F)]. The vertical change in temperature was less than 1°C per meter so that bv definition no thermocline was present. Nonetheless, there was no evidence that thermal stratification was ever completely broken up during the summer months. As mentioned previously, a similar type of stratifica- tion appears to be characteristic of numerous small artificial impound- ments. Fall Overturn Summer thermal stratification is broken up in the fall by wind action after the epilimnion cools to a temperature approximately that of the hypolimnion. Gradually, the entire lake begins to circulate, as the winds create water currents across the surface and compensating currents develop across the lake bottom. Water that has remained trapped in the lake depths all summer again comes in contact with the surface layers where free and dissolved carbon dioxide has an opportunity to escape and the dissolved oxygen supply is replenished. As fall progresses into winter, the lake water cools to 4°C (39.2°F) and below, and the colder upper layer becomes less dense: In the north a film of ice seals the surface and the lake is again thermally stratified, with the ice and colder water above the mass of water at 4°C. As long as ice covers the lake, very little circulation takes place. Some convection currents may be set up through the mild warming of water in contact with the bottom, but these warming forces are counteracted by colder water immediately under the ice. Once the lake is sealed from the air, the oxygen supply under the ice is dependent upon the photosynthetic activity of algae which in turn is supported by light transmission through the ice. 34 Artificial Aquatic Habitats Attempts to Upset Thermal Stratification Some attempts have been made to upset the thermal stratification of small lakes. ^' ^- For example, 180 hours of pumping of warm surface water into the bottom of a small German lake increased the temperature of the bottom water by 5°C.^ Also, this pumping initiated movements of water within the hypolimnion, thus causing an increase in its thickness as well as a shorter temperature gradient within another stratum, the thermocline. In an experiment in a 3.6-acre Michigan lake, water was pumped from the hypolimnion to the surface.^^ This caused a progressive increase in the depth of the epilimnion, a sinking of the thermocline at a nearly constant rate, and a decrease in the thickness of the hypolimnion as the bottom water was displaced. The upper limit of the thermocline was lowered from 13 feet to 25 feet, and the volume of the epilimnion was increased by 49.9 per cent. An attempt was made to follow the movement of the cold bottom water after its release at the surface. Apparently, the cool water became mixed rather thoroughly with surface water within the upper 4 to 5 feet. These experiments demonstrate that a large amount of energy is re- quired to modify normal thermal stratification in even a small lake. In lakes of moderate or large size, such a program would be highly im- practical. THERMAL STRATIFICATION AND RESERVOIR OUTLETS Many large reservoirs are equipped with outlet gates at or near the bottom of the impounding dams. Figure 2.6. Temperatures of water released through these gates range from 4° to 18.3°C (39.2° to 65°F), and may or may not contain sufficient oxygen for fishes. Usually if the water does not have an adequate amount, it become aerated a short distance below the outlet of the dam. In this location, trout are able to survive, and often grow very well, extending their range downstream until the water becomes too warm.-^ Where such a trout fishery has developed, it usually has been necessary to modify the original water- release program designed by the engineers, since to restrict the flow to only a few months of a year is impractical from the standpoint of de- veloping an artificial trout stream. In addition, it is worth noting that the release of cold water alters the bottom faunal pattern from large warm- water species to small cold-water species, such as the insect families, Tendipedidae, Simulidae, and Hydropsychidae, as well as snails and the scud Gammarusr^ The tailwater discharge below Dale Hollow Reservoir is an example of a man-made trout stream.-- This tailwater flows for 7.3 miles before it Thermal Stratification and Reservoir Outlets 35 LATE SPRING cold tailwater warm, oxygenated fish producing zone cool, transitional zone cold, oxygenated supports fish LATE SUMMER cold tailwater lower water level warm, oxygenated fish producing zone cool, transitional zone cold, oxygenated supports fish oxygen supply decreasing deficient for fish Figure 2.6. Storage reservoirs with deep water outlets. A water-release pro- gram that will furnish an adequate quantity of water cold enough for a year- round trout stream often supplies very fine fishing. However, trout must be stocked from hatcheries, as tailwater streams usually are not suitable for nat- ural spawnings. [Redrawn from Stroud, R. H., and Jenkins, R. M., Sport Fish- ing Inst. Bull, 9S (I960)] 36 Artificial Aquatic Habitats enters the Cumberland River in Clay County, Tennessee. The combined minimum flow when water is operating the three turbines is 5900 cubic feet per second. When the turbines are not in operation, there is a natural cold-water discharge of 19 cubic feet per second. Discharge schedules vary from year to year; shutdown periods of several days are common in the summer and fall, and levels of the tailwaters fluctuate within a 10-foot range. The water discharge below the Dale Hollow Dam is always clear ( turbidity less than 5 ppm ) , and the water temperature of the discharge ranges between 7.2° and 13.3°C (45° and 56°F). The minimum discharge of 19 cfs has maintained a water temperature cool enough for trout in the upper three miles of die tailwater during extended shutoff periods. However, the best periods for trout fishing are on weekends when the turbines are shut down and water levels are low. Dale Hollow Reservoir dam is 178 feet high, and the water depth at elevation 651 ( spillway level ) is 151 feet. The annual water-level fluctua- tion on this 30,000-acre lake is usually less than 25 feet. Excellent tailwater fishing for warm-water fish may occur where water is released from a reservoir at surface or near surface levels ( Figure 2.7 ) . Fish migrate upstream in the river formed from the overflow, and when they reach the barrier of the dam, they tend to remain in the tailwater pool. These tailwater fisheries never equal the fishing operations in the reservoir above the dam,-^ but this seems to be so because the fishermen are concentrated at the tailwater fisheries. Stroud and Jenkins -^ favor reservoir outlets located to release cold water (often deficient in oxygen) from reservoir depths because there is "a continuous discharge of oxygen-consuming decomposition materials with the colder, deep waters." This prevents stagnation and makes "maximum reservoir volume available for use by fish life." At the same time, the warm upper water is retained to promote fish production. OTHER FACTORS AFFECTING THERMAL STRATIFICATION Sometimes waters that enter lakes from feeder streams influence thermal stratification because such waters seek their appropriate density (weight) level. In south central Nebraska, a small reservoir built across Rock Creek (a stream originating from a large spring) always contained oxygen in the deeper water because the cold water entering from the stream moved along the lake bottom carrying dissolved oxygen with it. In winter, water from tributary streams may be colder than the lakes into which they flow, forcing lake water from deeper layers upward Other Factors Affcctiu}^ Thermal Stratification 37 LATE SPRING warm, oxygenated ^}iii fish producing zone cool, transitional zone cold, partially stagnant limited fish and invertebrates LATE SUMMER III lower water level warm, oxygenated fish producing zone cool, transitional zone warm tailwater cold, stagnant no fish or aquatic life Figure 2.7. Storage reservoirs with shallow water outlets. Warm-water fishes congregate in these tailwaters, having migrated upstream from below. Tail- water fishing is popular, and in a warm tailwater, fishermen may catch a variety of fishes. [Redrawn from Stroud, R. H., and Jenkins, R. M., S.F.I. Bull, 98 (I960)] 38 Artificial Aquatic Habitats toward the surface.-^ Cold, turbid waters entering Norris Reservoir (Tennessee) often formed a wedge of water between surface and bot- tom.^-' ^^ Water above and below this wedge was relatively clear, so that the vertical extent of the wedge could be defined on the basis of turbidity alone. In small ponds thermal stratification may be affected by blooms of plankton algae which form a near-surface blanket insulating the lower waters from light and the warmth of the sun's rays. For example, a farm pond adjacent to a barn lot near Illiopolis, Illinois, sampled in July 1939, showed an epilimnion 10 inches in thickness, containing a very dense "bloom" of plankton algae. The temperature throughout the epilimnion was 27.2°C (81°F), but at 13 inches below the surface the temperature was 21.6°C (71°F), a drop of 5.6°C (10°F) within 3 inches. Also, the dissolved oxygen was entirely gone at 13 inches below the surface. In spite of these extreme conditions, this pond contained bluegills, some of which were caught in traps set at the surface level. BIOLOGICAL PRODUCTIVITY OF WATER The biological productivity of water is a function of the nutrient ma- terials (organic and inorganic salts) dissolved in it and available from other sources. Although many experiments designed to test the value of inorganic fertilizers in pond fish production will be reviewed in Chapter 6, now it is important to stress that the addition of organic or inorganic plant nutrients to a body of water facilitates an increase in the production of phytoplankton, which, in turn, causes an increase in the production of zooplankton and insect larvae and, somewhat later, of fish. Furdiermore, increase in fish production is more pronounced among species that make direct use of the available phyto- and zooplankton organisms and insects than among those species of fishes that are piscivorous or have more specific food requirements. It can be demonstrated that, in natural waters, the chemistry of soils in the lake basin and its watershed are related to the water of the lake in question. The amounts of certain chemical compounds dissolved in natural waters are indicators of relative productivity. Several investigators have shown a positive relationship between alkalinitx' and fish production in lakes grouped as soft water (less than 50 ppm Methyl Orange alkalinity), medium (50 to 150 ppm) and hard (above 150 ppm). However, the greatest interruption of this relationship appears at about 40 ppm,-^ for above 40 ppm there seems to be no concise relationship between car- bonate content and fish yield. Also, none could be found between fish Lake Size and Productivity 39 yields and varying amounts of ionized hydrogen, carbon dioxide, or chlorides. Further, there was no relationship between sulfates and pro- duction until the sulfates exceeded 300 ppm.-^ Table 2.1 shows a productivity classification of natural lakes (Minnesota) on tlie basis of total alkalinity and sulfate ions.-*^ Table 2.1 A classification of natural lakes in Minnesota on the basis OF total alkalinity and sulfate ions (from moyle-0). Total alkalinity (ppm) Sulfate ion (ppm) Classification Productivity Fish Plant 0.0-20.0 0.0- 5.0 Very soft Low Low 21.0-40.0 0.0- 10.0 Soft Low to medium Low to medium 41.0-90.0 0.0- 10.0 Medium hard Medium to high Medium to high 91.0 or more 0.0- 50.0 Hard High High 100.0 or more 50.0-125.0 Medium alkali High High 100.0 or more 126.0-300.0 Alkali High High Minnesota ponds containing amounts of phosphorus below 0.05 ppm had low fish yields.^*^ Above a concentration of total phosphorus of 0.05 ppm, there was little difference in either average or maximum yield. Moyle concluded that the optimum concentration of total phosphorus might lie between 0.1 and 0.2 ppm; however, these phosphorus concentrations were usually associated with heavy algal blooms which may create a danger through their ability to cause sudden oxygen depletion. LAKE SIZE AND PRODUCTIVITY Prior to 1946, there seemed to be evidence of a straight-line negative logarithmic relationship between size of a lake and fish production, when all data then available were used.-^ These data included complete fish censuses of a number of small ponds, creel censuses as measurements of production on medium-sized waters, and commercial catches of fish on the larger lakes. No consideration was given to the possible effect of the geographical location of these waters and of regional soil fertility on production. Later, when these data were reworked and consideration was given to location and soil fertility, the apparent relationship between size and productivity disappeared.^ Information on yields from additional lakes (Minnesota) -^ in the form of average gill net ratios demonstrated that lakes of over 5000 acres in area were more productive than those of smaller sizes, while data from creel censuses indicated that lakes ranging in size between 500 and 1000 acres were more productive than those 40 Artificial Aquatic Habitats larger or smaller. One is forced to conclude that lake size alone has little significance as an index of productivity and that the water quality, the conformation of the lake basin, and the length of shoreline are much more important. Shallow lakes are more productive than deeper ones ^^' ^^ because the most productive zone is that influenced by the sun's rays. Where this layer is in contact with the lake bottom, one may expect to reach a high level of production. Other factors, such as the length of the growing season ^^ also influence productivity. LITERATURE 1. Anon., Ann. Repts. of the Upper Mississippi River Conservation Commit- tee (Mimeo), 1944-1961. 2. Bardach, J. E., Hydrohiologia, 7(4), 309-324 (1955). 3. Bennett, G. W., 7/7. Wildl, 1(2), 8-10 (1946). 4. Bennett, G. W., 7//. Nat. Hist. Surv. Bull, 26(2), 217-276 (1954). 5. Bennett, G. W., and Durham, L., 7//. Nat. Hist. Surv. Biol. Notes, 23, 1-16 (1951). 6. Carlander, K. D., Jour. Fish. Bes. Bd. Can., 12(4), 543-570 (1955). 7. Davidson, V. E., and Johnson, J. A., U.S.D.A. Farmers Bull, 1938, 1-22 (1943). 8. Grim, J., Allg. Fisch-Ztg., Jahrg., 77(14), 281-283 (1952). 9. Hansen, D. F., 7//. State Acad. ScL Trans., 35, 197-204 (1942). 10. Hasler, A. D., and Einsele, W. G., N. A. Wildl Conf. Trans., 13, 527-555 (1948). 11. Hayes, F. R., Jour. Fish. Res. Bd. Can., 14(1), 1-32 (1957). 12. Hooper, F. F., Ball, R. C, and Tanner, H. A., Am. Fish. Soc. Trans., 82, 222-241 (1953). 13. Humphrys, C. R., and Veatch, J. C, Mich. Sta. Uni. Ag. Exp. Sta. Water Bull, 8, 1-18 (1961). 14. Hutchison, G. E., "A Treatise on Limnology," Vol. 1, 1015 pp. New York, J. Wiley & Sons, 1957. 15. Jackson, H. O., and Starrett, W. C, Jour. Wildl Mgt., 23(2), 157-168 (1959). 16. Jenkins, R. M., Proc. Okla. Acad. ScL, 38, 157-172 (1958). 17. King, W., U.S.F.&W. Circ, 86, 1-20 (1960). 18. Maupin, J. K., Wells, J. R., Jr., and Leist, Claude, Kan. Acad, of Set. Trans., 57, 164-171 (1954). 19. Meehean, O. L., Jour. Wildl Mgt., 16(3), 234-237 (1952). 20. Moyle, J. B., Am. Fish. Soc. Trans., 76, 322-334 (1949). 21. Parsons, J. D., 7//. Acad. ScL Trans., 50, 49-59 (1958). 22. Parsons, J. W., Am. Fish. Soc. Trans., 85, 75-92 (1957). 23. Pfitzer, D. W., N. A. Wildl Conf. Trans., 19, 271-282 (1954). 24. Powers, E. B., Shields, A. R., and Hickman, M. A., Jour. Tenn. Acad. ScL, 14(2), 239-260 (1939). 25. Rounsefell, G. A., Copeia, 1946(1), 29-40 (1946). 26. Ruttner, F., "Fundamentals of Limnology," Univ. of Toronto Press, Toronto, Ont., Can., 1953. Literature 41 27. Starrett, W. C, and McNeil, P. L., Jr., ///. Nat Hist. Smv. Biol Notes 30,1-31 (1952). ' 28. Starrett, W. C, and Parr, S. A., ///. Nat. Hist. Surv. Biol. Notes 25 1-35 (1951). ' ^' ^ ^^ 29. Stroud, R. H., and Jenkins, R. M., Sport Fishing Inst. Bull., 98, 3-6 (1960). 30. Thompson, D. H., "A Symposium on Hydrobiologv," pp. 206-217 Univ of Wis. Press, Madison, Wis., 1941. 31. Welch, P. S., "Limnology," McGraw-Hill Book Co., Inc., New York 1935 32. Wiebe, A. H., Ecology, 20(3), 446-450 (1939). 33. Wiebe, A. H., N. A. Wildl Conf. Trans., 6, 256-264 (1941). Interrelationships of Fishes and Lake Habitats Several types of artificial aquatic habitats were described in the pre- ceding chapter. Now we will consider some of the components that make up an aquatic habitat, and the relationships of these components with fishes. Water in a habitat for fish must carry dissolved useful gases, minerals, and other substances of kinds and amounts nontoxic to fish. However, the habitat also consists of physical features, basically the contours of the lake basin, with depths, high ridges, rocks, gravel beds, silt areas, marl deposits, stumps, and fallen trees. Growths of submerged aquatic plants, filamentous algae, and shoreline vegetation are a part of the physical habitat as well as of the biological environment. Other parts of the bio- logical environment include the bacteria, plankton algae, fungi, aquatic invertebrate fauna, and a few kinds of vertebrates other than fish. Some of these organisms are foods, some are enemies, and others change with time— being enemies of small fishes first, and later, as these same fishes grow, becoming their food supply. As indicated in Chapter 2, artificial lakes, being proximate to man and recent in origin, harbor many abnormal and temporary ecosystems, since plant and animal lake inhabitants may be either slow or rapid in- vaders, and the stocking of fish by man is limited to the species he wants. In fact, man usually leaves it to other fish to find their own way into the lake he has created. Moreover, some aquatic forms that shun association with man seldom appear, and others that he dislikes are not permitted to enter ( or at least to remain ) . The status of a fish species in an artificial lake may be directly related to its ability to compensate for the point of greatest maladjustment with its environment. The population density of fish of its own kind or of other kinds may be a factor in maladjustment to a given environment, as there 42 Sewage Pollution and Fertility 43 is for all animals, a progressive decrease in the favorability of the en- vironment associated with a progressive increase in population density, until growth and reproduction arc inhibited.-"' Hey ^^ noted that when the two indigenous species of alga-eating tilapias (T. mossambica and T. sparrmani) were released in equal numbers in South African sewer ponds, T. sparrmani eventually disappeared. If a few T. mossambica were placed in a population of T. sparrmani, the former disappeared. Neither of these species can be considered as primarily predatory, but both will eat small fishes when they are available. The biological domination man exerts over most artificial lakes not only upsets interrelationships of aquatic organisms, but enters the picture in other ways, most commonly, perhaps, in water pollution from silt, from organic waste, and from chemicals. These pollutants are damaging to fishes in relation to the capacity of the recipient environment to absorb tlieir eJBFects without itself becoming greatly changed to the detriment of fish populations. Of the three types of pollutants, silt and chemicals are almost uniformly undesirable, while sewage pollution from organic waste may represent a mixed benefit: Organic sewage increases production once certain demands that it makes upon water are met. SEWAGE POLLUTION AND FERTILITY Some ponds and lakes receive sewage runoff from septic tanks, over- loaded tile disposal fields, and domestic-stock feed lots, or effluent from primary or secondary sewage works. In Europe, the use of municipal sewage as fertilizer for fish ponds -^ is widespread, and cities as large as Munich dispose of most of their effluent in this manner. Rainbow trout and carp from sewage-fed ponds are very acceptable as food in Germany and other countries. Detailed descriptions of the methods used in the propagation of fish in sewage-fed oxidation ponds are given by Kisskalt and Ilzhofer ^^ and by Wundsch.*^ In this country some interest has been shown in the development of oxidation ponds for disposing of the sewage of small communities. Also, many stock farmers are building ponds close to cattle barns and hog houses, so that animal waste can be piped directly into them. Although fish cannot be raised in most of these ponds receiving undigested sewage, they can be produced in supplemental ponds con- nected with the former. Those interested in unpolluted streams, ponds, and lakes should be made aware of the dangers associated with the predicted increase in human population, even if current practices are followed in sewage dis- posal. Some of these dangers will soon be apparent. A part of the fertilizer applied to crop lands is leached from the soil. In some locations as much as 10 per cent of tlie inorganic phosphorus 44 Interrelationships of Fishes and Lake Habitats applied to lands may later appear in streams draining these lands,^^ and the total phosphorus content of the stream water may vary from 10 to nearly 200 ppb.* Without considering the extent that land fertihzation may have influenced the phosphorus and nitrogen content of drainage water from southern Wisconsin land, Sawyer ^^ estimated tlie relationship between the nitrogen and phosphorus content of biologically treated human sewage and this drainage water. Treated human sewage supplied about 6 pounds of nitrogen and 1.2 pounds of phosphorus per person per year, and the wastes of 750 persons were equivalent to the agricultural drainage of one square mile of land area in southern Wisconsin on the basis of nitrogen; similarly, treated wastes of 212 persons were equivalent to drainage from the same area on a phosphorus basis.^^ Phosphorus was proved to be the most important item in the pro- ductivity of water.-^' ^- With the greatly increased use of detergents, which are largely complex phosphates, alkyl benzene sulfonates, and other surface active agents, the problems associated with the discharge of effluents from sewage plants, even where processing is complete, have become at least twice as great as they were previous to the beginning of the wide use of these detergents. Because of them, smaller amounts of sewage effluents will cause greater fertilization of aquatic habitats than formerly. It has been suggested that chemical methods may eventually be used to remove a part of the phosphates from sewage effluents. ^- While alkyl benzene sulfonate is not very toxic to warm-blooded verte- brates,-^^ concentrations greater than 1 to 2 ppm are toxic to sensitive fish and aquatic organisms, ^^ and excessive phosphates and nitrates may stimulate algal blooms to the extent that much of the esthetic value of a water area may be lost. Moreover, the fish population is subjected to constant danger, due to the fact that a sudden die-off of the algae might result in a severe oxygen deficiency or in the actual poisoning of the fish. These aspects of lake fertilization are discussed further in Chapter 6. Needless to say, pollution may create real nuisance problems and even dangers in the management of fishes in artificial lakes as well as in the navigation pools of large rivers. pH AND CHEMISTRY OF WATER No attempt will be made here to describe variations in the mineral content of impounded waters found throughout North America; rather we are interested in waters containing abnormal amounts of certain chemicals picked up from contact with natural deposits of minerals. As is to be expected, the mineral composition of pond or lake water is * Parts per billion. Effects of Water Temperature on Fish 45 roughly similar to that of the soils of the lake bottom and the surrounding basin. Fish are able to live in water having a pi I range from about 6 to 9 or 10. Aldiough most natural waters do not eontain ehemieals in eoneentrations great enough to limit the survival of fish, according to Neess,--^ at pH 5.5, fish develop hypersensitivity to bacterial parasites and usually die within a short time if the pH is as low as, or lower than 4.5. Moreover, very hard waters are sometimes toxic to fish. New clear ponds, in regions where surface waters are hard, may show an upper pH range of 10 or more when, in bright sunlight, their submerged plants or algae are active in photosynthesis. These plants use up all of the free carbon dioxide in die water and as much bicarbonate as is available to them, wdth the result that maximum alkalinity is attained which, if high enough, will cause the death of fish. In older ponds an accumulation of organic matter acts as a buffering agent against high pH. Sulfates in newly flooded stripmine ponds often cause them to be acid. If the stripmine is in a region where surface and ground waters are hard, an accumulation of calcium and magnesium and organic material may counteract the acidity so that these waters will eventually support fish and other aquatic organisms. For example. Sigmoid Pond in Kickapoo State Park, a former stripmine area located in central Illinois, contained 1340 ppm of sulfate on May 18, 1938 (an analysis by Illinois State Water Survey, unpublished). This pond had a total hardness of 669 ppm, and contained largemouth bass, crappies, bluegills, and green sunfish. The pH ranged between 8 and 9. Occasionally, flowing wells and springs contain high amounts of iron and sulfate, as well as methane and other gases that may make them uninhabitable by fish. Usually, however, the exposure of such waters to sunlight and aeration allows the precipitation of certain mineral elements and the release of gases. EFFECTS OF WATER TEMPERATURE ON FISH Temperature plays an important role in the aquatic environment in that certain organisms, including fish, are sensitive to the limitations of the natural range of water temperatures. In a broad sense, fresh- water fish can be separated into cold- or warm-water species. Ordinarily, one thinks of the trout as being representative of the cold-water species and accord- ing to James, Meehean, and Douglass, ^'^ rainbow and brook trout thrive in water with a maximum summer temperature approximating 70°F. Under certain conditions diey may tolerate higher temperatures for short periods of time, and in this the rainbow trout is more resistant than the 46 Interrelationships of Fishes and Lake Habitats brook trout to high temperatures. Fry ^^ states that 77.5°F is the lethal temperature for brook trout upon prolonged exposure. During the summer of 1951, surface temperatures of a pond fed by a small amount of spring seepage remained between 75° and 79°F for 24 days, and no loss of trout occurred. At the same time, the maximum bottom temperature was 74°F. Typical warm-water fish are the largemouth bass, bluegill, black and white crappie, and yellow bullhead. These fishes, in ponds and lakes, are almost never killed by high temperatures alone. Intermediate between the trouts and the fishes listed above are such species as the smallmouth bass, rock bass, walleye, northern pike, and the muskellunge. There is no question but that these are somewhat more sensitive to high water temperatures than are more typical warm-water species; however, there is some evidence to indicate that factors other than temperature limit their distribution in certain types of warm- water habitats. While tem- perature in itself may not be a limiting factor for most species, high ones are usually associated with other conditions which culminate in an un- satisfactory habitat. Water temperatures influence rate of metabolism and therefore the growth rate; they are often critical in their relationship to spawning and the development of normal embryos. A general knowledge of temperature requirements of common fishes is of value to a fishery biologist because with it he may be able to prevent the release of fish stocks in thermally unsuitable waters. More will be said of the physiological effects of temperature change on fishes in Chapter 8. EFFECTS OF TURBIDITY When rain falling upon the lands runs off into watercourses, it carries a greater or lesser amount of soil with it in the form of silt particles. In certain parts of the United States, these particles are so finely divided that, once they become suspended by water, they fail to settle. This is because the very fine particles carry an electrical charge and, therefore, tend to repel one another whenever they come close together. Since Oklahoma contains extensive areas where these colloidal soil particles are present, the problem of pond and lake turbidity is of considerable importance within that state. Irwin,^" writing of ponds in Oklahoma, states that in the clay-soil region, ponds had clear water for at least the first year if their basins were covered with vegetation at the time of impoundment. However, excavated ponds from which the vegetation had been removed had muddy water from the first. Also, older ponds, that had been drained, had had the silt removed, and then were refilled, usually had muddy Effects of Turbidity 47 water. In contradistinction, ponds that received runoff from sizeable feed lots or barnyards were usually clear of silt turbidity. Apparently, organic decay reduces the Brownian movements of the soil particles, probably through the neutralization of electrical charges. Although hay or fresh green vegetation introduced into a pond or lake caused a clearing of the silty water, the fresh green vegetation was the more effective of the two. Commercial fertilizers containing super- phosphate produced a good neutralizing agent involving phosphoric acid, and nitrate compounds also increased precipitation of soil particles. Any acid or other agent that ionizes in water or causes other compounds to ionize will bring about a release of positive ions that will neutralize the negative charges of the minute soil particles. If a sufficient number of positive ions are released, all of the negative charges will be neutralized, allowing complete precipitation of the soil particles. The continued presence of such a buffer will result in the continuous precipitation of colloidal soil particles. ^"^ Colloidal clay particles are not limited to Oklahoma soils, and are probably found in greater or lesser amounts in many states. In order to discover the direct effect of montomorillonite (hydrous aluminum siHcate) clay turbidity on fishes, Wallen ^^ made a series of experiments exposing fishes to turbidities as high as 225,000 ppm. A total of 16 common species was used. Most individuals of species exposed to more than 100,000 ppm turbidity had their opercular cavities and gill filaments clogged with clay particles, but some behavioral reactions were noted in common fishes within the range of 20,000 to 100,000 ppm. How- ever, very few turbidities resulting from natural conditions have been recorded that exceeded the lowest lethal turbidity in these experiments. Maximum natural turbidities for several streams in Oregon and Idaho were between 137 and 395 ppm ^^; maximum for the Rio Grande was 14,800 ppm. Although Whitewood Creek (South Dakota) was polluted at the rate of 48,400 ppm ^ and Coyote Creek (Oregon) ^i at 38,000 ppm, these turbidities were not from natural causes but rather from mining opera- tions. It must be concluded that natural turbidities are seldom if ever directly lethal to fishes. While high turbidities from soil particles may not be lethal to fishes, turbid waters may affect their growth rates. Growth of largemouth bass was considerably reduced in Oklahoma ponds that were turbid.^ The effect on growth of red-ears and bluegills was similar but less pronounced. Turbidity also affected the success of reproduction, particularly of large- mouth bass. It was also shown that the volume of basic food in clear ponds was approximately 8 times greater than in ponds of intermediate turbidities (average turbidities 40 to 90 ppm) and 12.8 times greater tlian in the muddiest ponds (average turbidities 110 to 205 ppm). These studies 48 Interrelationships of Fishes and Lake Habitats indicate that while natural turbidities in ponds seldom if ever cause direct lethal eflFects, over a period of years they may be responsible for poor production of fish and indirectly for the disappearance of certain species. OXYGEN AND CARBON DIOXIDE Both oxygen and carbon dioxide sometimes occur in w^ater in excessive and subnormal amounts with deleterious effects on fish. In bright sunlight abnormally high oxygen tensions ( supersaturation ) may occur within dense stands of submerged vegetation. On the other hand, unusually high carbon dioxide tensions may occur where rapid decay of organic material is taking place on the bottom of a pond or lake. Although high oxygen tensions are usually associated with low carbon dioxide tensions, this is not always the case. Fish living in a medium in which the tensions of oxygen and carbon dioxide change gradually, but markedly, with changes in depth or with time of day, are able to make certain physiological adjustments to com- pensate for changes in the amount of dissolved gases in their habitat. However, these adjustments cannot be made instantly. If forced to make rapid physiological adjustments to compensate for sudden severe changes in dissolved oxygen or carbon dioxide, fish may become deranged and die. Fish have been observed not only avoiding elevated concentrations but also reacting strongly to sudden very small changes in carbon dioxide tension.^- Any fisherman who has operated trap nets in ponds or lakes during summer months knows that fish sometimes make trips into the lower waters where dissolved oxygen may be low and carbon dioxide tension high."^ When caught in nets set in deep water, these fish may remain alive for some time at such depths, but if left too long they suffocate. Physiol- ogists have demonstrated the presence of oxygen in the swim bladders of some fishes and have been able to measure the adjustments in alkalinity of the fishes' blood, resulting from changes in tension of carbon dioxide. The length of time a fish may survive low oxygen tension varies inversely with the tension of carbon dioxide.* Interest in the effects of rapid change in carbon dioxide tension was stimulated by the death of fish in Norris Reservoir (Tennessee) in De- cember of 1937.-^ Tributary rivers were pouring ice-cold water into the lake at a time when the lake level was being lowered a foot per day. This river water, which was as cold or colder than the lake water, caused a pushing up (because it was heavier) of the bottom water of the lake. This upwelling and mixing of the carbon dioxide-saturated bottom water was indirectly responsible for the death of fish. Small shad moving about in this heterogeneous mixture of waters passed from high carbon Winterkill and Summerkill 49 dioxide tensions below die surface, to low carbon dioxide tension at the surface. These fish soon became affected by the rapid changes in carbon dioxide tension and died by the millions; larger fish rising to the surface from greater depths also became incapacited by sudden changes in carbon dioxide tension. However, it was significant that throughout the period when fish were dying, there was ample oxygen to support fish at all levels. Investigating biologists -^ conducted laboratory experiments to de- termine the cause of death of Norris Lake fish. Rock bass were placed in a hardware-cloth cage and lowered to the bottom of a water-filled 10- gallon carboy. The same number of fish were released in the carboy outside the cage. The carboy was left open so these latter fish could come to the water surface and gulp air. The water in the carboy was supplied with carbon dioxide to produce a COo tension above normal. Fish that were free to come to the surface of the water died before those that were held in the cage, thus indicating that rapid change in carbon dioxide tension from high at the bottom of the carboy to low at the surface affected the fish adversely. However, rock bass in the cage were able, by adjusting the alkalinity of the blood, to counteract the ill effects of high but constant carbon dioxide tension, and thus to extract oxygen as efiBciently as if the carbon dioxide tension were low. This situation held as long as the carbon dioxide tension remained fairly constant; but when the fish were forced to alternate between high and low tensions, they soon lost their equilibrium and died. The combinations of circumstances which produce the biological phenomena described above probably appear rather infrequently. More common are fish deaths occurring under ice in winter and in very weedy lakes during hot summer months. WINTERKILL AND SUMMERKILL The terms "winterkill" and "summerkill" are applied to sudden mortal- ities of fishes which occur in winter and summer, usually as a direct result of suffocation. Conditions that set the stage for a winterkill are, however, very different from those which result in an oxygen deficiency in a lake or pond during the summer. Winterkill In the north, winter ice forms a seal over lakes and ponds which pre- vents the exchange of gases between the water-air interface. Moreover, the penetration of light through ice is less than through clear water, and the light may be blanketed out entirely by a layer of snow upon the ice. When all photosynthetic activity is stopped because of insufficient light, 50 Interrelationships of Fishes and Lake Habitats the source of additional under-ice oxygen is gone, and in a relatively short time the current supply of oxygen may be completely used up by the respi- ration of living plants and animals and the demands of organic decay.^' ^^ In 1945, Greenback ^^ published results of a study of the physical, chemical, and biological conditions in ice-covered lakes (Michigan). He measured dissolved oxygen, pH, carbon dioxide, alkalinity, biochemical oxygen demand, and light penetration in these ice-covered lakes, to de- termine what factors or combinations of factors were responsible for the death of fishes, and to develop more efiFective methods of preventing winter fish kills. The amount of dissolved oxygen appeared to be the most important single factor in relation to death or survival. This oxygen con- centration might change gradually or rather suddenly, depending upon other conditions associated with the body of water in question. For ex- ample, at Green Lake ( Michigan ) Station 5, oxygen at the surface ( under ice) changed from 1.8 ppm on February 5, 1943, to 9.8 ppm on February 8, an increase of 8.0 ppm in 3 days, or at the rate of 2.7 ppm per day. The most abrupt decline was noted in Pasinski's Pond (Michigan) Station 27, where the oxygen fell from 12.3 ppm on February 12, 1940, to 2.4 ppm on February 14, at a rate of 5 ppm per day. A delicate balance often exists between the processes which produce oxygen (photosynthesis of plankton algae) and those that use it up.^^ As light is essential to photosynthesis, its transmission through the ice and snow covering a lake or pond is extremely important (Figure 3.1). Measurements of light penetration show that about 85 per cent will pass through 7.5 inches of clear ice, and as much as 11.5 per cent, through 15 inches of ice that is cloudy on top. However, 1 inch of crusted snow limited the light penetration through the snow only to between 10 and 17 per cent of the light that fell on the snow's surface, and 5 inches of dry snow allowed the transmission of only 2.5 per cent of the available light.^^ Clean fresh snow allowed the greatest light penetration, clean wet snow the next greatest, and granular snow the least. While the rate of photosynthesis is dependent on many factors, it is conceivable that there is a range of light intensity sufficient to stimulate a level of photosynthetic activity during which the oxygen output will exactly equal the oxygen demands of the aquatic environment. This is a dangerous condition because it may depreciate rapidly into a situation of oxygen shortage. There is reason to assume that the amount of light that penetrates 1.5 to 2 feet of moderately clear ice (without snow) is enough to satisfy the requirements for photosynthesis.^^ Further, the evidence is conclusive that a heavy snow cover on ice so greatly reduces the amount of light entering the water, regardless of the clarity of the ice, that photosynthesis of phytoplankton is completely stopped. Winterkill and Summerkill 51 Biologists and fish culturists liave tried to prevent the winterkill of fishes in various ways, most of which have been ineflFectual. Some methods explored for preventing winterkill of fishes are given below: 1. Aeration of Water under Ice. Many attempts have been made to blow air immediately under ice with pumps or blowers. This method is largely ineffectual/ ^ particularly for waters of any size, because little oxygen becomes dissolved in the water. Clear Ice 5 Inches Thick Cloudy Ice 15 Inches Thick 1 Inch of Snow Over Clear Ice 3 Inches Thick 5 Inches of Snow Over Clear Ice 3 Inches Thick Figure 3.1. Oxygen supply under winter ice depends upon the transmission of sufficient light for photosynthesis of plankton algae and rooted submersed plants. Light passes readily through clear ice and fairly well through cloudy ice. However, an inch of snow blankets out 83 to 90 per cent of the light and 5 inches of snow, 97 to 99 per cent. Winterkill of fishes is more common during winters when the snow on the ice persists for long periods than when it is light or melts between storms. 2. Aeration of Water above Ice. In 1935-36, the Michigan Institute for Fisheries Research attempted experimental aeration by pumping water from a lake and spraying it into the air where it fell to the ice and returned through holes cut in the ice.^^ This caused improvement in dissolved oxygen tension, but the effect was very localized, and tlie oxygen disappeared within 28 hours. 3. Pumping of Well Water. Well water at 50°F was run through wire-mesh and over an inclined trough to increase the dissolved oxygen 52 Interrelationships of Fishes and Lake Habitats from about 2 ppm to 4 or 6 ppm. The water was allowed to run into Pasinski's Pond (3.75 acres) through holes cut in the ice/^ and, over a number of days, opened a hole in the pond 8 to 10 feet in diameter. However, this pumping of aerated well water proved almost useless for preventing the death of fish, because the dissolution of oxygen was not ejBBcient. 4. Snow Removal. Manual removal of snow from hatchery ponds, although frequently impractical, has caused improvement in dissolved oxygen under ice. Furthermore, pumps and other equipment may be employed. At Green Lake ( Michigan ) water was pumped onto the surface of the ice, and melted the snow to slush for a one-acre area. There was an increase in the amount of dissolved oxygen in water under the ice, even though the slush rapidly became frozen. ^^ 5. Lamp Black. Tliis substance, spread on snow-covered ice from the air, melted the snow (through absorption of heat) and thereby allowed light penetration which resulted in improved oxygen conditions under the ice.-^ 6. Circulation of Bottom Water. A perforated plastic hose with small holes at spaced intervals was weighted and laid on the lake bottom to follow the long axis of the lake. The hose was closed at the distal end and attached to an air compressor at the proximal end. The compressor pumped air into the hose so that it bubbled out through the small holes along the entire length of the hose. These jets of air passing from the lake bottom to the surface of the water set up currents of water which eventually carried bottom water at about 39°F to the surface.^^ This warmer water eventually melted the ice and kept a strip of open water above the hose as long as the air compressor was operated, even though the air temperature was close to 0°F. When this system was operated at intervals in a lake subject to winterkill of fishes, no loss of fishes occurred. This method has been used to keep open water for ducks, and to prevent ice damage to piers, docks, and other installations. It may be the most successful technique yet devised for preventing winterkill,^- -^'^^'^^-^^ but Patriarche -^ demonstrated that in some lakes the circulation of water having a high biological oxygen demand increased the danger of winterkill. An increase in the oxygen supply of water covered by ice can come about only through photosynthesis. Thus, the maintenance of an adequate oxygen supply is dependent upon the presence and activity of green plant life, largely of the plankton algae, and this, in turn, depends upon the transmission of light for photosynthesis. It is conceivable that most of these algae might die or go into dormant stages if forced to remain in darkness (ice covered by snow) for an extended period, so that when sufiicient light for photosynthesis became available again, too few phyto- Winterkill and Sumyncrkill 53 plankton cells would be present to improve borderline oxygen conditions for fishes. Results of Partial Winterkill It is seldom that all of the fishes in a lake or pond are killed when sub- jected to adverse conditions under ice. This is because some are more resistant to low oxygen tensions than others and because adverse condi- tions throughout a lake or pond may not be uniform, so that certain ones in more favorable locations may survive while the rest may die. However, most game and pan fishes require larger amounts of oxygen than do the coarse ones— carp, buffalo, and bullheads. For this reason, a few of the more undesirable fishes may survive to repopulate the water. Studies of fish populations subjected to partial winterkill have been made by a number of biologists. In Michigan a partial winterkill was followed by changes in growth rate of the surviving fishes.- A dominant year class of bullheads developed in Lost Island Lake ( Iowa ) following a partial winterkill --' ^^; this same phenomenon was reported for Spring Lake near Savanna (Illinois). The fish population of 10-acre Gale Lake near Galesburg, Illinois, was examined about 20 months after a winterkill had occurred during January or February of 1945.^ Prior to the winter of 1944-45, the lake contained largemouth bass, bluegills, white crappies, carp, bigmouth buffalo, golden shiners, and catfish. When the ice went out in early March, 1945, about 4000 pounds of fish carcasses were collected. When the lake was censused by treatment with rotenone on September 16, 1946, it contained white crappies, green sunfish, black bullheads, carp, buffalo, and golden shiners. Table 3.1. Approximately 106,500 fish weighing 5275 pounds were collected. The larger fish shown in Table 3.1 are those which had survived the winter of 1944-45; the smaller indi- viduals of the same species represented the young produced in 1945 and 1946. It is inconceivable that this population could have ever produced satisfactory hook-and-line fishing. Studies of fish populations that have undergone partial winterkill il- lustrate well the danger of this phenomenon. Usually, it would have been far better if the adverse winter conditions had killed all of the fish. Tlien one could restock the lake with useful kinds and numbers of fingerlings and obtain satisfactory fishing within the second season after the winterkill occurred. Nonetheless, where the winterkill is partial and some desirable pan fish species survive, a stocking of a small number of sexually mature largemouth bass might result in the production of a dominant brood of these fish to prevent an overpopulation of the pan fish. Such a stocking probably could have been accomplished prior to the bass spawning season in a lake as small as Gale. Larger waters that have lost a part of 54 Interrelationships of Fishes and Lake Habitats their population from winterkill can be managed by stocking large num- bers of bass fry, which are often obtainable in quantity. The rapid de- velopment of a fishable population after a partial winterkill often rests with the ability or willingness of the lake or pond owner, whether an individual, a club, or a State Department of Conservation, to quickly appraise the damage and execute a plan to rectify it before the fish surviving the winterkill have produced young. However, once a lake becomes overloaded with a superabundant population of sunfish, crappies, bullheads, carp or buffalo fishes, much more radical measures are needed to restore fishing. Table 3.1 Census of all fishes in gale lake (10.0 acres), gale products recreation grounds, galesburg, illinois, september, 1946. Kind of Fish Total Number Total Weight, Pounds Average Weight Per Fish, Pounds Per Cent of Total Weight Fine Fish White crappie White crappie Green sunfish Green sunfish (large) ( small ) ( large ) (small) 76 42,500 17 4,580 97.25 752.74 3.75 75.12 1.279 0.018 0.221 0.016 47,173 928.86 17.6 Catfish Black bullhead Black bullhead (large) ( small ) 120 48,700 82.75 721.48 0.689 0.015 48,820 804.23 15.3 Rough Fish Carp Carp Buffalo Buffalo ( large ) ( small ) (large) ( small ) 219 1,331 31 424 1,572.75 583.75 626.13 618.96 7.182 0.439 20.198 1.460 2,005 3,401.59 64.5 Forage Fish Golden shiner 8,500 139.77 0.016 Grand total Per acre 8,500 106,498 10,646 139.77 5,274.45 527.45 2.6 SUMMERKILL Summerkill, as a major catastrophe, is less common than winterkill. On the other hand, there is some evidence that more fish die naturally in the summer period (and are never observed by man) than at any other time of year. There are several records of the disappearance of relatively Winterkill and Summcrkill 55 Strong year classes of white crappies during the summer months where the only evidence of their death was the fact that they suddenly dis- appeared from wing net catches and never again reappeared.^-' •'^' Crappies often are in their poorest condition in the summer, and manv apparently fail to recover. Summerkills comparable in extent to kills occurring under ice in winter sometimes take place in shallow weed-filled lakes during the hot, still nights of July and August. All of the summerkills that I have observed (that did not involve pollution from outside sources) occurred after periods of several days during which skies were cloudy or partly cloudy, temperatures ranged in the 80's and 90's both day and night, and winds were calm or nearly calm. Under these weather conditions, the dissolved oxygen that may be abundant in a weed-choked lake during the daytime may disappear entirely during the calm hot nights with the resulting wholesale death of fishes. Usually, some fishes survive summer oxygen shortages, and these may be seen gasping for air at the lake surface as die first light of the approaching dawn makes them visible. Often a quiet period lasting several days and nights may be broken off by a violent storm which restores the oxygen supply, lowers the water temperature, and stops any further death of fishes. Probably, high water temperatures, darkness, and rapid organic decay in shallow weed-filled lakes combine forces to produce summerkills. Another type of summerkill of fishes is caused by the decay of "blooms" of toxic algae (usually bluegreens). These toxic algae are concentrated by winds, ^^ or they develop from the stimulus of nitrates and phosphates originating from organic pollutants. Death of fishes may be caused by oxygen deficiencies, by toxic substances released from decaying blue- green algae, or both.-^ The death of domestic stock, forced to drink die alga-filled water, has been attributed to these toxic substances. High or low oxygen tensions produced by unusual circumstances some- times will cause the death of fishes. During April, 1940, a loss of fish was observed at the south end of Lake Waubesa (Wisconsin), and in the Yahara River below this lake.^-^ At that time an algal bloom of Chlamij- domonas was concentrated in the south end of the lake and produced oxygen to a level of 30-32 ppm at the lake surface. The death of fishes was attributed to the presence of gas emboli in the gill capillaries which blocked blood circulation. Black crappies, bluegills, northern pike, wall- eyes, white suckers, and carp were killed. In October, 1936, a heavy mortality of fish was reported for the Yahara River below Lake Kegonsa ( Wisconsin ).-i These fish died from an oxygen deficiency caused by the decay of an almost pure culture of Aphani- zomenon flos-aqiiae. The fish were crowding close to shore and were gasping at the surface until they finally expired. The bluegreen alga, A. 56 Interrelationships of Fishes and Lake Habitats flos-aquae, is known to release a very toxic substance when it dies and decays; and a secondary cause of death may have been direct poisoning. Jackson and Starrett ^^ described locahzed kills of fishes ( mostly gizzard shad ) in Lake Chautauqua on July 9, 1953, that apparently resulted from localized oxygen deficiencies. At 6:20 a.m. the dissolved oxygen content at one point was only 1.6 ppm, and later several fish were observed that presumably had died of asphyxiation. The weather was hot and the lake very quiet. OTHER DANGERS OF IMPOUNDMENTS Fishes in small artificial ponds and lakes may be decimated by some "accidents'" that occur because of the physical aspects of these impound- ments and the fact that men are careless by nature. These "accidents" are mentioned here so that they may be recognized and avoided. Loss OF Ponds Because of Burrowing Animals Small ponds are sometimes subjected to washouts through the activities of burrowing crayfish, muskrats, rats, and other burrowing mammals. These animals usually work in the dam, digging tunnels above the normal water level of the pond. No damage appears until a heavy sudden rain raises the pond level well above normal, and the tunnels become water channels through which water escapes to the downstream side of the dam usually taking with it a section of the earth fill and all of the water and fish in the pond. It is usually impractical to bury wire mesh or metal sheeting in small dams to prevent damage from burrowing animals. The best solution is to maintain a constant vigilance and trap or poison the rodents when they appear to be damaging the pond dam. Burrowing crayfish are sometimes killed by dropping one or more crystals of crude copper sulfate in their holes or "chimneys," or by adding either one teaspoonful of carbide powder or two ounces of stock dip solution to each burrow and then tamping the burrow shut.^ On ponds larger than 3 or 4 acres, the dams are so wide at the top that there is little danger from burrowing animals. Wind Action When artificial lakes and dams are too large to be subject to damage from burrowing animals, prevailing winds acting on such a wide surface can create another danger by blowing parallel to the long axes of the lakes, thus causing waves and currents that cut earth from the fills at the water line. Unless a fill is protected by rip-rapping of concrete, rocks, or by a floating boom, the action of the waves may gradually cut away the dam. Wind and wave action can occur in any part of the lake, often Other Dangers of Impoundments 57 cutting away at one shore and filling up a nearby bay or channel. Where rip-rapping is impractical, booms or deflector structures can be used to stop severe wind and water erosions. Wind-driven ice causes considerable damage in northern lakes. Dangers from Insecticides Since the discovery of DDT during World War II, new dangers for fishes and other aquatic organisms have come with the many new in- secticides. The toxicity to fishes of some of these insecticides commonly used on agricultural crops is very high (see Table 6.3), and carelessness in application, particularly in crop dusting or spraying by plane, can cause locahzed damage to fish. The passage of the Miller Act in 1954 (Public Law 518) established tolerances for residues of insecticides, fungicides, and herbicides in those agricultural commodities involved in interstate commerce. These regula- tions have reduced carelessness in the application of, or in the use of excessive amounts of insecticides. This, in turn, will reduce to some extent the danger of fish kills. LITERATURE 1. Beall, H. B., W. Va. Cons., Apr., p. 32 (1959). 2. Beckman, W. C, Am. Fish. Soc. Trans., 78, 82-90 (1950). 3. Bennett, G. W., III. Nat. Hist. Siirv. Biol. Notes, 19, 1-9 (1948). 4. Black, E. C, Fry, F. E. J., and Black, V. S., Can. Jour. Zoo/., 32(6), 408- 420 (1954). 5. Buck, D. H., Okla. Fish. Res. Lab. Rept., 56, 1-62 (1956). 6. Burdick, M. E., Wise. Cons. Bull. 24, 21-23 (1959). 7. Carlander, K. D., Jour. Wildl. Mgt., 16(3), 258-261 (1952). 8. Cooper, G. P., and Washburn, G. N., Am. Fish. Soc. Trans., 76, 23-33 (1946). 9. Ellis, M. M., Westfall, B. A., and Ellis, M. D., U.S. Fish and Wildl Ser. Res. Rept., 9, 1-122 (1946). 10. Fry, F. E. J., Proc. N. E. Atlantic Fish Conf., Mimec, 1-29 (1951) . 11. Greenbank, J., Ecological Mono., 15, 343-392 (1945). 12. Hansen, D. F., ///. Nat. Hist. Surv. Bull, 25(4), 211-265 (1951). 13. Hemphill, J., Ariz. Game ^ Fish Dept., 8 pp. (1954). 14. Henderson, C., Pickering, Q. H., and Cohen, J. M., Sewage Ind. Wastes, 31,295-306 (1959). 15. Hey, D., Proc. Int. Assoc. Theor. b- Appd. Limnology, 12, 737-742 (1955). 16. Ingram, W. M., and Prescott, G. W., Am. Midland Nat., 52(1), 75-87 (1954). 17. Irwin, W. H., Okla. Agri. and Mech. Coll Bull, 42(11), 1-16 (1945). 18. Jackson, H. O., and Starrett, W. C, Jour. Wildl Mgt., 23(2), 157-168 (1959). 19. James, M. C, Meehean, O. L., and Douglas, E. J., U.S.F.W.S. Cons. Bull, 35, 1-22 (1944). 58 Interrelationships of Fishes and Lake Habitats 20. Kesskalt, K., and Ilzhofer, H., Arch. Hijg. imd Bah., 118, 1-66 (1937). 21. Mackenthun, K. M., and Herman, E. F., Am. Fish. Sac. Trans., 75, 175- 180 (1948). 22. Moen, T., Iowa Coiiserv. 19(5), 37. 23. Neess, J. C, A7n. Fish. Soc. Trans., 76, 335-358 (1949). 24. Neil, J. H., Purdue Univ. Engng. Extn. Serv., 94, 301-316 (1957). 25. Nicholson, A. J., Australian Jour. Zool, 2(1), 9-65 (1954). 26. O'Donnell, D. ]., Midwest Wildl. Conf., Mimeo., 9 pp. (1947). 27. Patriarche, M. H., Jour. Wild. Mgt. 25(3), 282-289 (1961). 28. Powers, E. B., Shields, A. R., and Hickman, M. E., Jour. Tenn. Acad. Set., 14(2), 239-260 (1939). 29. Rasmussen, D. H., Prog. Fish-Cult., 22, 185-187 (1960). 30. Rose, E. T., and Moen, T., Am. Fish. Soc. Trans., 80, 50-55 (1951). 31. Sawyer, C. N., New Eng. Water Wks. Assoc. Jour., 61(2), 109-127 (1947). 32. Sawyer, C. N., Sewage and Indust. Wastes, 24(6), 768-775 (1952). 33. Scidmore, W. ]., Prog. Fish-Cult., 19, 124-127 (1957). 34. Schmitz, W. R., Wise. Cons. Bull, 24, 19-21 (1959). 35. Schmitz, W. R., and Hasler, A. D., Science, 128 (3331), 1088-1089 (1958). 36. Smith, M. W., Jour. Fish. Res. Bd. Canada, 16(6), 887-895 (1959). 37. Starrett, W. C, and McNeil, P. L., Jr., III. Nat. Hist. Surv. Biol. Notes, 30, 1-31 (1952). 38. Swartley, A. M., Ore. Dept. Geol and Min. Indust. Bull, 10, 26-27 (1938). 39. Tusing, T. W., Poynter, O. E., and Opdyke, D. L., Toxicology and Ap- plied Pharmacology, 2(4), 464-473 (1960). 40. Wallen, I. E., Okla. Agr. and Mech. Coll Bull, 48(2), 1-24 (1951). 41. Ward, H. B., Ore. Dept. Geol ir Min. Indust. Bull, 10, 4-25 (1938). 42. Whitmore, C. M., Warren, C. E., and DoudorofF, P., Am. Fish. Soc. Trans., 89, 17-26 (1960). 43. Woodbury, L. A., Am. Fish. Soc. Trans., 71, 112-117 (1942). 44. Wmidsch, H. H., Handh. der Binnenfischerei Mitteleuropas Bd., 6, 139- 262 (1926). 4 Carrying Capacity, Productivity, and Growth The carrying capacity of a container (a pail or basket) is hmited by the height of its sides and its diameter. Not so well defined, however, is the ability of an environment to support life. The term "carrying capacity" was probably first used in game management to express the maximum population of game animals supported by a limited range during a period covering at least the four seasons of one year.^^ Before we define carrying capacity further, it is important to distinguish between this term and saturation. An adult animal population that tends to be uniform over a wide area may reach a saturation point. Saturation point is defined as a uniform maximum density of grown individuals attained by a species, even in the most favorable local environments. However, saturation also implies a degree of intolerance of animals to "piling up," an interaction between individuals that may have little connection with other environ- mental conditions. Thus, saturation should not be confused with carrying capacity, which always implies a tendency toward uniformity over a wide area. Carrying capacity when applied to fishes in aquatic habitats may be defined as the maximum pounclage of a given species or complex of species of fishes that a limited and specific aquatic habitat may support during a stated interval of time. Since adverse environmental factors during certain seasons might actually control the maximum poundage of fish, seasonal adversity could establish the carrying capacity. However, as fishes rarely can be seen readily, or estimated by direct observation, little is known of the effects of seasonal adversity on fish populations. We believe that food is often limiting to population size in fishes, but other factors may be of equal importance. Therefore, at present the concept of carrying capacity is largely theoretical. 59 60 Carrying Capacity, Productivity, and Growth CARRYING CAPACITY AND STANDING CROP In contrast to carrying capacity, which emphasizes maximum poundage and a stated interval of time, the term standing crop, is applied to some- thing very definite, namely, the poundage of a given species or complex of species of fishes present in a body of water at a given time. When one 1200 - • 900 - • • • •• 500 a; < ^ 300 ^ 200 o • • • • • t I • • • • • • • • • % % • • 100 - 70 - • — i 1 1 u. 3 4 5 6 7 89 Number of Species 13 24 Figure 4.1. Relationship between standing crops of fishes and num- bers of species in midwestern reservoirs. [From Carlander, K.D., /. Fish Res. Bd. Canada, 12 (4) (1955)] drains a pond and makes a census of the fish, the census total is the standing crop of that pond at the time it was drained. The same fish in the same pond may be censused at a later date to give a different standing crop figure, influenced, perhaps, by a change in the relative abundance of various kinds of fishes present. Still, both census figures represent standing crops of this pond. In theory, the standing crop might be lower than, equal to or in excess of the carrying capacity of the pond. The relationship of numbers of fishes to carrying capacity and standing crop is not well understood (although, in general, large numbers of fishes Carrying Capacity and Standing Crop 61 are usually associated with small individual sizes of those fishes and vice versa). In theory, a body of water in which the fishes represented the greatest range of species and sizes would offer the maximum in efficient utilization of available food (Figure 4.1), although it is conceivable that the kinds and relative numbers of fishes within any given size range might not be paralleled by an equal abundance of acceptable food for this size range. Thus, a part of a population representing a certain size range of one or more species might be stunted, while in the same species, other sizes might be growing rapidly. Surface Area The carrying capacity ( in pounds ) of a body of water for a specific fish population seems to be largely a function of surface area rather than depth or volume; probably the zone of light penetration at the surface produces the bulk of the food supply (directly or indirectly) for fishes. There seems to be much evidence that carrying capacity is directly related to fish food production, which, in turn, is related to the basic fertility of the water, and conditions allowing the capture of this fertility by the food chain. Experimental Testing Little is known regarding the carrying capacity of any water for in- dividual species of game or pan fish, although European fish farmers engaged in raising commercial fishes for market have recognized that there are production limits of ponds for carp and other commercial species. It is common practice in fish farming to stock ponds with only a sufficient number of fish to produce a marketable product at the end of one or two growing seasons. Boccius ^^ wrote: "It has been fully proved that a given space of earth can produce only a certain quantity; so only can a given space or quantity of water produce a certain quantity, either of vegetable matter or animalcules; and curious as it may appear, yet it is as true as curious, that by storing only the proper number of fish adapted to the water, the weight in 3 years will prove equal to what would have been had twice the number been placed therein, so that the smaller number produces the same weight as the larger, from a given quantity of water. By overstocking the water, the fish become sickly, lean and bony." Swingle and Smith ^^ described an experiment in which two ponds were stocked with 6500 newly hatched bluegill fry per acre in late spring, and when the ponds were drained in November, the fish had grown to an average weight of slightly less than one ounce and the total populations in each of the two ponds amounted to approximately 300 pounds per acre. The fish were returned to the refilled ponds and when the ponds 62 Carrying Capacity, Productivity, and Growth were drained two years later, these fish still averaged about an ounce each and the ponds' populations still weighed about 300 pounds per acre. In another series of experiments,"^"' three ponds were stocked with 1300, 3200, and 6500 bluegill fry per acre. When these ponds were drained in November of the same year, the fish in the first pond averaged 4 ounces, those in the second slightly less than 2 ounces, and those in the third approximately one ounce. The total weight produced was approximately 300 pounds per acre in each of the three ponds. In 1939 these same authors ( Swingle and Smith •'^' ) published the re- sults of other experiments having to do with the carrying capacity of waters : "In May 1936, one pond was stocked with bluegill bream fry at the rate of 26,000 per acre, weighing 2 pounds 5 ounces. Another pond was stocked with year-old fingerlings at the rate of 13,000 per acre, weighing 180 pounds. . . . When drained the following November, the former pond produced at the rate of 105 pounds of fish per acre and the latter at the rate of 92 pounds per acre. One pond gained 103 pounds per acre, while the other lost 88 pounds per acre due to overstocking." In a second experiment. Swingle and Smith used a pond of 1.8 acres over a period of 2 years. In the spring of the first year (1935), they stocked 4485 fish of eight common pond species, weighing 40 pounds, 9 ounces. At the end of that year they collected 22,069 fish weighing 293 pounds, and 4 ounces. Early in the spring of 1936 they stocked 236 fish of the same species in this pond, weighing 24 pounds, and 7 ounces. At the end of 1936 they collected 30,405 fish weighing 296 pounds, and 2 ounces. In these experiments the variation in total numbers and weights of fish stocked seemed to have little effect upon the total weights of fish found in the ponds at the end of one growing season. Rather the total weights of these populations were adjusted upward or downward until they approached a rather uniform level for individual ponds, probably associated with the food production capacities of these ponds. Factors Affecting Poundage We have seen that the poundage of fish supported by a pond or lake of constant size may remain fairlv constant, in spite of the numbers of fish stocked. This is true only within limits; and the carrying capacity of a lake or pond for fish may vary (1) with variations in the fertility of water, (2) with age of water if age represents change in chemical com- position, (3) with fertility of watershed soil if a change brought about through erosion or artificial fertilization is carried to the pond in runoff water, (4) with changes in the kinds of fishes, or in the relative abundance of certain kinds and sizes of fishes. Carrying Capacittj and Standing Crop 63 Fertilization. Fisheries literature contains listings of many censuses of fish populations made through the draining of ponds and lakes or through the use of rotenone. Many of these censuses have been republished by Carlander ^^ in his compilations of growth and population statistics. A great deal has been published on the increase in the standing crops of fishes resulting from the use of various fertilizers.^' ^^' -^' ^^' ^^' ^" These studies furnish evidence that various inorganic and organic fertilizers introduced into ponds will temporarily increase the standing crop of most fishes, although there is evidence that one species may be benefited through the use of fertilizer to a much greater extent than another inhabiting the same water.-^ A pond fertilized for a period and then left without the addition of fertilizer will show a reduced standing crop of fishes 3 or more years after fertilization is stopped."^ This indicates that some of the fertilizer is no longer available, either because it has been washed out of the pond or has become bound up in insoluble compounds in the pond bottom. How- ever, if the standing crop of fishes, even though reduced, is still not as low as it was prior to the beginning of fertilization, this indicates that fertilizer in an available form has accumulated in the pond bottom. This accumulation of fertilizing materials may go on as a natural process (even if a pond or lake owner adds no organic or inorganic fertilizers) through the death of plants and animals in the pond, through the ac- cumulation of dust and leaves blown in from outside sources, and through the addition of nutrients leached from the soils of the pond watershed. Chemical Basis for Fertility. Moyle ^- demonstrated a positive rela- tionship between the presence of varying amounts of certain chemicals (total phosphorus, total nitrogen, and total alkalinity) in the surface waters of Minnesota lakes and the poundage of fishes supported by those lakes, although this could be expressed as a direct relationship only in the case of total phosphorus. Kinds of Fishes. Standing crops of fishes vary greatly on the basis of the kinds of fishes making up a population and the relative abundance of each of several kinds ( Figure 4.2 ) . Standing crops of fishes in Illinois ponds varied from 75 pounds per acre in soft-water-Ozark-hills ponds of southern Illinois where the population was largemouth bass and green sunfish, to 1100 pounds per acre in a black-soil-flood-plain pond in central Illinois where the population was composed of crappies and bigmouth buflFalo. In Iowa the standing crops of fishes ranged from 28 to 1235 pounds per acre.^^ Where the standing crop exceeded 300 pounds, usually bullheads or buffalo were present. Populations of bluegills usually ex- ceeded 100 pounds per acre. In Kentucky poundages ranged from 200 to 1000 pounds per acre in unfertilized ponds. ^^ A number of ponds have been censused one or more times, and these 64 Carrying Capacity^ Productivity^ and Growth Trout Walleye Rock Bass Pike Y. Perch Lm. Bass Ch. Catfish Pumpkinseed Quillbacks Drum Bowfin Crappies Suckers Bluegill Sunfishes Bullheads Carp Buffaloes Giz. Shad Standing Crop in Pounds Per Acre 4 6 10 20 4060 100 200 400 10002000 10 J I 1 ^////////// m 10 100 Mean for Lakes and Reservoirs Maximum 1000 Figure 4.2. Standing crops of named fishes in North American lakes and reservoirs. Usually these fish were in combination with other species. This figure furnishes a rough approximation of the relative efficiency of the species listed. [From Carlander, K.D., /. Fish Res. Bd. Canada, 12(4) (1955)] show the influence of kinds of fish upon the standing crop. Ball,* re- poisoning Ford Lake (Michigan) for a second time, found 2.4 times the weight of fish which had been recovered when the lake was poisoned 10 years previously. In the earlier census, the yellow perch was dominant in the population while in the latter census, the bluegill was dominant. Ball Carrying Capacity and Standing Crop 65 concluded that the difference in the poundages of fish in the two censuses was due to the fact that the perch is largely piscivorous in its feeding habits, whereas the blucgill is largely dependent on invertebrates for food and thus is closer to tlie primary food chain. Fork Lake, a pond of 1.38 acres in central Illinois, was censused at the beginning of a cropping experiment when an undesirable population of fish was poisoned, and again 4 years later when the pond dam was washed out.» At the time of the first census. Fork Lake contained 5350 fish weighing 774 pounds or 539 pounds of fish per acre. By weight, carp and bigmouth buffalo made up 47.5 per cent, bullheads (plus 4 channel catfish) 41.2 per cent, and largemouth bass and panfish, 6.3 per cent. At the time of the dam failure, an estimate of the population was 10,300 fish weighing 260.9 pounds or 189.1 pounds per acre. By weight, 64.7 per cent of the population was largemouth bass and 35.3 per cent blue- gills. This population had been subjected to heavy wing net-fishing for bluegills. The earlier population containing carp, buffalo, and bullheads was 2.85 times as heavy as that composed of bass and bluegills. Duck Pond (3.05 acres), an isolated part of an old flooded stripmine in Vermilion County (Illinois), was censused at two widely separated times. At the time of the first census in 1940, the pond contained 11,269 fishes of 30 species weighing 2051 pounds or 672.5 pounds per acre. Tlie population was composed of 3.0 per cent bass, 12.2 per cent pan fish, 0.6 per cent catfish, 23.5 per cent rough fish (largely quillbacks and carp), and 60.7 per cent forage fish (gizzard shad). In a second census made in 1945, the population consisted of 3150 fishes of 18 species weigh- ing 689.1 pounds or 229.7 pounds per acre. This population was composed of 2.6 per cent largemouth bass, 33.1 per cent pan fish, 1.3 per cent catfish, 41.0 per cent rough fish, and 22.0 per cent forage fish (gizzard shad). The wide discrepancy in the total poundages of fishes in the two censuses is difficult to explain. There were more pounds of bass in the first census (62.4 pounds as compared with 18.2 pounds) and more pounds of pan fish (251.2 pounds to 205.2 pounds). However, the large differences were in the poundages of rough fish (479.8 pounds in the first census, 282.2 pounds in the second) and forage fish (1245.0 pounds of gizzard shad in the first census, 150.0 pounds in the second). Ap- parently at the time of the second census, the populations of rough fish and gizzard shad were considerably below the carrying capacity of the pond for these species. Arrowhead Lake, an artificial pond of 2.6 acres on the grounds of the Ilhnois State 4-H Club Camp, University of Illinois Allerton Estate near Monticello, Illinois, was stocked in 1948 with 22 fingerling bass, 26 adult bluegills, 7 adult warmouths, and 103 black bullheads. This pond was censused by drainage in the springs of 1950, 1952, 1953, and during K P O h iz; ^-^ % W UJ < H !/3 Q < H ffl ►J 1-1 o w u hJ <1 J hJ I— 1 w ^ o Q hJ ^ J I— 1 ^ w O ^ rt rt Q <: ;z; <'3 o w W EQ U) U n H c U CO in G5 C CD U CM lO 05 4-J \^J CO IS O ^ Ph CO CO CO 0) s :z: 1^ "^3 bJO fl -a S t)JO G 0) a 00 CO CM 05 CO ^ 00 05 1—1 o G5 CO o 00 TP CO CO lO CM Ttl I— ( 1—1 fM (M CO TJH 1—1 o CO CM 00 CO CO i-H CO i> 00 00 CO o CM O CO CO t- l-l xt^ (M CI 1—1 00 00 I— 1 CO CM* Tf5 1— ( ^ (M CO I— 1 ^ -C! CM •^ s oi 'S O Oh lO ^ CM O o 00 tdO CO lO CM 1> CM CM xo ^ Ir- CO 00 1—1 CO CM in CO CO CM CM CM 00 (M 05 CM t^ CO CO CO "—I 13 3 O) CO CO lO CO CO CM CM 1—1 lO CO lO 00 lO CO CO CO CO lO lO CM CO 00 CO « ^ m o H o < Pk CN On -a >> d) ;-! -a c o o c c T3 o 'o en S-H ■ O £« C O _ c ^ a O ^ c/^ M • -^ C K^ t/3 O 3 Jl? «J I: o -gx) - C3 O -^ c ° 2 -s s ^^ o 5 ?► (u c/D c^ O 5 H CI CO 1—1 66 Carrying Capacity and Sfandiiig Crop 67 October in 1955. With the exception of about 700 sniallmoutli bass that were stored in the pond from October, 1952, to Marcli, 1953, tlie fish population has consisted of the four species hsted above. In the 1950 census (Table 4.1), the bluegills and the bullheads dominated die popula- tion; and when fish were restocked following die first census, only 1093 bluegills and 1069 bullheads were returned. In die 1950-1952 period, the bass made the greatest gains (80 fish expanded to 1057), die bluegills increased about 2.5 times, and the bullheads dropped from 1069 to 63 individuals. This reduction in bullheads reflected poor reproductive suc- cess and a heavy hook-and-line yield. Following the March, 1952 census, fish replaced in the pond were 541 bass weighing 72.1 pounds, 569 blue- gills weighing 140.6 pounds, 16 warmouth weighing 4.2 pounds, and 36 black bullheads weighing 25.6 pounds. The fishing in Arrowhead Pond in 1953 was rather light until October when fishermen discovered that smallmouth bass taken in draining another lake were being stored in Arrowhead. Then, it appeared that considerable poaching occurred. Following the 1953 census, 104 largemouth bass weighing 56.1 pounds, 834 bluegills weighing 91.6 pounds, 427 warmouth weighing 41.3 pounds, and 30 bullheads weighing 14.8 pounds were returned to the pond. During die process of draining the pond in March of 1955, the outlet valve was opened wide at night by someone unknown. The next morning, the fish were scattered along the outlet channel for several hundred yards, and footprints indicated that some pre-dawn collection of fish may have taken place. Thus, the 1955 census may be short some large bass and large bluegills. The four censuses of Arrowhead Pond showed a range of standing crops from 164.3 to 254.5 pounds per acre. The total poundage was lowest when the bass were the most numerous and highest when bluegills and bullheads were abundant. All of these fish except the warmouths appeared to be in competition, each species ready to "take over" the pond if an opportunity should arise. The fish population of Ridge Lake ( central Illinois ) has been censused by draining 8 times in the past 20 years ( Bennett ^^ and unpublished data). Numbers and weights of fish per acre taken in these censuses are listed in Table 4.2. The time interval between each of the first 5 censuses was two vears; between the fifth and sixth and the sixth and seventh censuses, three years, and between the seventh and eighth censuses, three and one-half years. Each September, 1951 through 1955 inclusive, the water level of Ridge Lake was lowered to reduce the surface area during the fall months so that the fish populations exposed in the 1953 and 1956 censuses were hardly comparable to the others. Table 4.2 shows that after 1945 the population was composed largely of bass and bluegills. No bluegills were stocked until 1944, a year after the 1943 census. War- 68 Carrying Capacity, Productivity, and Growth mouths were stocked in 1949 and channel catfish, in small numbers in 1951 and 1955. Neither were numerically very abundant because of low success in reproduction; in fact, only a few young catfish were ever observed in the last three censuses. Other fish entered from the small feeder stream or came upstream over the spillway during floods. Table 4.2 Numbers and weights of fish per acre taken in 8 draining CENSUSES OF RIDGE LAKE, COLES COUNTY, ILLINOIS. By 1951 AN accumulation of silt in THE UPPER LAKE BASIN HAD REDUCED THE LAKE SURFACE AREA FROM 18 TO 17 ACRES. Larg ;emouth Bass Bluegill Warmouth Num- Weight, Chan Num- inel Cats Num- Weight, Num- Weight, Weight, Censuses ber Pounds ber Pounds ber Pounds ber Pounds Spring, 1943 265 48.2 Spring, 1945 91 39.6 559 7.0$ Spring, 1947 139 31.5 3702 193.3 Spring, 1949 113 50.4 1095 86.9 Spring, 1951 84 49.9 2887 105.2 51 4.0 Spring, 1953 * 116 26.6 440 58.3 15 3.8 38 27.8 Spring, 1956 * 132 37.5 1011 119.9 37 4.6 14 36.4 Fall, 1959 137 30.5 5451 161.7 122 13.2 10 20.2 Av.t 138 41.7 2739 136.8 80 8.9 12 28.3 Bullheads Num- Weight, Carp Miscellaneous Num- Weight, Total Num- - Weight, Num- Weight, Censuses ber Pounds ber Pounds ber Pounds ber Pounds Spring, 1943 1 0.4 1 0.2 267 48.8 Spring, 1945 30 23.8 12 1.9 692 72.3 Spring, 1947 27 9.0 3 22.1 7 0.4 3878 256.3 Spring, 1949 3 2.2 3 0.7 1214 140.2 Spring. 1951 9 3.7 tr 0.9 3031 163.7 Spring, 1953 * tr 0.3 2 0.1 611 116.9 Spring, 1956 * 1 0.8 tr 0.3 1195 199.5 Fall, 1959 tr 0.1 3 20.8 tr tr 5726 246.4 Av.t 12 6.5 2468 154.6 * Population influenced by September drawdowns. t Data for 1953 and 1956 not included in average. t Not included because bluegill population newly introduced. Table 4.2 shows that no two censuses were very similar, either in numbers, or pounds of fish per acre. The poundage of bass in 1943 when almost no other fish were present was exceeded by only two subsequent censuses. Exclusive of the drawdown period, the lowest poundages of bass appeared in the 1947 and 1959 censuses when the bluegills were most abundant, both in numbers and in pounds per acre. Bluegills larger than about 2.5 inches ranged in number from 440 to 5400 per acre and in weight from 58 to 193 pounds per acre. Carrying Capacity and Standing Crop 69 The standing crops of fisli recorded in the 1947, 1949, 1951, and 1959 censuses (which are most nearly comparable to one another, with both bass and bluegills present and no drawdowns ) ranged from 140 to 256 pounds per acre. The liighest poundage (256) represented more tlian an 80 per cent increase over the lowest poundage ( 140 ) . After each census, oil of the cotchahle bass icere returned to the lake, and the bluegill populations were drastically reduced, usually to less than 200 per acre of the larger fish. The population after two, three, or four growing seasons ( 1959 census ) reflected the struggle for dominance between the bass and bluegills. From Table 4.2 one is led to believe that fall drawdowns of the lake affect both species: the bass througli a poundage decrease with litde change in numbers, the bluegills through a decrease in both numbers and poundages, but with a more severe effect on numbers. Thus, the drawdowns were more favorable to bass than to bluegills. The eight censuses of Ridge Lake parallel the four censuses of Arrow- head Pond in exposing what appears to be competition, primarily between largemouth bass and bluegills in which the bass would rather quickly lose out except for the culling of bluegills on each census. Ridge Lake is a highly favorable habitat for bluegill reproduction and survival, but poor in nutritional resources for bluegills of desirable sizes. Growing Season. Swingle and Smith ^^ state: "After the fish used in stocking have spawned once, more small fish are present than can be adequately supported by the food that the pond is producing. Hence a pond rapidly reaches its maximum carrying capacity, usually within one year." The length of the fish growing season in the southern part of the United States may be more than 10 months long, whereas in the northern states the fish growing season may be less than 4 months (Figure 4.3), and northern lakes and ponds may be covered with ice from 3 to 5 months. The length of the growing season affects the time of population growth required for it to approach the carrying capacity of an un- populated body of water. RalP found that the total weight of fish re- corded at the end of the third year did not vary greatly from that of the second year. This indicates that two growing seasons are usually sufficient for a population to approach the carrying capacity of a body of water in the northern part of the United States. Other Factors Related to Standing-Crop Size. The relationships be- tween the standing crops of fishes and certain environmental and fish population characteristics have troubled fishery biologists for years. Re- cently, using regression methods, Carlander ^^ attempted to determine whether certain environmental factors may affect standing crops of fishes. He found no relationship between standing crop and lake area; an in- 70 Carrying Capacity, Productivity, and Growth crease in standing crops with decreasing average depths (may not be entirely significant), and a significant increase with increasing hardness of the waters. There was also an increase in standing crop with an increase in number of species present (Figure 4.1). 70 60 H 50 80H 70 60-1 50 80i 70 60 50 I I I I I I I I I I Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. MAXIMUM International Falls, Minn. GROWING SEASONS FOR FISHES Madison, Wis. 11 to 12 months J I L J I L I I Figure 4.3. Variation in length of the fish growing season, based upon the observation that growth in warm-water fishes is very slow at temperatures below 55°F. (Records from U.S. Weather Bureau, 5-year average.) As stated previously, Moyle ^- was able to show that the standing crops of fishes in Minnesota increased with increases of phosphorus, nitrogen, and total alkalinity. Fish of Useful Sizes. In passing judgment on the value of a fish popula- tion for sport angling, it becomes necessary to set up arbitrary standards for fish of useful sizes as opposed to those too small to interest anglers. Immediately, we enter an area of controversy among fishery biologists as Fish Production 71 well as among fishermen. In 1939, Illinois biologists ^~ set up arbitrary standards for useful sizes for some eommon panfish and catfish: 6 inches or larger for bluegills, and other sunfish, 8 inches or larger for black and white crappies, 7 inches or larger for bullheads, and 12 inches or larger for channel catfish. At that time, the length limit of 10 inches for largemouth bass was still in force in Illinois; however, a bass should be 9 inches or longer before it is large enough for table use. In some states bluegills of over 5 inches ^*' •' ^ and bass over 7.2 inches were considered of edible sizes, and bullheads were considered to be of an edible (or salable) size in Michigan at approximately 7 inches total length.^' Walleyes and northern pike are too small to be useful unless the walleyes are about 12 or 13 inches long and the northern pike, 16 to 18 inches. The decision as to what constitutes a fish of useful size may best be made by the fishery biologist rather than the fishing public. Just because the fishing public will take 5-inch bluegills does not mean that they would still do so if enough bluegills of 6 inches or larger were available to satisfy their desire for fish. In taking 5-inch bluegills, fishermen are demonstrating that the 5-inch length is the minimum size for which they can find use. Tliis should not be the goal of the fishery biologist. Fish management should be able to produce fish of such sizes that the sporting aspects of fishing are satisfied and the end product (in this case the fish) is large enough for table preparation. When the head, fins, and tail are removed from a 6-inch bluegill, the part remaining is barely large enough to make an attractive morsel. A 5-inch fish would scarcely be of interest, unless bones were cooked sufficiently to be eaten without separation from the flesh. Even then the work of cleaning and scaling is large for such a small return. The only reason for recommending a minimum crappie size of 8 inches is that these fish have small bones that are almost impossible to separate from the meat in fish smaller than 8 inches. As mentioned above, the potential angling value of a fish population may be defined on the basis of numbers of fish of useful sizes, that are present in a population. A large standing crop of fishes may mean nothing from the aspect of potential angling if these fishes are too small to be usable. FISH PRODUCTION Definition of Production The term production is generally applied to the increase in ninnher of individuals and/or the weight of fish flesh added during a limited period. For example, if a new pond containing no fish were stocked in March 72 Carrying Capacity, Productivity, and Growth with 1500 bluegill fry weighing a total of 1.0 pound and in November when the pond was drained 200 pounds of bluegills were collected, the fish production for that season would be 199 pounds plus as additional poundage of fish that had been lost through natural causes during the season. In addition, flesh added to fish that later died and decayed or were eaten by predators, must be included in a total production estimate. Thus, if this same pond were restocked in March of the next season with 203 pounds of bluegills and drained in November, with a population weighing 204 pounds, the production the second year would be 1 pound plus a poundage lost as above. The term "production," therefore, is more definitely associated with yield than with standing crop, although it is never entirely synonymous with yield unless one is willing to assume that, for a given period, there has been a complete replacement of fish flesh equal to that removed through predation and "natural" deaths as well as through human pre- dation ( fishing, netting, spearing -^ ) . According to Carlander,^^ "Since the annual rate of turnover probably varies less from one fish population to another than does the standing crop, standing-crop data are probably fairly good estimates of fish production." However, Clark ^^ states that, "the magnitude of the standing crop at any moment does not give a measure of the rate at which production is going on since it (standing crop) is determined by the difference between the rates of production and destruction over the whole previous history of the population up to the time considered." (material in parentheses mine) Food Conversion Various kinds of fish are able to convert foods (of several kinds palatable to them) into flesh at various rates. Maximum efficiency in food conversion is attained when food is available for consumption at a rate between maintenance requirements and the maximum a fish is able to eat in a specified period. When food is scarce, a fish may expend too much energy in finding the food and, therefore, be unable to approach maximum efficiency. Where food is super-abundant, the fish may consume more than it can digest and assimilate so that a loss of efficiency results. Thompson ^^ stated that at 70°F 2.5 pounds of minnows are required to produce one pound of bass. When larger amounts were fed to the bass, the food was used less efficiently and conversion values were 3.8 for largemouth bass and 4.5 for smallmouth bass.^^ Markus ^^ has shown that the rate of digestion in largemouth bass is very slow at temperatures below 65°F, but that it increases rapidly be- tween 65° and 90°F. Tliompson ^^ used Markus' temperature-digestion rate curve along with mean monthly temperatures at each of seven dif- ferent localities within the range of largemouth bass in the United States Fish Production 73 to compute the total quantity of minnows which a 10-inch bass could digest in a year at each of these locations. Since the maximum yield is probably 2^i"f>portioiial to the total of potential digestion, it may be possible to show the relationship between carrying capacity and potential yield at diflFerent latitudes. Carrying this idea further, Thompson pub- hshed a table showing the theoretical eflFect of latitude on potential annual sustained yield (Table 4.3). Table 4.3 Effect of latitude on annual yield as esti- mated FROM MEAN MONTHLY TEMPERATURES OF different localities.^^ (From Needham, J. G., "A Symposium on Hydrobiology," the University of Wisconsin Press, Madison, Wisconsin, 1941.) Maximum Annual Yield as Percentage of Carrying Locality Latitude Capacity Vilas County, Wisconsin 46° North 21 Madison, Wisconsin 43° North 39 Urbana, Illinois 40° North 50 Cairo, Illinois 37° North 74 Memphis, Tennessee 35° North 86 Jackson, Mississippi 32° North 102 New Orleans, Louisiana 30° North 118 Relation to Standing Crop Aetual data on the relationship between production (as expressed by fish yields ) and the standing erops of fishes are as yet inadequate to test Thompson s theory as expressed in Table 4.3. Where data on yields and standing crops are available for comparison, there are other factors that obscure a clear relationship, such as fishing pressure, apparent unco- operativeness on the part of fish, known differences in food chains, etc. For example, the yields of bass at Ridge Lake were influenced more by the presence or absence of available foods than by fishing pressure ^^; but even in years when the highest yields were taken ( about 65 or 70 per cent of the available weight of bass in the lake), there was no indication that these yields reduced potential yields for following seasons. A recent study of a smallmouth bass population in a gravel-pit pond ^^ that produced hook-and-line yields of more than 100 pounds per acre for two successive years and then in the third year produced a yield of 80 pounds per acre, suggests that the maximum annual yield of fish in the region of central Illinois may be nearer 100 per cent of the carrying capacity than 50 per cent as given by Thompson (Table 4.3). The small- 74 Carrying Capacity, Productivity, and Growth mouth bass yield from this pond for any one season was almost wholly dependent upon fish spawned during the two preceding years. When the population was censused after 4 years of high yields, the standing crop at the beginning of the fifth year (fish caught by fishermen in April, May, and early June plus the fish taken in a June census of all remaining fish ) was approximately 100 pounds per acre which is considered relatively high for this kind of fish in this type of water (gravel pit) in central Illinois. Largemouth bass are less predictable than are smallmouth bass. Certainly, the fishing pressure was heavy enough at Onized Lake (II- hnois ) ^ in 1939 and 1940 to insure a crop of fish by hook-and-line methods, large enough to tax the production capacity of that body of water. The yield of largemouth bass here was 53.0 pounds per acre in 1939 and 19.8 pounds per acre in 1940. The weight of bass caught in April, May, and June of 1941, when added to the weight of bass taken in the total fish census of June 24, 1941, suggested that the standing crop of bass at the beginning of 1941 may have been between 50 and 60 pounds per acre or about 2.5 times the weight of bass taken in 1940 when the total fishing pressure exerted on the lake was 1647 man-hours per acre. On the basis of the 1939 yield of 53.0 pounds per acre, the carrying capacity of Onized Lake for bass could be estimated to lie between 50 and 100 pounds per acre, which is anything but specific. However, the fact that yields of largemouth bass are influenced largely by factors other than angling pressure makes it unsafe to estimate its turnover, production, or carrying capacity in a mixed population of fishes. Bluegill yields from Onized Lake for 1939 and 1940 were 174 and 66 pounds per acre, respectively. The estimated standing crop at the begin- ning of 1941 was around 153 pounds per acre, which must have been con- siderably under the carrying capacity of Onized Lake for bluegills. More than 6500 bluegills were taken in the final census, June 24, 1941, which was more than 3200 per acre, a sufficient number of active digestive tracts to convert food to flesh quickly if an excess of food (over mainte- nance needs) was available. These feeble attempts to unravel the basic relationships between stand- ing crops, observed yields, and productive capacity in populations com- posed of several species of fishes emphasize the need for more data on fish populations composed entirely of one species. Current information demonstrates the validity of these concepts and their importance in understanding fish-population dynamics. Estimating ProdtjCtion A fairly close estimate of total production of fish flesh may be obtained for one or two growii.g seasons provided the fish are tagged or marked Relative Plumpness of Fish 75 when released at the beginning of the period, and are collected by draining for a census at the end. In the final census, marked fish are counted to determine natural mortalit) , and of course fishing mortality within the period must be measured through a creel census. Marked fish in the "draining" census are weighed individually to calculate the gain in flesh made during the period. Unmarked fish are weighed, as they also represent production. Total production equals: ( 1 ) flesh gained by recaptured marked fish, plus ( 2 ) flesh produced by new recruits entering the population plus ( 3 ) flesh gained by fish lost through natural mortality plus (4) flesh gained by fish captured by anglers. An estimation of flesh gained by marked fish eventually lost through natural causes may be made if one assumes that these fish remained alive for half the period in question. Under this assumption their gain per in- dividual would equal one-half that of marked fish of a comparable size range. Loss of production through natural deaths among new recruits is an unknown quantity, although it might not represent a very large poundage of fish flesh. Estimated production in long-lived species is probably more accurate than that in short-lived species because the annual turn-over (recruitment and death rate) is smaller. Using the method outlined above for bass in Ridge Lake (Illinois) with a period limit of two years, the gains are as follows: in the the 1941-1943 period, 58.5 pounds per acre; in the 1943-1945 period, 34.2 pounds per acre; in the 1945-1947 period, 37.2 pounds per acre; in the 1947-1949 period, 59.8 pounds per acre; and in the 1949-1951 period, 53.5 pounds per acre.^^ However, these estimates of production for two-year periods cannot be used to estimate the maximum production for a single year. RELATIVE PLUMPNESS OF FISH Everyone who has an opportunity to handle large numbers of fish soon becomes aware of the variation in the plumpness of individual fish in any species. Usually, this variation in the relative condition of each of several species of fish inhabiting a body of water is greater than that among individuals of one species; for example, bass in a lake may be in good flesh while sunfish may be thin. While there is some variation in the plumpness of individuals of a single species at any one time, larger changes in relative plumpness for that species may follow annual cycles or even longer periods, in the latter instance perhaps reflecting variations in feeding conditions or population densities. All of these variations may 76 Carrying Capacity, Productivity, and Growth be expressed numerically when weights and lengths of individual fish are used for calculating some form of condition factor. Methods of Measuring Condition In order to describe the relative plumpness of a fish as a numerical value, a formula involving the relationship of surface ( length ) to volume (weight) is applied to the fish. The solution of this formula gives a number known as a condition factor, that stands for a measurement of relative plumpness: A fat fish of a given species and length will show a higher condition factor than a thin fish of the same species and length. By calculating these condition factors for an adequate number of fish of one species and of a limited range in length, taken from a single body of water during a limited period of time, and averaging them, one may arrive at an average condition factor for the species under the experi- mental conditions adhered to above. In this way, the range of relative plumpness for a species may be defined, and cycles of relative plumpness associated with seasonal feeding or other kinds of behavior may come to light. Coefficient of Condition. One of the first formulas developed by fisheries workers -^ for figuring relative plumpness required the use of the weight of a fish in grams, and its standard length in centimeters ( the standard length of a fish is the distance from the tip of the snout to the base of, but not including, the tail fin ) . In this formula K, the "Coefficient of Condition," is equal to the weight of the fish in grams times 100, divided by the cube of the standard length in centimeters, thus: ^ _ 100 W When this formula was used for figuring the coefficient of condition for fish with laterally compressed but deep bodies, such as the white crappie, or the bluegill, the numerical value of K usually ranged from 2.00 to 4.00. Fishes with bodies of lesser depth, such as the largemouth bass, usually had K values ranging from 1.00 to 3.00. The numerical values for K meant nothing in themselves but allowed comparisons between individuals or groups of fishes. Index of Condition. The use of the Coefficient of Condition K had one great drawback, namely that the average fisherman and even most fish- eries biologists were unable to "think" of fish in terms either of weight in grams or of body lengths (without tails) in centimeters.-^ Fishermen were used to measuring the maximum length of a fish with its mouth closed and the tail pinched so that the longest rays of the tail were parallel to the body axis. Lengths of fish were measured in inches and fractions of inches Relative Plumpness of Fish 77 and weights in pounds and fractions of pounds. As the numerical values derived from condition formulas arc useful for comparisons only, Thomp- son and Bennett ^^ developed a new condition formula in which the total length of a fish was taken in inches and tenths of inches and the weight in pounds and liundredths of pounds. This formula required neither con- versions of centimeters to inches nor grams to pounds, and the entire fish was measured rather than some fraction of the total length. Tenths of inches and hundredths of pounds were used instead of quarters of inches or ounces in order to facilitate rapid calculations. The formula was as follows : Index of Condition, C = — where W represents weight of the fish to the nearest hundredth of a pound, and L represents total length to the nearest tenth of an inch. When calculations are made using the lengths and weights of largemouth bass between 5 and 15 inches of total length, an Index of Condition of 3.5 to 4.5 denotes a fish in poor flesh, 4.6 to 5.5 one of average plumpness, and 5.6 to 6.5 a very fat fish. In fish such as the bluegill, which is deep in proportion to length and laterally compressed, the figures for condition are higher. In bluegills from 5 to 8 inches of total length, an Index of Condition figure of 7.0 or below denotes a fish in poor flesh, 7.1 to 8.0 one of normal plumpness, and above 8.0 one of unusual plumpness. Condition Factor of E. M. Corbett. A system used by the English for figuring condition was the "Condition Factor" invented by E. M. Corbett and issued by the Salmon and Trout Association, Fishmongers Hall, E.C. 4, London (date unknown). Corbett used total length in inches and weight in pounds and ounces, but actually converted fractions of inches into tenths and ounces into hundredths of pounds. Using essentially the same formula as that proposed by Thompson and Bennett, Corbett multi- plied his numerator by 100,000 instead of 10,000 to get rid of the decimal point. His formula is as follows: o J-.- IT ^ m7 100,000 W Condition Factor, C.F. = — '—- • where W = weight in pounds and L = length in inches. This formula gave a condition factor as a whole number. For example, a bluegill having an Index of Condition of 7.22 with the Thompson and Bennett method of calculation was equal to C.F. = 72 with the Corbett system. Applying the Corbett system to bass and bluegills in South Africa, A. Cecil Harrison arrived at the same range of condition figures (times 78 Carrying Capacity, Productivify, and Growth ton) for thin, a\orage, and fat fish that Thompson and Bennett found for lUinois fish of those species. Condition Factor of Cooper and Benson. Still another method of figuring condition is that of Cooper and Benson -^^ Coefficient of Condition, R = ^, ■ Where W = weight in grams, and L = total length in inclics. This mixed method of fi^urins^ condition \\as used on small trout con- venientlv weighed in scrams, whereas measurino; boards were calibrated in inches and tenths. Conversion was as follows: to the English system (CF.), R X 22.038 — C.F.; to the Thompson and Bennett system, R X 2.2038 — C. Condition Cycles Some fish species follow c\cles of condition associated with seasons. For example, the bluegills in a pond in central Illinois *^ showed high condition during Ma\' and earlv June at the beginning of the spawning season, follo\\od b\- a gradual chop in condition throughout the summer and fall until a low point was reached in October or No\ember. Over the winter, the condition gradualh" rose, but the most rapid rise took place in earh' spring, during the months of March, April, and Ma\-, when bluesiills were feeding hea\il\' on dipterous lar\ae and cladocera. Some of the loss in condition of bluesfills, boiiinnins; in late Mav and extend- ine throushout the summer \\as undoubtedlv associated with the lonff spawning season of this fish, which began in Mav and extended into early September. A seasonal condition cvcle for white crappies in a lake in the same region-^ was quite difierent from the bluegill cycle of condition. Lake Decatur ( Illinois ) crappies of 6 to 8.5 inches usually showed their highest condition in the fall and winter, and the condition of these fish dropped sharph" from earlv spring to June or July. Following a low point, usually in Julw the condition of the crappies began to rise in August and con- tinued to rise until winter. Several studies on condition in largemouth bass suggest that these fish show no seasonal cxclo of plumpness. There is e\idence, however, that the average condition of a bass population may change rather sud- denlv with changing feeding conditions. For example, in 1941 at the time of an extensi\e natural die-ofi" of die pond weed, P. foUosus, in a pond,*' the bass had an average condition of 5.00. After the plant die-oft' beo-an, thev became verv fat and their average condition rose to 6.15. Growth 79 Condition and Growth Rate Higli condition of fishes is usually associated with rapid growth, but this is not always so, particularly where fish are living in very soft water in which there may be such a shortage of calcium as to curtail the growth of the fishes' skeleton. In some other locations, relatively rapid growth seems to be associated with moderately low condition, at least during a part of a year. Use of Condition in Management An important value of condition factors is in their use in determining the well-being of a fish population in w^hich one has a special interest, either as a sport fisherman or as a lake owner. Often, when fish are in poor condition (exclusive of lows of normal cycles), it may mean over- population or disease. A high condition may mean a sparse population or a high temporary food supply. When either of the extremes of condition may appear, the situation may bear investigation. The condition of any component of a population of fishes is related to the relative abundance of food for that group, and this relative abundance of food may be related to high natural food production of the aquatic habitat; but more especially to the number of individuals among which a specific quantity of food must be divided. Thus, rapid growth, high condition, and large average size of a species may be essentially a function of natural or artificial cropping, because the type and intensity of cropping that occurs may determine the amount of food an individual fish is able to gather in a given period of time. Average condition figures have been calculated for most species of fresh-water fishes of interest to anglers. ^-^ These condition figures may be used by lake and pond owners as well as fishery biologists as a basis for comparison with the condition of fish that are members of a local popula- tion. GROWTH Although a new-born Great Dane or Chihuahua pup may be expected to grow to a rather definite size within a period of about a year and then remain the same size throughout the rest of its life, no amount of whole- some food would cause a Chihuahua to approach the size of a Great Dane, nor would a severe shortage of food prevent a Great Dane pup from greatly exceeding the size of a mature Chihuahua in the one-year period. However, growth in fishes is unlike growth in most warm-blooded animals in that it is relatively indeterminate and follows no exact pattern of attaining a maximum size in relation to a specific length of time. Thus, 80 Carrying Capacity, Productivity, and Growth m o . 0.2- • I— I CD Homewoocl Lake No Food Competition 1934 1935 1936 1937 1938 1939 Figure 4.4. Three- to five-year-old stunted bluegills from Homewood Lake (Illinois) more than tripled their weight in one season after release in Fork Lake where adequate food was available. [From Bennett, G. W., Thompson, D. H., and Parr, S., III. Nat. Hist. Surv. Biol. Notes, 14 (1940)] Growth 81 although every species of fish probably is characterized by a maximum size (length and weight), this size is so much greater than the average maximum attained by any given individual of any selected species in most waters that fishing contests with prizes for the largest examples of each kind of fish have flourished and will continue to flourish as long as man is interested in angling. Growth stoppage in fishes is not associated with sexual maturity as it is in most warm-blooded animals, and fishes continue to grow throughout life, although growth is relatively slower in larger and older fishes than in smaller and younger ones. Effects of Starvation Under conditions of near-starvation, fishes may remain the same size for an indefinite period, and after months or years of life on a maintenance diet, they still retain the capacity to grow rapidly to large average sizes should an abundance of food suddenly become available.^' ^^ This was demonstrated with bluegills when fish 3 to 5 years old and averaging 0.08 pound each were moved into a renovated pond where food com- petition was probably absent ( Figure 4.4 ) . The fish grew to average about 0.40 pound each in one growing season, although they had been badly stunted in past years. Growth of Fish in New Waters Fish often grow rapidly and reach exceptionally large sizes when first introduced into new waters. This superior growth is largely due to an abundance of food and space and possibly an absence of parasites and other biological forces which may slow down the rate of growth in waters where these fish have been present for some years. One of the most interesting of introductions was that of the importation of certain game and food fishes from Europe and North America into the waters of South Africa.^' -' ^ The importation of trouts into South Africa came early with the brown trout in 1892 and the rainbow in 1897. Intro- ductions of exotic fishes were carp from England in 1896, European perch in 1915, and largemouth bass from an English hatchery in 1927. Fish importations to South Africa from the United States were small- mouth bass from Maryland in 1937, bluegills from Maryland in 1938, and spotted bass from Ohio in 1939. The live-bearing top minnow gambusia appeared in South Africa in 1936 from an unknown source. Following are some records of catches of warm-water fishes of known ages, that had been stocked in South African waters: A largemouth bass caught on January 19, 1949, by Mr. D. S. Stewart and reported by Mr. H. Manson, Hon. Secretary of the White River Angling Society, was 23 inches long, girth 17 inches and weight 7 pounds 82 Carrying Capacity, Productivity, and Growth 10 ounces. Growth calculations from scale measurements indicated a growth to 9 inches the first year, then 15 inches, 18, 20, 21, 22 and 23 inches; its age was 7 years plus part of an additional summer. Official record largemouth bass from Paarde Vlei Lake, Sommerset West, in July, 1936, weighed 5 pounds 1 ounce, was 19 inches long and was stocked in 1930 as a fingerling. These bass growth records are similar to examples of maximum growth for largemouth in northern United States ( increment of about one pound per year ) , but no example of largemouths of 8 to 10 pounds which are fairly common in the United States has been recorded for South Africa. This may be related to the genetics of the original stock, the source of which (in the U. S.) is unknown. A bluegill was caught on March 17, 1947, by Mr. H. F. Palmer that weighed 3 pounds 1 ounce in a dam (pond) at Butha Buthe in Basutoland. "The fish could not swim upright in 9 inches of water." It could not have been more than 8 years old because the first bluegills were imported in 1938 and the first fingerlings were distributed in 1939. At least one bluegill exceeding three pounds is recorded for the United States, so here again maximum sizes may be comparable in South Africa and North America. Factors Affecting Rate of Growth In comparing growth rates of fishes in various parts of the United States, fisheries biologists have emphasized the importance of (1) the genetic growth potential of a given species, (2) a large available food supply per individual fish, (3) and the length of the growing season, e.g., length of the period when the water is warm enough to allow rapid digestion and assimilation of food. All of these growth-controlling factors are important, but in the production of fishes of above average size, a large source of available food per individual fish seems to exceed in im- portance the length of the growing season (Figure 4.5). Often a large supply of available food is present for the few fish that are restocked in renovated lakes, and they grow phenomenally during the first season before population expansion has reduced the food supply per individual fish.22 In studying the growth of largemouth and smallmouth bass in Norris Reservoir, Jones -^ concluded that although the agricultural growing season at Knoxville, Tennessee, was almost 7 months, the bass growing season was only 4 months. He assumed that because the bass stopped growing they could not grow after late September or early October, even though the water was still warm. Jones apparently misinterpreted the effects of what presumably was a temporary fall shortage of bass foods in Norris Reservoir, and he assumed that these effects represented a definite bass growing season which was much shorter than the agricultural growing season. Others have shown that bass may grow rapidly during Growth 83 4.0- m CD I 3.0 gi 2.0 1.0- 10 fish per acre-foot 300 800 2800 " 4500 " 6500 30 60 90 Days of Growth Figure 4.5. Relationship between growth and growing time in days of small walleyes in ponds, showing population density in fish per acre-foot. These growth curves are actually a measure of relative abundance of food. [From Dobie, J., Prog. Fish-Cult, 18(2) (1956)] April, September, and October in waters several hundred miles north of Norris, Tennessee, provided the water is above 60°F and an abundance of food is available.^ The length of the growing season for fishes which rather closely parallels the agricultural growing season, but is somewhat shorter, limits the annual growth cycle to a definite period of weeks or months (Figure 4.3). Tlie amount of growth actually taking place during this time depends (within limits) upon available food resources. At one extreme, a fish might approach the maximum rate of gain for a given species in a location where food was abundant and the growing season was less than 3 months; at the other extreme, a fish might remain at a 84 Carrying Capacity, Productivity, and Growth constant weight throughout a growing season of 8 or 9 months if it were hving under conditions that furnished no more than a maintenance diet. There are aspects of a short growing season that may have an important bearing on management practices. In some Michigan waters where the length of the growing season does not exceed 5 months and ice and snow cover the lakes for 4 or 5 months, bluegills seldom mature sexually until the third summer of life.'^ This means that all phases of management that bear any relationship to time of sexual maturity of the fishes must be adjusted accordingly. Viosca ^- described the growth of warm-water fishes— largemouth bass, spotted bass, and crappies— in the International Paper Company reservoir at Springhill, Louisiana, where the growth period for these fishes is limited by draining and refilling operations rather than the length of the natural growing season. The lake basin (270 acres) is pumped full in April and May, and newly-hatched fry are brought in with the water. The lake is drained in September or October to make room for waste water from the paper mill so the growing season for fry of various fishes ranges from 5.5 to 6.5 months. Fish that enter the reservoir through the pumps as fry in April and May produce a "field day" for anglers by late summer. In 1949, largemouth bass (age group 0) ranged from 11.1 to 13.8 inches and 11.4 to 24.5 ounces; spotted bass ( 10 times as numerous as largemouth) ranged from 6.1 to 10.6 inches and 1.4 to 11.6 ounces. In 1950, crappies reached sizes as large as 9.8 inches and weights of 12 ounces in 6.5 months. These crappies averaged a weight increment of 1.8 ounces per month, which is very near their maximum rate of growth. The accumulation of nutrients from the decayed paper waste added to the productivity of this reservoir. There is evidence, both from laboratory feeding experiments and field studies,^- that fish supplied with quantities of one or several live foods alternate between periods (weeks) of heavy feeding with rapid growth and periods of little or no food consumption with subsequent growth stoppage. Fish, like other animals, find a heavy diet without variety unattractive. In the laboratory experiments cited above, bluegills that stopped feed- ing were being fed earthworms at the rate of 7 to 8 per cent of their body weight per day. However, other bluegills receiving earthworms in smaller quantity (3 to 5 per cent of body weight per day) did not stop feeding. A continuous study of the ecology and growth of one or several species making up a small fish population will show that there are times when certain types of food are abnormally abundant and that this abundance is often reflected in unusual growth. For example, in August, 1941, a sudden die-oflF of heavy growths of submerged aquatic plants in a pond Growth 85 in central Illinois ^ was followed during August, September, and October of 1941 by a rapid growth of largemouth bass. These plants were pro- tecting a large population of small fish which suddenly became easily available. Bass that were between 10.5 and 11.0 inches before the plant die-off, averaged 13.0 inches in October; those about 7.0 to 8.0 inches before the die-off averaged 10.5 inches in October; and the surviving members of the current year class averaged nearly 6.5 inches by October. No comparable growth rate increase was shown among bluegills, even though the pond developed a bloom of plankton algae following the death of the higher aquatic plants. In this instance, a large supply of food suddenlv became available to the bass with no comparable increase in foods for bluegills. The cause of death of the rooted aquatic plants was unknown, but its effect was highly favorable to a species of fish having little direct ecological relationship to aquatic plants. Interpretation of Growth from Fish Scales Variations in growth rates and the occurrence of growth stoppage are recorded on the scales of the fishes. When a fish is growing rapidly, the circuli (fine lines of new material) laid down on the edges of the scales are relatively coarse and spaced far apart; on the other hand, when growth is slow, the circuli are fine and close together. A fish subjected to a period of starvation not only loses body flesh, but erosion with resorption of material on the edges of the scales may also take place. However, on a maintenance diet where the condition of a fish remains constant, there appears to be neither increment nor erosion of the scales. Thus, the correct interpretation of the marks on the scale surface will give an accurate growth history of a fish. The years of a fish's life are recorded as a series of annuli or distinct rings laid down around the focus or center of the scale, each one repre- senting a year. The circuli are between the focus of the scale and the first annulus and are also between the other annuli. A newly-hatched fish may be scaleless, but if it is of a scaled species, the scales soon form. Once the fish becomes covered with scales, the number remains constant throughout life, and to form a covering for the fish, the scales must grow as the body grows. Any natural or artificial phenomenon that will stop feeding and growth of a fish for about 14 days or longer will be followed, once growth is resumed, by the appearance of an "annulus," on the margin of the scales. The so-called "true annulus" was first called a winter ring because it was not visible until new circuli were laid down in the spring when the fish had resumed growth. Further studies of annulus formation showed that some fishes in some locations did not begin to grow until the middle of summer ^^ so that the winter ring became a summer ring. Hubbs and Cooper -" recorded a double annulus in green and long-eared sun- 86 Carrying Capacity, Productivity, and Growth A Figure 4.6. Abnormal growth rings on fish scales. A. Yearling blueliU from Fork Lake, 1938 year-class, taken September 22 1939, age 16 months, showing well-defined annulus (I) and talse annulus (F). The false annulus was formed on this scale about July. B. Largemouth bass, 10.1 inches, weight 0.49 pound, female taken April 20, 1941, before the 1941 annulus had formed. This is a 1938 year-class fish in which the 1939 annulus (I) and he 1940 annulus (11) are separated only in the anterior field ot the scale. Growth 87 fishes in Michigan. They believed that the outside member of the double ring was the true annulus, whereas this outer ring in actuality probably was a mark laid down during the first early peak of spawning. The 1938-brood bluegills at Fork Lake (Illinois) '-^ were growing very rapidly in 1939 and during the summer growing period of that year, many members of this brood laid down two or three distinct "annuli" on their scales ( Figure 4.6a ) : formation of the true annulus was completed by the last of May; the second annulus (false) first became visible during the latter part of June and the third annulus (false) appeared on the scales of a small per cent of the population in July and August. There was no satisfactory method of separating false from true annuli.^- It is my belief that the false annuli appeared on the scales of the Fork Lake bluegills because they went "off their feed" for short periods. Some abnormalities of growth, which are reflected on the scales of a fish and cause difficulty in the correct interpretation of age and growth, are: (1) False annuli— ialse rings having all of the characteristics of true annuli, but which form during the middle of die growing period and after the true annulus has formed for the current year ( Figure 4.6a ) . (2) Skipped annuli— wheve the position of the annulus for one year coincides with that of the preceding year, e.g. the fish does not grow during one growing season ( Figure 4.7b ) . (3) Overlapping annuli— where growth in length through one growing season is very small with no corresponding increase in plumpness. The annulus for one year coincides in part with that of the next, but in part (usually in the anterior field) is separated by 4 or 5 circuli (Figure 4.6b). Without detailed growtli information, a scale reader is likely to consider die second component of the double ring as a false annulus. (4) Close spacing of annuli— where growth for one season is small and two annuli are separated entirely, but by only a few circuli. Without growth information, a scale reader is liable to consider the outer annulus as false. Scales, spines, bones, and otoliths of fishes have been used successfully in age determinations. Studies of these parts from fishes of known ages prove that most species usually lay down a single growth ring each year. However, there are some exceptions, such as European carp, which fre- quently form extra "annuli." But even the scales of carp may be inter- preted on the basis of "growth patterns" for successive growing seasons, provided a specific population is being studied intensively over a period of several years. In the wake of the many excellent studies on the age and growth of fishes, the methodology for using scales ( and other structures such as ear stones, vertebrae, spines, and opercular bones that show annual rings) 88 Carrying Capacity, Productivity, and Growth Fieure 4.7. Normal and abnormal growth rings on largemouth bass scale! A. Normal scale pattern for 1938 year-class largemouth bass irLk Lake (Illinois) after 2+ years of growth. Th.s bass was 9.6 inches weighed 0.45 pound, a male taken August 30 1940. B. A 1938 yelr-cfass bass in which the second (11) and third (III annul, coincide. This fish was 15 inches, weighed 2 pounds, a fen-al^ t»^^" Tulv 8, 1942 from Fork Lake. This could not have been a 1939 yeai^ class fish because there was no 1939 year-class m this pond ( 1938 year-class original fry were sexually immature m 1939). Growth 89 in an interpretation of growth, has become standardized.^'^ In this chapter, the abnormalities of growth have been stressed because the more one knows about a population of fishes the more accurate one's interpretation of growth. For example, Dr. R. W. Larimore found a mark on the scales of \^ernard Lake warmouths -^ that corresponded in time to a period when a drag line was dredging the shallows of the pond from which these warmouths were taken. Without a series of fish collections before and after the drag line operation, it would have been impossible to explain the "false annulus" that appeared on the scales of many warmouths, apparently caused by the "feeding disturbance" resulting from the opera- tion of the drag line. There is more to reading scales than counting rings. One must become familiar with the usual variations in the marks and ridges on scales of specific species, as well as the potential for abnormalities on these scales and causes for the abnormalities. LITERATURE 1. Anon., Inland Fisheries Dept., Union of So. Africa, Cape Town, 1, 1-47 (1945). 2. Anon., Jour. Cape Pise. Sac, 1(2), 14 (1947). 3. Anon., Jour. Cape Pise. Soc, 3(9), 12-14 (1949). 4. Ball, R. C, Am. Fish. Soc. Trans., 75, 36-42 (1948). 5. Ball, R. C, Jour. Wildl. Mgt., 16(3), 267-269 (1952). 6. Ball, R. C, and Ford, J. R., Agri. Exp. Sta. Mich. St. Coll Bull, 35(3), 384-391 (1953). 7. Ball, R. C, and Tait, H. D., Mich. St. Coll Agr. Exp. Sta., Tech. Bull, 231, 1-25 (1952). 8. Bennett, G. W., ///. Nat. Hist. Surv. Bull, 23(3), 373-406 (1945). 9. Bennett, G. W., ///. Nat. Hist. Surv. Bull, 24(3), 377-412 (1948). 10. Bennett, G. W., ///. Nat. Hist. Surv. Bull, 26(2), 217-276 (1954). 11. Bennett, G. W., and Childers, W. F., Jour. Wildl Mgt., 21(4), 414-424 (1957). 12. Bennett, G. W., Thompson, D. H., and Parr, S. A., 7//. Nat. Hist. Surv. Biol Notes, 14,1-24 (1940). 13. Boccius, G., "A Treatise on the Management of Fresh-Water Fish," J. Van Voorst, London, 1841. 14. Brown, W. H., Tex. Game, Fish and Oyster Comm., 1-21 (1951). 15. Car lander, K. D., "Handbook of Freshwater Fishery Biology with the First Supplement," pp. 1-429, Dubuque, Iowa, Wm. C. Brown, Inc., 1953. 16. Carlander, K. D., Jour. Fish. Res. Bd. of Can., 12(4), 543-570 (1955). 17. Carlander, K. D., and Moorman, R. B., Proc. of the Iowa Acad, of Sci., 63, 659-668 (1956). 18. Clark, G. L., Ecol Mono., 16, 321-335 (1946). 19. Clark, M., Jour. Wildl Mgt., 16(3), 262-266 (1952). 20. Cooper, E. L., and Benson, N. G., Prog. Fish-Cult., 13(4), 181-192 (1951). 90 Carrying Capacity, Productivity, and Growth 21. Cooper, G. P., and Latta, W. C, Pap. Mich. Acad. Sci., Arts b- Letts., 39, 209-223 (1954). 22. Grice, F., Am. Fish Soc. Trans., 88(4), 332-335 (1959). 23. Hansen, D. F., III. Nat. Hist. Surv. Bidl, 25(4), 211-265 (1951). 24. Hansen, D. F., Bennett, G. W., Webb, R. J., and Lewis, J. M., III. Nat. Hist. Surv. Bull, 27(5), 345-390 (1960). 25. Hile, R., U.S. Bur. Fish. Bull, 48(19), 211-317 (1936). 26. Hile, R., Am. Fish. Soc. Trans., 75, 157-164 (1948). 27. Hubbs, C. L., and Cooper, G. P., Pap. Mich. Acad. Sci., Arts ir Letts., 20, 669-696 (1935). 28. Jones, A. M., Am. Fish. Soc. Trans., 70, 183-187 (1941). 29. Larimore, R. W., Ill Nat. Hist. Surv. Bull, 27, 1-83 (1957). 30. Leopold, A., "Game Management," pp. 1-481, Chas. Scribner's Sons, New York, 1933. 31. Markus, H. C, Am. Fish. Soc. Trans., 62, 202-210 (1932). 32. Moyle, J. B., Jour. Wildl Mgt., 20(3), 303-320 (1956). 33. Swingle, H. S., Ag. Exp. Sta. Ala. Poly. Inst. Bull, 264, 1-34 (1947). 34. Swingle, H. S., Jour. Wildl Mgmt., 16(3), 243-249 (1952). 35. Swingle, H. S., and Smith, E. V., Ala. Poly. Inst. Ag. Exp. Sta., Mimeo, 1-6 (1938). 36. Swingle, H. S., and Smith, E. V., N. A. Wildl Conf. Trans., 4, 332-338 (1939). 37. Swingle, H. S., and Smith, E. V., Ag. Exp. Sta. Ala. Poly. Inst. Bull, 254, 1-30 (1947). 38. Tanner, H. A., Am. Fish. Soc. Trans., 89(2), 198-205 (1960). 39. Thompson, D. H., "A Symposimn on Hydrobiology," pp. 206-217, Univ. of Wis. Press, Madison, Wis., 1941. 40. Thompson, D. H., and Bennett, G. W., Ill Nat. Hist. Surv. Biol Notes, 11,1-24 (1939). 41. Tiemeier, O. W., Kan. Acad. Sci. Trans., 60(3), 294-296 (1957). 42. Viosca, P., Jr., Am. Fish. Soc. Trans., 82, 255-264 (1953). 43. Williams, W. E., Am. Fish. Soc. Trans., 88(2), 125-127 (1959). 5 Reproduction, Competition, and Predation If the habitat is suitable for fish, the success of various components of a fish population may depend upon the interrelationships of three forces, namely, reproduction, competition, and predation. Reproduction produces new individuals, not only to replace losses of mature animals but also to enter the trophic cycle— the young fishes feeding on lesser animals and themselves becoming food for other larger ones. Thus, the products of reproduction push the population toward expansion. In opposition, the forces of competition (inter-specific and intra-specific ) and predation tend to counteract population expansion. This interrelationship of re- production, competition, and predation is normal and necessary to die well being of the population, although considerable variations exist. Thus, some populations, that appear engaged in a constant struggle for exist- ence, may show comparatively little fluctuation from year to year in actual number of surviving individuals. However, other populations may require a cycle of two or more years to achieve maximum abundance, while still others may fluctuate irregularly between maximum and mini- mum limits. Geological evidence has shown that fishes have existed on the earth's surface for many milHons of years. Yet in spite of the fact that they have furnished food for many forms of vertebrates— fishes, amphibians, reptiles, birds, and mammals including man— and some kinds of invertebrates, they must be considered as one of the more successful groups of vertebrates. A number of species of fishes alive today are scarcely changed in form from those found as fossils in deposits representing the Devonian Period, 360 million years ago. 91 92 Reproduction, Competition, and Predation REPRODUCTION Most of the vertebrates that prey upon fishes are more recent (geo- logically speaking) than their victims. Thus, it is conceivable that as pred- ators of fishes e^'olved and accumulated, the reproductive potential of fishes expanded to compensate for greater and greater losses through predation, until the reproductive potential became very high. Less than 100 years ago in the United States man began to dominate other verte- brates, and he purposely or inadvertently upset the normal relationships between fish predators and their prey. The high reproductive potential of a fish without an accompanying high predation rate became a detri- ment to the well-being of a fish species in that too many individuals survived for the available food supply. Tliis high reproductive potential of fishes has remained unchanged, and its significance must be appreciated in any plan for the management of a species. Reproductive Potential of Fishes Common warm-water fishes produce numbers of eggs in an inverse rela- tionship to the amount of protection that they give the sex products after they are released. In species such as the European carp which offer no protection to its sex products, a single female may produce several hundred thousand eggs. In contrast, the stickleback, which builds a com- plicated nest of plant material and then actively guards it, may lay a few hundred eggs. Between these extremes are the sunfishes depositing their eggs in depressions which they have made in the river or lake bottom and then attempting to guard against the predatory activities of their own kind, and other kinds of aquatic predators. Here the number of eggs is intermediate, ranging from 5000 to 10,000 in largemouth and smallmouth basses to 20,000 to 50,000 in the crappies and larger sunfishes. Jenkins "^^ cited an example in which 50 adult crappies with a reproduction potential of 590,000 produced a population of one-year-old fish of 200,500. Actual counts of eggs produced by various kinds of warm-water fishes have demonstrated that the reproductive potential of every species is more than adequate to replace the losses of the adults that produced them. For example, one pair of bluegills may easily produce 50,000 to 75,000 fertile eggs in a life time. The survival of only two of these embryos, through development, hatching, and growth to sexual maturity, is neces- sary to replace the loss of the parents. In order to investigate the success of natural reproduction of some common nest-building fishes. Carbine ^^ siphoned off the fry from the nests of largemouth bass, rock bass, common sunfish, and bluegills in Deep Lake, Oakland County, Michigan, and made counts of the number Reproduction 93 of fry collected from each nest. He found that numbers of largemouth bass fry varied between 751 and 11,457, with an average of 4375 per nest (5 nests); rock bass from 344 to 1756 with an average of 796 per nest ( 9 nests ) ; common sunfish 1509 and 14,639 per nest ( 2 nests ) ; and blue- gills from 4670 to 61,815 per nest with an average of 17,914 (17 nests). On the basis of the number of nests being used by these centrarchids during the 1938 season, the minimum number of fry produced in Deep Lake (surface area 14.9 acres) was estimated as follows: bluegill 6,610,- 000; common sunfish 1,518,000; rock bass 46,000; and largemouth bass 164,000. As this lake probablv would not support a fish population of more than 8000 to 10,000 individuals of useful sizes, it is obvious that any one of the 4 species listed above produced enough young fish in 1938 to overpopulate the lake. Other pan and sport fishes produce large numbers of eggs. Female warmouths ranging in size from 3.5 to 7.0 inches contained from 4500 to more than 50,000 eggs per fish."^- The female walleyes in Lake Gogebic (Michigan) -^ ranging from 16.0 to 22.7 inches yielded from 37,000 to nearly 155,000 eggs per fish. On the average, 34 walleye females from Gogebic yielded 28,112 eggs per pound of body weight. Wisconsin muskel- lunge from 25 to 53 inches in length were reported to produce 22,000 to 180,000 eggs per fish at each spawning, and northern pike are known to produce about the same numbers. ^^ As mentioned above, only a small fraction of the young produced by any fish species survives. For example, in the spring of 1941, Ridge Lake, Coles County, Illinois (18 acres), was stocked with 100 sexually mature largemouth bass.^^ Thirty-eight schools of young were observed in early June, each containing at least 2000 individual free-swimming fry; a con- servative estimate of the total was 76,000. When the lake was drained in March, 1943, almost 2 years later, sHghtly more than 4000 of these young bass were still present and even with this number, Ridge Lake was over- populated with bass. Heaviest predation probably takes place during the first few weeks of life when the fish are very small and relatively helpless. This was sub- stantiated by studies on the spawning of northern pike in Houghton Lake ( Michigan ) in 1939 and 1940.^^ Weirs were installed to catch fish in tlie spawning migration of northern pike into the ditches tributary to the north bav of Houghton Lake and to trap the returning young pike from the ditches. In 1939, 125 female and 280 male adult pike migrated into the ditches, and 7239 young pike were caught migrating toward the lake. In 1940, 65 females and 81 males migrated into the ditches, and only 1495 young migrated out. In both years, newly-hatched pike fry were about equally abundant in the ditches, but in 1939, minnows and perch 94 Reproduction, Competition, and Predation were allowed to migrate into the ditches on only one day, while in 1940, minnows and perch could enter the ditches during the entire period of the investigation. In 1939, 58 young per spawning female returned, while in 1940 this dropped to 23. Predation on young fish begins in the embryo stages and continues through one or two growing seasons for the slow-growing or small species. Predators large enough to use fast-growing large species for prey may be relatively rare after the first few months of growth. Spawn Production and Number of Spawners If natural reproduction of fishes is so successful, why have populations of many important game and food fishes continued to dwindle? The answer seems to be that although many more young of these fishes are produced than a body of water may support, the survival rate of these young between the time that they first begin to develop and the time when they grow too large for easy predation is inadequate to balance the natural and accidental death rates of adults. Fish embryos and newly-hatched fry are vulnerable to many decimating forces: sudden temperature changes, disease, absence of adequate food, turbidity, aquatic fungi, and fluctuating water levels, and a host of aquatic animals that would use them for food. There are many "accidents" that may eliminate very small fish. Carbine's study of predation by perch on the young of northern pike, described above, is a concrete example of interspecific predation which may have been the most important cause for a reduction of northern pike in Houghton Lake. In many situations where the survival of spawn of an important species of fish is inadequate to maintain a population of these fish, no amount of stocking will help because (1) hatchery-reared fish small enough to be supplied in quantity are more vulnerable to predation than naturally spawned fish of the same sizes, and ( 2 ) fish too large to be preyed upon by most kinds of predators can be reared in such small numbers in hatchery ponds that they are insignificant when they are released in large natural waters (Houghton Lake has an area of 20,044 acres). Predation rates may vary from very high to very low, while reproductive potentials remain uniformly high. The end result of these counter forces is to obscure the relationship between number ( or poundage ) of spawning adults and the number of young produced (Figure 5.1). For example, suppose that in a given spawning season the survival rate of bass fry (to a length of one inch) from 10 spawning pairs was 90 per cent and the average number of eggs produced was 2000 per female. Thus, 10 X .90 X 2000 = 18,000 fry produced. Reproduction 95 In another year, the spawn of 100 pairs ot bass producing an average of 2000 eggs per female was subject to such heavy predation that the survival rate was only 5 per cent. Thus, 100 X .05 X 2000 = 10,000 fry produced. In these illustrations, twenty spawners in the first spawning season would produce more 1-inch fry than would 200 spawners in the following season. <: •73 p; :=! o 1953 1955 2 1 Pond No. 2 1 3 Pond No. 3 2 Pond No. Stocking rate of adult cai-p Production of young carp Survival of adult carp Figure 5.1. Production of carp in small ponds at Lake Mills, Wisconsin. These experiments demonstrate that there is no consistent relationship be- tween the number of adult fish stocked (pounds) and poundage of young carp produced. The poundage of young brought forth by the smallest poundage of adults usually equals or exceeds the poundage of young produced by larger numbers of adults (pounds). Also, the total poundage of all carp produced in individual ponds is not consistent from year to year. [From Mraz, D., and Cooper, E. L., Jour. Wildlife Mangt., 21(1) (1957)] In comparing annual estimates of schoohng bass fry with the numbers of sexually mature bass known to be present in Ridge Lake in each of 10 years (1941-1951), I was unable to show any correlation between number of spawners and the fry produced other than an indication of a negative relationship, e.g., in several years when the numbers of spawners 96 Reproduction, Competition, and Predation were smaller than average, the numbers of fry produced were larger than average. ^^ Mraz and Cooper ^^ also found little correlation between the number of brood fish stocked in ponds and the strength of the resulting year classes (Figure 5.1). Age and Sexual Maturity The age of a fish at the time of sexual maturity varies with its size and the latitude of its habitat. According to Swingle and Smith/*" bluegills as small as one-half ounce have been known to spawn when 1 year old (in Alabama) and where food was extremely plentiful, young bluegills weighing 2 ounces spawned when only 5 months old. Largemouth bass as small as 6 ounces and crappies as small as 2 ounces have been known to spawn when 1 year old (Alabama). Eschmeyer -^ stated that some of the largemouth taken from the Clinch River below Norris Dam (Tennessee) would probably have spawned at 1 year of age. Farther north in central Illinois, James ^^ making a histological study of the gonads of largemouth bass and bluegills concluded that while a few male bass produced small numbers of sperm at 1 year, none of the females produced mature eggs. Many of the yearling bass that James studied were more than 10 inches in length and weighed 0.50 to 0.60 pound each. The larger and medium-sized 1-year-old bluegills produced mature eggs or sperm, but those less than 2 inches contained only small oocytes, in- dicating that they were sexually immature. North American centrarchids imported to South Africa have had their seasons reversed and some showed rapid attainment of sexual maturity.^^ Smallmouth bass reproduced at 17 months and bluegills at 7 months when their seasons were reversed south of the equator. These fish orig- inated in Maryland and Ohio. In the north central states, most of the common game and pan fish require one year (bluegill, rock bass), two years (crappie, largemouth, smallmouth), or three years (northern pike, walleye) to reach sexual maturity. The age of a fish at sexual maturity is important in planning stocking rates for new or renovated lakes or ponds. Sex Ratios Most studies of sex ratios of the individuals composing isolated popula- tions of fresh-water fishes have shown that more males than females are to be found among the young of the year, but among older fish the dominance of females was so great as to leave little doubt that the males die off much faster than females. This is not only true in the pike and perch families but also in some of the nest-building centrarchids.^^ Reproduction 97 External Sex Characteristics In many kinds of fresh-water fish, secondary sex characteristics develop as the adult fishes approach the spawning season. These characteristics make it possible to separate the sexes, either through differences in coloration (Figure 5.2), or through structural differences (such as tuber- cules on the heads of some male cyprinids or a complete change in body appearance in some male channel catfish and salmonids). In many species, male secondary sex characteristics are so definite as to make Figure 5.2. Sexual dimorphism in hybrid crappies. During the spawning season the male crappie (lower) becomes very dark, particularly on the head, ventral side, and fins. accurate sexing of males easy; in others only long experience can develop proficiency in sex determination. An ability to separate the sexes may be useful and important when stocking new or renovated waters because it allows one to set up balanced or unbalanced sex ratios of selected kinds of fish; also, one may be relatively certain of the sex of individuals released in ponds for the production of hybrids between two closely related species. Spawning Nearly all of the warm-water fish begin spawning during the first 6 months of the year; some have short spawning periods lasting only a few days; others may spawn for 1 or 2 weeks, while still others may spawn intermittently over a period as long as 3 or 4 months. 98 Reproduction, Competition, and Predation Among the earliest spawners is the northern pike which spawns at water temperatures of 40° to 46°F. Walleyes spawn at 45° to 50°F, yellow perch a little after walleyes and muskellunge at 48° to 56°F.^* The pikes, walleyes, and perch begin spawning soon after the ice goes out in the early spring. Eschmeyer ^^ described the spawning of walleyes at Lake Gogebic ( Michigan ) : "At Lake Gogebic walleyes arrived on the spawning shoals immediately after tlie break-up of the ice in the spring. Usually they occurred in small numbers at first, followed by rapidly increasing numbers as the water warmed. . . . "Almost all spawning activity which has been observed at Lake Gogebic has occurred within the shallow-water strip along the shoreline —exposed by the lowered water level. . . . The entire shoreline along which spawning occurs is densely wooded and is washed almost continuously by waves as a result of its exposure to the prevailing northwest winds. . . . Since spawning occurred almost exclusively at night, much of the activity of fish on the shoals was observed with an automobile spotlight powered by a storage battery carried in the rowboat from which the observations were made. The eyes of walleyes reflect light, to appear as bright orange-red globes, thus greatly facilitating the location of fish on the shoals. "Walleye spawning seasons reported by other workers in various local- ities extend from late March to early June, but always include a portion of April or May." The nesting habits of the sunfishes (including largemouth and small- mouth basses and black and white crappies) are among the most in- teresting of warm-water fishes. Breder ^^ described the nesting behaviour of many members of this family. Some nest in groups, such as bluegills which select areas of shallow water 1 to 4 feet in depth and exposed to the direct rays of the sun for at least a part of the day. Here they scoop out their shallow nests often located so close together that only a narrow ridge of earth separates one nest from others surrounding it. Bluegills have a long nesting season; Fork Lake ( Illinois ) bluegills became ripe in late May and some ripe fish were collected every month tliereafter until mid-September. Bluegills usually show two peaks of spawning activity, the first and largest at the beginning of the spawning season and a second peak a month or so later, usually in early July in the north. However, some bluegill reproduction continued throughout most of their long spawning period. Some warmouths were found to be in spawning con- dition from about May 15 until August 15 in central Illinois ^- although no fish were completely spent until September 1. Both largemouth bass and crappies have short spawning seasons com- pared to bluegills and other sunfish, although the spawning period of crappies is somewhat longer than that of the largemouth. ^^ Crappies and Reproduction 99 largemouth begin spawning in late May or June in the north, but some- what earher in the south. The water temperature seems to have an im- portant eflFect in initiating spawning activity of all the centrarchids. Also, sudden drops in water temperature associated with periods of cold spring weather may stop all spawning activity and sometimes kill the embryos alreadv in the nests. Bullheads and catfish also sweep out nests and ofiFer protection to embryos and young fish. Channel catfish spawn during the period from late May to mid-July in Missouri,^^ with a peak occurring in early June and one in late June. Bullheads usually spawn during late May and early June as far north as central Iowa and Illinois. Hybridization Hybridization within certain families of warm-water fishes is not un- common. Hybrids have been observed in many groups of fishes including the Cyprinidae (minnows) and Esocidae (pikes), and they are relatively common in the Centrarchidae (sunfishes). The artificial production of hybrids in the laboratory is not difiicult if one is able to obtain ripe or nearly-ripe individuals of species capable of hybridization. In nature, hy- brids may be produced accidentally, and they are much more common among fresh-water fishes than among salt-water species. ^^ However, as the stimulus of a ripe male appears to be necessary in some species to induce the females to release eggs, the possibility seems remote that sperms from an outside source might fertilize the eggs thus released. There are recorded instances, however, of female carp releasing ripe eggs without the stimulus of a male fish. No similar behavior has been reported for members of the sunfish family, although it is difficult to see how ripe females could retain all of their eggs, due to the pressure within the fishes' abdomen. Probably most of the unspawned eggs are resorbed. Among closely related genera "mating behavior patterns" and "con- sciousness of species" may fail to prevent a certain amount of promiscuity, particularly in crowded populations where there is competition for spawning ground space. Under these conditions, hybrids may be as common as one in 10 of the young produced in a spawning season, al- though the production of hybrids rarely exceeds 1 or 2 per cent. Some use has been made of hybridization in fish management, e.g., hybridization of the northern pike, with the muskellunge, to produce the "silver musky" ( which is highly regarded by some fishermen ) , or the use of hvbrid sunfish to produce larger individuals with a low reproductive potential. Several studies of natural and artificial hybrids ( Figure 5.3 ) of various species of sunfishes have been made.^^^ 37, 48, 54, cs, is Sunfish hybrids have been reported between species in the following hst: green sunfish, blue- ■ asKSSBsKSK'?'-— ass*-. Figure 5.3. Hy- brid sunfish are pro- duced in the labora- tory. A. Ripe eggs from a female of one species are gently squeezed into a petri dish. B. Immediately milt from a ripe male of another spe- cies is sprayed over the eggs. Eggs and milt are gently mixed and then left for two minutes. C. Fertilized eggs are then placed in other petri dishes — a few hundred in each — and washed with aged tap wa- ter. When eggs are washed, they adhere to the bottom of the dish. [From Child- ers, W. F., and Ben- nett, G. W., 7^/. Nat. Hist. Surv. Biol. Notes, 46 (1961)] 100 Reproduction 101 gill, pumpkinsced, orange-spotted sunfish, longear sunfish, red-ear sunfisb, and warmoutb. Hubbs and llubbs ^" using li>'brids of bhiegill X green sunfisb and pumpkinsecd X green sunfisb determined diat under nearly identical conditions tbe hybrids grew faster and attained larger sizes tban eitber parent species. Ricker ^'•' stocked new ponds witb hybrid sunfisbes from male bluegill and female red-ear parents. He reported that in tbe absence of competition from other species these hybrids weighed between 3 and 4 ounces at the end of their first growing season and a pound after three growing seasons. Krumholz ^^ working witb tbe same hybrid (bluegill male X red-ear female) found that the hybrid was heavier in relation to its length than either of the parent species. Most of tbe Fi hybrid sunfish that have been studied have demonstrated unbalanced sex ratios, usually in the direction of 70 to 95 per cent males. -^ Some authors have reported no oflFspring from Fi hybrid parents."'' When crossing members of the Fi generation, numbers of Fo young are reported as varying from a few to as many as occur in natural spawning of pure species. -^^ ^^ Back crosses of hybrid sunfish to parent types have been accomplished in the laboratory, -^' ^^ and intergradations of hybrid sunfish types found in nature suggest that backcrosses as well as Fi hybrids are relatively common.'^ Recently, all possible crosses and reciprocal crosses between bluegill, red-ear sunfish, and green sunfish (Figure 5.4) were produced artifically by fertilizing the ripe eggs of one species with sperm from another (Figure 5.3).-^ After development in the laboratory, newly-hatched fry were placed in isolated ponds and allowed to grow for one or more years. A number of backcrosses and outcrosses were made and in one instance eggs from an Fi hybrid of red-ear male X green sunfish female were fertilized with sperms from a bluegill, producing an outcross that was one-half bluegill, one-fourth red-ear, and one-fourth green sunfish. As yearlings, these fish could be separated into four rather distinct types. From these observations and experiments, it is conceivable that the hybrid sunfish may show some of the advantages in pond management that hybrid corn and hybrid domestic stock have shown in agriculture. It may be possible to develop a hybrid sunfish with characteristics better suited to the needs of modern angling than any sunfish species now in existence. A start in this direction was made by Ricker and Krumholz when they used bluegill X red-ear hybrids for stocking farm ponds in Indiana. Selective Breeding Changes through selective breeding have been accomplished in carp, goldfish, and some other species of "domestic" fish raised in fish-farming 102 Reproduction, Competition, and Predation operations. Most of this work has been done in Asia and Europe, where fish are raised for food or display. These fish are usually not exposed to "wild" conditions; when they escape to compete with wild fish, they either die or revert to their original wild type as the European carp has done in this country. Fish that have been raised in hatcheries for many years have probably been subjected to unintentional as well as intentional selection. Hatchery RED-EAR SUNFISH R-B Fj HYBRID^ Figure 5.4. Hybrid sunfish produced by fertilizing eggs of one species with milt from a different one. In this experiment, fish were limited to green sunfish, bluegills, and red-ear sunfish. Hybrids showed characteristics intermediate be- tween those of parents but were usually heavier-bodied. Males were more abundant than females in most hybrid combinations. [From Childers, W. F., and Bennett, G. W., INHS Biol. Notes, 46 ( 1961 ) ] personnel may consciously choose the heaviest-bodied and fastest-growing fish for egg production; at the same time they may unconsciously select the fish that are most easily handled, e.g., those that have lost most of their "wildness." This type of selection has made hatchery trout somewhat less desirable than wild fish ^^ for sport fishing and has reduced their ability to survive under "wild" conditions. Attempts at selection for improving species of warm-water fishes ap- parently have accomplished little. However, artificial selection may be operative in heavily-fished, isolated small lakes and ponds that receive Stocking 103 no stock from outside sources. Fish in waters that receive 500 or more hours of fishing pressure per acre per season and that escape being caught are probably more wary than those that are caught. If wariness is associated with an inlierited behavior pattern, it might in time become incorporated in all of the fish in this population. No one lias devised a means for testing this theory, but it is conceivable that an isolated large- mouth bass population exposed to this type of selection for many gen- erations might furnish low annual production to anglers. Unsuccessful Reproduction Control of overproduction of young fish often is one of the major problems in fish management. In this respect, low production or non- production of certain hybrids may be an asset. Although most fishes have a high reproductive potential, as described on page 92, this potential may not be realized, either because of unfavorable environmental con- ditions, actual predation, or intense intraspecific competition of young. Eschmeyer -^ described unusual concentrations of dead walleye eggs observed in 1948 in Lake Gogebic. He was unable to determine causes for the low viability of these eggs. In Fork Lake (Illinois) the produc- tion of 40 bluegill nests w^as almost completely wiped out by predation from stunted yearling bass.^^ Channel catfish and flathead cat usually fail to reproduce in ponds. '^^ Under certain conditions this inability to reproduce might be an asset, in that one could control the numbers of these catfishes in ponds through stocking, which is impossible when using any of the species of bullheads. Observations of the limiting or curtailing of fish reproduction that can be traced to identifiable causes are extremely important because methods may be suggested for curtailing the survival of the young of a less de- sirable species while improving that of a more desirable one. STOCKING Many tests of stocking procedures have been made since 1935. How- ever, the recommendations of the states and provinces throughout the United States and Canada do not reveal much agreement among fisheries biologists upon the kinds and numbers of fish useful for stocking new waters or renovated old ones. This is due in part to dissimilarities among the fish habitats of North America, to differences in the biology of certain game and pan fishes in widely-separated parts of their present ranges, as well as to regional variations in the popularity of certain fishes. Also, some of the divergences in recommendations may be related to differences in the objectives of stocking. For example, some biologists stock to produce excellent but short-term angling as quickly as is possible; others 104 Reproduction, Competition, and Predation to produce satisfactory angling over a longer period of time; others, to favor one species; others, to give equal status to two or more species. These variations in objectives can be accommodated only by diflFerences in stocking methods. Fishes Used As early as 1944, evidence was available to show conclusively that warm-water fishes raised in hatcheries were valuable only for stocking new or renovated waters, or for stocking "waters seriously depleted as a result of overfishing, pollution, or other special circumstances." ^^ The Bass-Bluegill Combination. The bass-bluegill combination was re- born in the 1930's, although it had been used in ponds by Dyche before 1914 and by Barney and Canfield before 1922.*^' -^ In theory this com- bination seemed excellent: both largemouth bass and bluegills would be available for sport fishing; the bluegills would convert small invertebrate animals in the pond into bluegill flesh and the small bluegills would serve as food for the bass, the latter controlling excessive numbers of bluegills, so that the few that survived would grow to large average sizes. As with most theories involving specific behavior patterns for animals, the bass-bluegill combination did not always follow the original theory. The bass often were unable to control the expansion of bluegill numbers, and as Dyche -^ observed, the number of bass were more often controlled by bluegills than vice versa. Shortly, however, biologists discovered that this combination furnished satisfactory fishing for both bass and bluegills as long as neither species was allowed to become overabundant. Interest was immediately directed toward stocking specific ratios of these fish because these ratios had an influence upon the length of time required for a newly-stocked pond to reach overabundance, either of bluegills or bass. In the southeast, the bluegill was more easily controlled by bass than elsewhere, and both species reproduced when one year old.'^- Thus, stock- ing ratios were 1 to 15 in favor of bluegills. According to Swingle and Smith,"^^ fertilized ponds should be stocked with 100 bass fingerlings and 1500 bluegill fingerlings per acre, unfertilized ponds with 30 bass finger- lings and 400 bluegill fingerlings per acre. These ratios apparently were not considered satisfactory for the Southwest as Brown ^^ recommended either 200 to 400 largemouth bass per acre alone or in conjunction with about the same number of bluegills or other pond fish. In the north central states, the bass were usually unable to attain sexual maturity at the age of one year, although Clark -- reported that they did so in Kentucky. Bluegills were sexually mature the next season after hatching. Thus, when fingerling bass and fingerling bluegills were stocked together in the same pond, the bluegills reproduced during the first Stocking 105 spawning season, while the bass did not, and by the time the bass had reached sexual maturity after a second growing season, so many yearhng and two-year-old blucgills were present that thcv limited the survival of bass cmbr\'os and fr\'. To prevent this carlv shift toward an overpopula- tion of bluegills, mid-continent fishery biologists stocked ratios to give bass a better chance to survive and reproduce: 100 largemouth bass fingerhngs to an equal number of bluegill fingerlings. During the first season, the bass fingerlings would prey upon the bluegill fingerlings re- ducing their number to a low figure but still not so low as to preclude the survival of a small number of bluegills to spawn the following season. Furthermore, the pond was not too crowded with bluegills by the second season to prevent a successful spawn of bass when they became sexually mature. Fishing for bluegills is ponds stocked originally with 100 bass fingerlings and 100 bluegill fingerlings might not be satisfactory until after individuals of the first bluegill brood spawned in the pond, had reached a length of 6 inches, usually late in the third summer. This process could be speeded up one full season by stocking fingerling or fry- size bass at the rate of 100 per acre along with adult bluegills ( that would spawn the same season as stocked) at the rate of 10 to 30 per acre. Clark -- stated that this was the only satisfactory ratio for stocking bass and blue- gills in Kentucky; more recent findings do not support Clark's specific statement for a satisfactorv ratio. ^^^ ^^ In soudiern Illinois, six ponds on the lands of the University of Illinois, College of Agriculture Dixon Springs Experiment Station were stocked with stunted largemouth bass, 6 to 10 inches long and with bluegill finger- lings averaging 1 inch in length.^- Then their progress was followed closely for six years. Three of the ponds scheduled to receive fertilizer were stocked with 89, 91, and 109 bass and 1127, 1250, and 1630 bluegills per acre, respectively. Three unfertilized ponds were stocked with 21, 30, and 36 bass and 310, 396, and 412 bluegills per acre, respectively. A nearly complete creel census throughout the 6-year period showed that bass fishing was best during the first year. By the second season bluegills were of worthwhile sizes (6 inches) and the bluegill fishing (rate of catch) improved for the next few fishing seasons, reaching a peak in about 5 years. Often it is impractical or impossible to obtain enough fry or fingerling bass for stocking new artificial impoundments of 100 to 1000 acres at rates of 100 small bass per acre. In these instances, stocking might be done at rates of 1 adult bass per 3 to 20 acres of water along with bluegills at a somewhat higher rate. Usually, after the first spawning season, young bass can be collected in numbers in all parts of the lake, and it is nearly impossible to find any bluegills. Two seasons later the lake might furnish excellent fishing for both bass and bluegills. 106 Reproduction, Competition, and Predation For artificial impoundments of intermediate sizes between farm ponds and large reservoirs, a stocking of 2 to 10 adult bass per acre, followed by a stocking of 100 small bluegills per acre after the bass had produced young, usually gave satisfactory fishing one or two seasons later. In the northernmost states and southern Canada, the bass-bluegill com- bination was nearly useless because waters were infertile and growing seasons were short; thus, both species of fish required three growing seasons to reach sexual maturity. Also, the bluegills showed a strong tendency to overpopulate and become stunted. Rawson and Ruttan ^- stated that yellow perch grew better than bluegills in Saskatchewan and recommend stocking ponds with yellow perch and bass or northern pike. Rail- suggested stocking ponds in Michigan with 100 fingerling bass and 10 adult bluegills per acre. Ry so doing, the bass would have forage immediately instead of two or three seasons later. Later Rail and Ford ^ stated that the largemouth bass-golden shiner combination was more satisfactory for the production of bass fishing in Michigan than was the bass-bluegill combination. Neither Rrown and Thoreson ^' in the nortli- west ( Montana ) nor Saila ^^ in the northeast ( New York ) would recom- mend the bass-bluegill combination for ponds in these regions because results were unpredictable. A perusal of published material on the bass-bluebill combination will show that it apparently is most successful in the southeast. Farther north, results may be good but good results are not necessarily a certainty, al- though records of one bass-bluegill pond in Illinois showed a high yield of both bass and bluegills for 12 years after stocking.^^ This pond of 2.5 acres was stocked originally with 100 bass fingerlings and 100 bluegill fingerlings. In Illinois, the bass-bluegill combination usually moved in the direction of an overpopulation of bluegills. If ponds were stocked with equal numbers of bass and bluegill fingerlings per acre, this condition of over- population of bluegills might not arrive for 5 to 12 years. However, bluegills were very efficient in controlling populations of largemouth bass after bluegill numbers reach 1000 or more per acre,^^ particularly if these bluegills were small. Control was exerted through predation of small blue- gills on bass eggs and fry in the nests. Stunted bluegills would gather around a bass nest guarded by a male and sooner or later a bluegill would venture close enough to cause the male bass to give chase. While the male was away from the nest, other bluegills entered and fed upon bass embryos or fry. The bluegills scattered when the male bass returned, but soon the process was repeated and before long the eggs or young bass would be greatly reduced or eliminated. This was the only period in the life cycle of largemouth bass when these fish were highly vulnerable to bluegill predation, but if bluegill predation on bass embryos and fry were StockinE 107 t> consistent and efficient for several years in succession, the bass population of a lake might dwindle to a few old fish which lived well but were unable to produce a new year-class of young bass, because of an ever-increasing population pressure by stunted bluegills. Small bass are also vulnerable to crappie prcdation where the latter are abundant. Young crappies are much more inclined to hide in aquatic vegetation than are young largemouths, and for that reason the latter are usually more vulnerable to adult crappie predation than are small crappies. An overpopulation of stunted crappies (or bluegills) plus a few large bass unable to reproduce successfully was self perpetuating and con- tinued until the bass were caught or died of old age. Only drastic thinning of the stunted fishes allowed the bass to bring off a successful spawn. In contrast, an overpopulation of small bass might be controlled in a single season of fishing, provided the fishermen were educated to the necessity of taking these fish in spite of their small size.^^ If bass could be expected to prey upon bluegills exclusively, either through "normal" feeding habits or through a special taste for them, the bass-bluegill combination would be considerably more dependable than at present. However, bass actually select a variety of foods (including the larger aquatic insects, crustaceans, and fishes), and the evidence is that they will feed upon crayfish and their own young in preference to bluegills. Therefore, in order to control bluegills indefinitely, bass need assistance, several types of which will be described later. Other Combinations of Fishes. There is reason to assume that other combinations might be more satisfactory from the standpoint of angling and easier to manage than the bass-bluegill combination. Since large- mouth, smallmouth, and spotted bass are fairly omnivorous in their feed- ing and can get along well on crayfish, large aquatic insects, and their own young, any one of the three bass species stocked in a pond by itself should produce bass fishing without the complications of controlling blue- gills or other fish stocked with bass. Experiments testing the ability of each bass species to survive in its own pond have proved very satisfactory, particularly if the population is set up originally by stocking several year- classes to prevent the development of a dominant year-class, which might become stunted. This was quite easily done by stocking and assortment of bass larger than 10 inches in addition to 100 fingerfing bass per acre. At the first spawning season after stocking, the adult bass produced young; however, the fingerlings already present prevented the develop- ment of a dominant brood. Experiments have shown that populations of bass by themselves are as large, or larger in pounds per acre than are bass populations in combination with other species of fish. Several fisheries biologists have tested largemouth or smallmouth bass 108 Reproduction, Competition, and Predation in combination with one or several species of minnows. Ball and Ford^ tried a combination of largemouth bass and golden shiners which proved to be very successful. After 5 years, there were still plenty of shiners present as well as bass. Bluntnose minnows were able to maintain a popu- lation over at least a three-year period when confined in a very clear pond (gravel-pit) with smallmouth bass. This pond contained some rooted aquatic vegetation. Smallmouths taken from this pond showed a higher average condition than others taken from a nearby pond which contained only smallmouths. The largemouth-bass-red-ear-sunfish combination has been tested ex- tensively in Indiana ^^^ ^^ and Illinois. ^^ In no case where only largemouth bass and red-ears were present has there been any evidence of over- population of red-ears. However, the red-ear sunfish is somewhat less desirable as a sport fish than is the bluegill because the former prefers live bait fished deep and will seldom hit artificial flies or poppers at the water's surface. Larimore ^- combined largemouth bass with warmouths in stocking more than 15 ponds in central Illinois. This combination was very satis- factory. The warmouths showed little tendency to overpopulate and both warmouths and bass grew rapidly to useful sizes. Jenkins ^^ concluded from his observations in Oklahoma that large- mouth bass, channel catfish, warmouth, and red-ear sunfish produced more harvestable-sized fish in comparison with their total standing crops than any other warm- water fishes. In north-central Nebraska several ponds of 5 acres or larger have been stocked with a northern pike-bluegill combination. The pike were re- leased as fingerlings and bluegills as adults so that the latter furnished young bluegills for pike forage during the first season.^''^ Recently, some experiments have been started to test the usefulness of muskellunge (Figure 5.5) in the control of overpopulation of sunfishes. Smallmouth bass do well in warm-water ponds if they do not have to compete with some of the more prolific warm-water fishes, such as blue- gills, green sunfish, and black bullheads.^- For example, smallmouths maintained a large population in a 14-acre central Illinois lake until ex- cessive numbers of green sunfish prevented their successful reproduction. This lake was made by damming a pasture ravine and contained no sand or gravel in the shoal areas. Several studies have been made of combinations involving three, four, and five species of fishes, such as the bass, bowfin, and bluegill combina- tion of Krumholz ^" or the bass, bluegill, warmouth, and channel catfish combination used in Ridge Lake.^^ The study of Tliompson '^^ of an unfertilized fishing club lake in Macoupin County, Illinois, illustrates that the bass, crappie, bluegill, and bullhead combination in a lake with Stocking 109 limited shallows may be productive of substantial annual yields, if the lake is fished intensively when the fish are biting ( annual yield was about 100 pounds of fish per acre of lake ) . Every practicing fishery biologist will discover, sooner or later, at least, one pond or lake that, although unmanaged or "mismanaged," still pro- duces as good or better fishing than any lake or pond toward which he may be directing his management efforts at that time. 1 have found several. In some, the high production was transitory, and they eventually became poor fishing waters, usually because of overpopulation. There Figure 5.5. Muskellunge are sometimes stocked in small numbers in artificial lakes as a "bonus" fish and as an aid in the control of sunfish populations. were and are, however, a few others in which there seems to be a delicate relationship between reproductive success of the fishes and the natural food supply, perhaps through the action of predators of which 1 am unaware. In these waters, bluegills may range between two and three to the pound, with scarcely any of smaller sizes, or green sunfish may grow to 8 or 9 inches and may be represented by a few hundred fish instead of tens of thousands of 3- to 5-inch fish. These lakes always seem to contain many small bass, usually from 6 to 9 inches. There may be some unrecognized relationship between the physical environment and the fish and aquatic organisms that inhabit it. In later chapters, we will discuss this possibility. It is the author's opinion that one would be naive to expect any com- bination of fishes stocked in a man-made lake or pond to be productive of good fishing for an indefinite period of time. Too many of the integrated 110 Reproduction, Competition, and Predation forces and counter-forces that are active for promoting the well-being of a fish population in a primitive environment are absent from a man-made and man-dominated lake. Changes in Fishing There are many records of lakes and ponds that once produced ex- cellent fishing but which lost their productiveness after a few years be- cause of the introduction and expansion of undesirable fish or the over- population and stunting of desirable ones. Krumholz ^^ working on hybrid sunfish in ponds in Indiana found that 39 of 78 ponds stocked originally with hybrids and largemouth bass contained, after two years, other species of fish than were originally stocked. After questioning the owners of 29 of these ponds, 26 admitted that they had introduced other kinds of fish. Among the fish that were introduced were largemouth bass, bluegills, black and white crappies, green sunfish, orange-spotted sunfish, longear sunfish, and warmouth, and in two instances, smallmouth bufiFalo. Most of the pond owners explained their actions with the statement that they wanted to catch a greater variey of fish. Ball and Tait^ state: "The knowledge and effort necessary to maintain, over a period of time, a pond producting fast-growing bass and bluegills are beyond the scope of the average pond owner. Consequently, it is recommended that ( 1 ) small ponds be stocked in such a way as to pro- duce the species desired in the greatest edible weight in the shortest time, (2) these fish to be harvested, and (3) restocking be done instead of attempting to maintain a 'balanced' pond over a period of years." This may be a practical solution if ponds are supplied with a drain outlet and fish for restocking are readily available. In recording the history of Fork Lake, a farm pond in central Illinois,^ it was discovered that fishing had been considered good from 1926 to 1930 but had become poor through the development of large populations of black bullheads, carp, and buffalo fishes. At the time that Fork Lake was censused in 1938, it contained 5350 fish weighing 774 pounds (539 pounds per acre) and consisting of 16 species. Only 145 fish of the 5350 were of such species and sizes as to interest anglers. Stocking for Improvement of a Population Biologists are not in complete agreement on the value of adding fish stocks for the improvement of a population that, from the standpoint of fishing, is somewhat less than optimum. Viosca,'^^ after censusing ponds in Louisiana, concluded that the stocking of ponds which already contain fish may cause almost no change in a population of fish. He cited an ex- ample of a pond stocked with 1500 largemouth bass fingerlings which Stocking 111 after a year or so yielded only 6 stunted individuals weighing a total of 1.85 pounds as against 12,505 bluegills and green sunfish weighing 201 pounds. A deeper pond nearby was stocked with 4500 largemouth bass fingerlings. Here a mixed crappie population which was already present dominated bass and other fish. Later, when the pond was censused the proportions were 8.4 pounds of crappies and 1.4 pounds of sunfish to each pound of bass. According to Viosca: "—this type of evidence completely discredits the idea that artificial restocking will restore the largemouth bass population of a pond dominated by other species." In contrast, Swingle, Prather, and Lawrence ''^ state that "Since partial poisoning is normally required in populations containing too few bass and all small bass in the pond edges are killed, marginal or sectional poisoning in the spring or summer is detrimental unless followed by restocking of bass." However, stocking of fish after a partial poisoning operation is not com- parable to a situation such as Viosca described, because partial poisoning makes living space available for the stocked bass. Lagler and DeRoth ^^ came to the same conclusion as Viosca, after stocking fin-clipped bass fingerlings in the Loch Alpine ponds ( Michigan) ; none of the 4000 bass fingerlings stocked in 4 years were seen again. These ponds had uncontrolled outlets, and the small bass were free to move out. Cooper -^ on the fish stocking policy for Michigan says that this state has largely dispensed with plantings of warm-water fishes such as bass, bluegills, perch, walleyes, and northern pike. In the past, plantings of these warm-water game species were distributed among hundreds of lakes on a rather orderly schedule. While the state-wide totals of fish stocked were large, the yearly plants to individual waters were small: bluegills were stocked at the average rate of 35 fingerlings per acre; largemouth bass at the rate of 2.4 fingerlings per acre and smallmouth bass at the rate of 1.9 fingerlings per acre. At the same time (1947) large numbers of fingerlings of these species could be seined in most of these lakes. In 12 of these lakes selected on the basis of public interest, intensive seining operations on shoal areas indicated populations of young fish ranging from 103 to 1760 per acre with an average of 742 per acre for the 12 lakes. This so far overshadows the maximum stocking efforts of state personnel as to make their efforts of little consequence. By stocking 300 adult bluegills per acre in Kentucky farm ponds over- populated with largemouth bass, Clark -- produced satisfactory fishing for both bass and bluegills. Swingle and Smith '''' state that for ponds con- taining stunted bluegill populations a stocking of 100 bass fry or finger- lings per acre plus "proper fertilization" and heavy fishing for bluegills to reduce their numbers will correct this condition in a few months. This statement appears to be counter to the findings of Viosca ^^ and Lagler and DeRoth,^ ^ except that "proper fertilization" may increase the available 112 Reproduction, Competition, and Predation food for bluegills or at least produce enough additional plankton to allow some survival of bass. Even with a high rate of survival of stocked fish, corrective stocking will not eflFectively improve a fish population unless the number of fish stocked per acre is large enough to approach that found in "superior" populations of these same fish.-^ However, this is not always easy to achieve. Thus, if bass fry were stocked at the rate of 100 per acre into a population of stunted bluegills and the survival rate of the bass was 75 per cent ( a very unlikely assumption ) , they soon might grow large enough to feed upon the smaller bluegills, reducing food competition among the bluegills and thus facilitating the growth of the larger ones to useful sizes. However, in most instances of bass-fry stocking, the evidence in- dicates a low survival of the bass fry. Thus, the larger the young fish are allowed to grow before they are stocked from a hatchery, the better are their chances of survival. The problem here is that hatchery production of large fingerlings as compared to fry might be 1 to 1000, so that the fingerling production of the hatcheries of an entire state might be able to stock only a relatively small number of ponds at the rate of 100 large fingerlings per acre. The kinds of warm-water fishes that are prone to overpopulate and stunt in certain waters often may be introduced successfully into existing fish populations with the release of only a few adults. Under severe com- petition, these adults are incapable of producing a substantial population in one or two years, but after three or more years, these fish may appear in large numbers. Bluegills, red-ear sunfish, green sunfish, white crappies, and black bullheads have all been observed to increase in this manner after a small number of individuals were introduced into a dominant population of other fishes. Corrective stocking is always more dependable where a part of the population has been removed by poisoning or through mechanical means (seining, etc.). Tliis procedure reduces the standing crop of fish to a point below the carrying capacity of the water and leaves food and space for the fish that are introduced. Corrective stocking should not be attempted unless a source of fish is available for introducing a large enough number of a given species to approach the carrying capacity of the water for that species, e.g., 100 bass per acre, 300 to 1000 bluegills per acre, etc. Stocking to Improve a Food Chain This type of stocking is done in an attempt to improve the production of game fish in an existing fish population by increasing the forage for these game fish. The introduction of the threadfin shad into large southern Stocking 113 reservoirs was an example of this type of stocking.^- McCaig, Miillan, and Dodge ^'^ recorded that the introduction of smelt into Quabbin Reservoir ( Massachusetts ) furnished an improved food supply for lake trout so that the latter reached a length of 18 inches in their fourth year instead of the fifth as they had done before the smelt were abundant. This type of stocking should be considered very carefully before it is done because: (1) food chains of fishes are very complex and the intro- duced species may not serve the purpose intended; moreover, (2) forage fishes that are capable of expanding their populations in the face of existing populations of predatory species already present may constitute a danger from overpopulation as the gizzard shad has done in some waters. Invertebrates such as crayfish, scuds, and insect larvae are sometimes stocked in new ponds and small lakes, and these stocking attempts are frequently successful. Failures in Stocking Fish Biologists in various parts of the country have developed stocking numbers and ratios of fishes intended to give satisfactory results in fish- ing returns. However, these stocking recommendations sometimes result in poor fishing or no fishing. There may be one or several reasons for these unsatisfactory results: (1) Poor fishing in new impoundments may result from unauthorized stocking prior to or after the lake or pond has been stocked with tested ratios of kinds and numbers of desirable fish. Krumholz ^^ mentions the problem of maintaining uncontaminated hybrid ponds in southern In- diana. In Illinois, and probably in most states, there are fishermen who spend a part of their time as amateur fish managers; one of their main activities is the promiscuous stocking of small numbers of most of the kinds of warm-water fishes to be caught on hook-and-line. If one finds bluegills where red-ears were stocked or largemouth bass where there should be smallmouth bass only, one may be observing the work of an amateur fish manager. Many fishermen release left-over live minnows into streams or lakes; the contamination from the minnows may be of minor importance if none of them are goldfish, carp fingerlings, or suckers. (2) Some contamination takes place, usually in lakes or ponds near large urban centers of population, through the release of aquarium fishes when the owners tire of caring for them. Goldfish usually survive but guppies and other tropical fishes cannot winter over except in the deep south. New waters that have been contaminated with local warm-water fishes soon become useless for fishing unless through accident a reasonable number of bass are also present. As discussed previously, corrective stock- 114 Reproduction, Competition, and Predation ing of a water area that already contains fish is largely useless; it is better to kill all the fish and start again with correct numbers of selected fish. (3) Poor results may be obtained because the fish stocked in new or renovated ponds died after they were released "in good condition." Even though the fish released were active and swam into deep water in a normal manner, they may have sustained injuries through handling prior to release that later resulted in their death. For example, Brown ^^ found the survival rate of bass fingerlings in bass-bluegill stocking experiments ranged from 47.1 to 83.3 per cent. Other investigators have obtained 69.2 to 100 per cent survival "^^ and a 75 per cent survival for one pond.^*" A striking example of this type of injury was observed by the author while tracing the final dispensation of the original adult and yearling bass released in Ridge Lake (Illinois) in 1941.^^ These bass consisted of 58 adults from Crab Orchard Lake near Carbondale, 42 adults from Lake Chautauqua near Havana, and 335 yearlings from Lake Glendale near Robbs. The 58 Crab-Orchard-Lake bass were caught and held in wing nets, April 27-29 inclusive, moved into a tank truck supplied with an air compressor, and transported to Ridge Lake, a distance of 150 miles by road. The weather was unseasonably warm for April. Although the fish appeared in good condition when released, 44 of these fish (75.8 per cent) were believed to have died, soon after release, from injuries sustained in capture and transportation. These 44 bass were not caught by anglers in 1941 and 1942 and were not present when the lake was drained in 1943. The 42 adult bass from Chautauqua Lake (also caught in wing nets) were the survivors of a much larger number taken in March and April and held indoors for several weeks in aerated holding tanks. Fish injured in netting operations were removed and discarded. On May 1, bass re- maining in the tanks were transported in the early morning to Ridge Lake, a distance of 145 miles. Sixteen of the 42 bass disappeared without a trace and probably died from injuries (38.1 per cent). The 335 Lake Glendale yearlings (5.0 to 7.0 inches) were seined on June 17, held overnight in a holding net staked out in the lake, and then were hauled in aerated tanks to Ridge Lake— a road distance of 180 miles. The weather was hot, and a few of the fish died in transit, but those released appeared to be in good shape. In this group, 317 of the 335 bass (94.6 per cent) disappeared. Some may have been eaten by the larger bass, but with only 100 of the larger bass released in 18 acres of water, the fish were not crowded and predation probably was not heavy. The as- sumption that these fish died from improper handling was further sub- stantiated by the high survival rates of marked bass which were recorded Predation 115 in later years when the lake was drained in March, the fish censused, and the marked fish returned immediately to the refilling lake basin. One of the Chautauqua Lake fish that survived the original road trip and stock- ing in 1941 later survived 4 lake drainings and fish censuses and was caught by a fisherman in 1949. Stroud "^^ tagged fish released in Massachusetts ponds in order to measure the recovery from angling of largemouth and smallmouth bass, pickerel, bullheads, white perch, and yellow perch. He recorded several instances of high mortality shortly after stocking, presumably from injury during handling prior to release. Fingerlings or fry may be more or less sensitive to rough handling and poor water conditions during transportation and stocking than yearling or adult fish. However, stocking ratios of bass and other fish mean little if the stocking mortahty is abnormally high. A 10 per cent loss of bass or bluegills in a specific stocking ratio might mean little, but a 75 per cent loss of the bass (or bluegills) through unsuspected injury during trans- portation and stocking would certainly influence the future dynamics of a bass-bluegill population. Most hatchery men and biologists recognize the need for extreme care in the handling of fish to be stocked. The problem is largely related to the shortness of the season for moving fish (late fall and to a lesser extent early spring ) and the magnitude of the operation. Often, inexperi- enced help must be used in the handling and moving of fish which con- sequently become overheated or exposed to other unfavorable conditions before their final release in new locations. Some hatchery ponds are too badly silted for fish to be removed without having to hold them in silt-filled water for comparatively long periods. The danger here lies in the high-oxygen demand and the products of anaerobic decay in this silt. Thus, unless an outside source of clear water is directed into the pond to reach the fish concentrated in the seines, high mortality is almost certain to follow. PREDATION The Role of Predation in Fish Management At the beginning of this chapter we noted that the fishes have inhabited the earth for more than 350 million years and during this period have been relatively successful. Part of their success was due to die develop- ment of a high reproductive potential tliat allowed them to out-strip the predators that evolved to feed upon them. As each higher class of verte- brates appeared, certain members became predators of fishes, so that in a modern fresh-water environment, fish predators are represented by a variety of vertebrates each modified for predation activity in or on the 116 Reproduction, Competition, and Predation water. With a little thought anyone can name at least 10 common fish predators such as: garfishes loons pikes and muskellunge ospreys l3ullfrogs cormorants snapping turtles gulls alligators terns watersnakes pelicans mergansers eagles kingfishers mink ducks and geese otter herons men These predators, with the exception of man, are opportunists, willing to catch and feed upon wdiatever fish are available. Although most of them prefer taking fish as large as they can capture and hold, the predators of small fishes are much more numerous than those of the larger ones. In fact, among fishes of the largest sizes (such as the muskellunge, lake sturgeon, and salmon) only bears and men are predators. Most numerous of all are the animals that prey upon fish eggs and newly-hatched fry, because not only are many small fishes of all kinds included in this grouping, but also several kinds of invertebrates, espe- cially well represented by the predaceous aquatic insects, such as beetle larvae and dragonfly nymphs. With all of these aquatic animals actively foraging upon small fish, most of the losses from predation occur while the fish are very small. These losses represent tremendous numbers of individuals but relatively small amounts of fish flesh. In the primitive environment where man was sparsely represented, predators of fishes were much more numerous than they are now, except perhaps in the more remote parts of North America. Some idea of the extent of predation in remote regions may be gathered through estimating the food requirements of small temporary concentrations of mergansers or cormorants. For example, about 1000 cormorants were observed to be feeding on Chautauqua Lake, a U. S. Fish and Wildlife Refuge near Havana, Illinois, during the fall of 1954. These birds were present throughout a period of 3 weeks. Studies of cormorants in captivity have demonstrated their voracity; an adult cormorant requires a maintenance diet of about 1 pound of fish per day, and it can eat more than 2 pounds per day if the opportunity arises. Using a food consumption estimate of 1.5 pounds of fish per bird per day, the Chautauqua Lake flock must have been consuming 1500 pounds of fish per day or for the 3-week period, a total of 31,500 povmds or 15.75 tons of fish. Although these fish may have been mostly gizzard shad if available, it is well to remember that if Predation 117 largemouth bass or crappies were more accessible than the shad, the cormorants would have been eating bass or crappies instead. This be- havior pattern of most predatorv animals of taking the available prey that can be conquered at a given time and place has the partly or wholly beneficial eflFect of reducing excessive numbers of concentrations of prey animals, which, in itself, is an important function."^ We see the direct effects of curtailed fish predation among the fishes stocked by man in artificial impoundments. These fish still reproduce as though they were subjected to the usual complex of decimating factors; but because many types of natural predators are relatively scarce where man controls the environment, there is insufficient culling of the fish population, and overpopulation, excessive food competition, and stunting are commonplace. These conditions eventually eliminate certain kinds of game fish from a fish population and stunt other kinds so badly that scarcely any grow to sizes large enough to interest anglers. For a long time anglers looked upon fish predators as direct competitors with themselves for the fishes of our streams and lakes, and they de- stroyed garfish, watersnakes, turtles, mergansers, herons, pelicans, and other known fish-eaters whenever the opportunity appeared. There is little doubt that some of these fish-eaters, particularly fish-eating birds, may make serious inroads on abnormal concentrations of fish such as are found in hatchery ponds or in cold-water streams where numbers of hatcherv-reared trout have been stocked. In most other waters their impact upon fish populations is beneficial. Man's own activities and attitudes regarding fishes have in part been responsible for the poor fishing that has plagued him. This situation stems from his substitution of a new type of predation for that which occurs in nature. Man preyed upon large fish, but protected and pampered small ones. This new type of predation and protection coupled with the fact that no change occurred in the fishes' reproductive potential resulted in excessive survival and competition among the young. In this competition bass and other game fish lost out to hoards of stunted crappies, sunfish, and yellow perch. Thus, many of the techniques of fish management that will be discussed in Chapter 6 are simply methods of population control or environmental manipulation used to prevent the development of dominant populations of some kinds of fishes and to stimulate the dominance of other kinds. Many observations have been made to show that where predators of fish ( other than the fish themselves ) are reduced, a prey species of fish may actively control the survival of its normal ( fish ) predator. I have produced evidence that young bluegills may control the survival of young bass.^ Carbine ^^ demonstrated that perch and minnows may control the survival of aelvins and juvenile northern pike; and Eschmeyer -^ cites several 118 Reproduction, Competition, and Predation recorded instances of yellow perch, minnows, sturgeon, catfish, and suckers eating walleye eggs and fry. COMPETITION Several types of competition occur among the fishes in an aquatic habitat. Although the most common rivalry is probably for food, com- petition for space in specific habitats, as when sunfish vie for nesting areas, may be somewhat more obvious. Not much is known of the extent and importance of any specific type of competition in a specific habitat, but we can demonstrate the end results by tlie changes that take place in crowded populations where several types of competition are severe. Competition for Food Most fishes depend upon a wide variety of food, rather than upon a restricted diet. Thus, if a seasonal or localized shortage of one food occurs, a species may shift to another type.^^- ^- Anyone who has had occasion to study the stomach contents of individuals of any single species of fish collected over a period of several months or seasons, has no doubt marveled at the changes in the kinds of foods as well as in the quantities of a single aquatic organism sometimes found in a single stomach. One is almost led to believe that the taking of some foods becomes habit during certain seasons. It is often very difficult to recognize competition between two kinds of fishes for a specific food organism as was described by Johannes and Larkin ^^ for rainbow trout and red-side shiners. In this case, competition was recognized only because a study had been made of the feeding of the trout before the red-side shiner had become abundant. To what extent is an available food utilized? Patriarche and Ball ^^ emphasize the importance of the "forage ratio" of Hess and Swartz^^ which is the ratio of the percentage of occurrence of an organism in an aquatic population to its percentage of occurrence in the stomach of a fish species. If this ratio varies significantly from 1:1, it should be due to either a difference in availability or a difference in preference. Allen ^ offered an "availability factor" for forage ratio and Leonard ^^ suggested that the forage ratio be used as a measure of availability only. In many cases where a specific food organism is abundant there is little question as to its availability to fish; in others it is impossible to measure the difficulty involved in the capture and ingestion of an abundant organism. Most aquatic biologists agree that the bacteria and algae are at the base of the food chain. The bacteria use complex waste materials in the water, and the algae are able to utilize inorganic salts, carbon dioxide, and water in sunlight, to make carbohydrates and proteins, which are Competition 119 used as a source of food by odier organisms. However, the food chain from bacteria or algae to the larger fishes does not consist of a single series of hnks, but many. A few of these food chains may be dominant during one season or under certain environmental conditions while others may replace them at another time or season. Thus, at certain times, foods more suitable for one species of fish may be more abundant than at other times. This may be reflected in changes in condition, growth rate, and in stand- ing crop of the fish. Larimore^- could not show definite food competition between large- mouth bass and warmouths inhabiting the same lakes, although these two species often fed on similar types of organisms. Warmouths tended to feed on organisms on soft bottoms in shallow waters and along banks while largemouths fed more on the surface organisms and free swimming forms in deeper or more open parts of the lakes. Studies of the food habits of closely related fishes may show similarities, yet with certain important differences. Ball and Tanner ^ in studying the foods of bluegills and pumpkinseed sunfish from the same waters, dis- covered that the pumpkinseeds selected a larger proportion of molluscs and hard-bodied insects than did the bluegills; while the bluegills ate larger amounts of aquatic vegetation than did the pumpkinseeds. The selection of these types of foods by bluegill X pumpkinseed hybrids was intermediate between the two parent species. Both parent types were feeding upon about the same range of foods, but distinct preferences for certain types were clearly evident. In fishes of widely different food habits, such as largemouth bass and bluegills, there may be some evidence of food competition at times and under certain conditions. For example, bass and bluegills in one pond competed for insects when fish or crayfish were not available for the bass.^^ Under these conditions, however, bass ate more flying insects and bluegills more larval aquatic forms. In this particular situation, it was impossible to evaluate the degree of competition between these two species. Competition for Space When fish are forced to compete for living space, there is evidence that in some species (and perhaps all species) growth rate and reproduction are affected adversely. Anyone who has kept goldfish or tropical fish in aquaria indoors and then has placed these fish in an outdoor pool during a summer period, has had a demonstration of the change in growth rate brought about by increased space. Usually, a part of the increased growth rate is due to an improved diet, but even where aquarium fishes are receiving a completely balanced diet, their growth appears to be affected by the amount of space for each fish. 120 Reproduction, Competition, and Predation Experiments designed to expose the causes o£ reduced growth and reproduction in crowded fishes have furnished evidence that where ade- quate food was available, inhibited growth and reproduction was due to ammonia and other material excreted into the water by the fishes them- selves."^^' ^'^' ^^' ^- In most of the experiments in which conclusive results were obtained, the fish were tropical aquarium fish, goldfish, European carp, or one of the species of buffalo— fish that have a reputation for intermittent production of year classes. Goldfish stocked in small ponds at the beginning of the growing season at the rate of 200 4-ounce fish per acre produced large numbers of young, while those stocked at the rate of 2000 or more of 4 ounces or larger produced few or no young. "^^ Originally, it was thought that the goldfish ate their own eggs and young in the ponds stocked with the larger num- bers of adults; however, examination of adult females showed that eggs were well formed but never laid. Later, it was discovered that each time during the summer that adult goldfish were moved from their regular ponds into new ponds freshly filled with water, that the fish spawned within 48 hours. On the basis of these and other experiments. Swingle '^^ postulated the presence of a repressive factor composed of a secretion or excretion pro- duced by the goldfish themselves that inhibited final development and deposition of eggs, although these eggs were already practically mature. Additional experiments showed that the inhibitory material was ex- creted into the water by the goldfish and that when this water was moved into new ponds it retained its ability to inhibit reproduction, even when it was diluted 2:1 with fresh water. Similar tests using carp and bigmouth buffalo gave results comparable to those from goldfish experi- ments. Swingle believed that largemouth bass were affected by an inhibitory factor. Certainly there is evidence that the production of young in this species is never directly related to number of spawners; usually there appears to be an inverse relationship between bass fry and number of adults available for spawning.^^ It was also assumed that overcrowded bluegills depressed the produc- tion of largemouth bass through the secretion of a repressive factor. If this were so, why were bluegills able to build up overcrowded populations and depress bass reproduction without curtailing their own reproduction? More experimental work must be done before exact evidence is available to prove or disprove this hypothesis, particularly when crowded bluegills have been observed repeatedly to feed upon bass eggs and fry. Yashouv ^^ placed two small carp in each of a number of aquaria con- taining 26-27 liters of water. These fish were fed 10 per cent of their Competition 121 body weight per day and held at water temperatures warm enough for growth. Water was changed in all aquaria at two-day intervals through- out experiments extending from 43 to 80 days. Controls received no other treatments, but experimental a(|uaria received varying amounts of meta- bolic water obtained by stocking a small container with a large number of fish for 30 to 40 minutes or until thev had difficultv in breathinij^. Fish in J ^ CD aquaria receiving metabolic water gained slightly or lost weight wliile control fish gained 150 to 275 per cent of original weight. As yet, there is no exact agreement among investigators as to the causative agent responsible for tliis inhibition of growth and reproduction. One believes it is a hormone-like substance, another a substance that created a vitamin deficiency. When fish-conditioned water was given to rats for drinking they lost weight and died after 45 to 50 days.^- These rats displayed characteristic symptoms of Vitamin Bi deficiency. In the Far East where fish are grown in boxes in streams there is no growth inhibition, although the density of fish in the boxes amounts to as much as 50 per cubic meter. Here the metabolic waste is carried away by the current. Yashouv ^- thinks that this action of metabolic waste may represent a defense mechanism (for slowing population growth) of the existing population. Mraz and Cooper ^^ found little relationship between population density of adult carp (within the range of 75 and 450 pounds per acre) and the weight of young carp present with them at the end of the first summer (3 months), Figure 5.1. Adult carp were stocked in May just prior to the spawning season, and the ponds were drained in August or early September. These adult fish always produced a spawn; when less than 100 pounds of adult carp per acre were stocked, the fish usually gained in weight; a loss in weight followed, when adults were stocked, at rates of 150 to 450 pounds per acre. Average size of these young carp ranged from 2.7 to 5.0 inches after 3 months, and the weight per acre of young carp ranged from 98.4 to 308.7 pounds. As the carp were stocked just prior to the spawning season, there was no inhibition in spawning, but growth of adults and young may have been affected later. Among game and pan fishes in hatchery ponds no clear effect of crowding upon growth (where adequate food was available) and repro- duction has been demonstrated, although one may assume that accumula- tion of waste may function as a growth inhibitor. Larimore ^- demonstrated a very significant difference in the numbers of ova carried by female warmouth living under different conditions. For example, a female warmouth of 5.3 inches from Venard Lake (Ilfinois) contained 40,400 ova in various stages of development, while a female of the same length from Park Pond contained only 12,500 ova. Venard 122 Reproduction, Competition, and Predation Q in O < > O o < O H u^ ^ / VENARD LAKE / / 56 - PARK POND o Jan-Mar r=98 A Apr-Jun r=.97 o / 48 n Jul- Aug r=64 / - 40 / r O A - 32 o / o ▲ X X ^ X ^ X ■^ Ax ^ A a/ / - 24 a/ ^ /^ X A • 16 o/ A / - 8 / ^A 1 ^ / / / / 1 1 1 1 1 - 3.0 4.0 5.0 6.0 7.0 TOTAL LENGTH OF FISH, INCHES Figure 5.6. Estimated number of ova from warmouths of various total lengths in collections from Venard Lake and Park Pond (Illinois) in 1949. Scattergram, regression line, and coefficient of correlation (r) are given for each collection group. Each graphic symbol represents one female from which the number of ova was estimated. Venard-Lakc warmouths belonged to a pop- ulation of fish that was expanding rapidly. In Park Pond the warmouths were subjected to severe competition for food and space, and many were heavily parasitized. [From Larimore, R.W., III, Nat. Hist. Survey Bull, 27(1) (1957)] Competition 123 Lake was supporting a rapidly expanding fish population, while the fish in Park Pond were more crowded and more heavily parasitized (Figure 5.6). Competition for Specific Habitats This type of competition may be more common than is the general crowding of fishes, particularly in the case of competition for spawning areas by sunfishes, or competition for limited shallow bottom areas for bottom-loving bullheads. Control of a specific habitat within an aquatic environment may allow a single species of fish to hold dominance over other components of a fish population. Starvation Most fishes are well equipped to withstand prolonged periods of starva- tion. In some laboratory experiments by Dr. Marian F. James,^^ bass were held in aquaria at room temperature without food for several months. During this period they lost nearly half of their body weight, and some died of starvation. Some were brought back to their original weights through repeated force feeding of small amounts of food. After having been starved for 2 or more months, these bass were no longer interested in food and would pay little or no attention to live minnows released in the aquaria with them. Other laboratory experiments indicated that in order to maintain a con- stant weight bass required about 1 per cent of their body weight per day in tlie form of fish. These fish were able to live for an indefinite period on a maintenance diet with no indication of ill health. It is not unlikely that food ingestion at this level may occur frequently in populations of stunted fish. Inter- and Intraspecific Competition In considering competition among the fishes in an aquatic habitat, one usually considers competition between the several species first, but intra- specific competition— competition between individuals belonging to a single species— may be more continuous or severe than that between species. In describing the fish inhabiting Lake Gogebic, Eschmeyer -^ stated that walleye and the northern pike dominated the game fish population. Other species of game and pan fish were present in small numbers: largemouth and smallmouth bass, black crappies, rock bass and seasonally, brook trout. Young of the yellow perch were abundant, but adults were relatively scarce. On the basis of Eschmeyer's study of various phases of the life history of the walleye in this lake, it is probable that intraspecific competition among walleyes was more severe than inter- specific competition between walleyes and other kinds of fish. 124 Reproduction, Competition, and Predation In Onized Lake (Illinois) where heavy fishing controlled the numbers of larger fish, except largemouth bass and bluegills," there appeared to be severe inter- and intraspecific competition among the )'Oung o£ all species present. Once these small fishes reached sizes large enough to interest anglers they were thinned bv fishermen. A suggestion of intraspecific competition was indicated among the blue- gills of Fork Lake ( Illinois ) ^^ where older bluegills were eating a higher percentage of plants than were the yearlings. Here the older bluegills were believed to be less active than the yearling fish in seeking animal foods, and thev were apparenth' using plant material as a substitute. REPRODUCTION, COMPETITION, AND PREDATION In the preceding paragraphs I have attempted to illustrate several aspects of the life cycles of fishes— reproduction, predation and com- petition—which, when integrated with one another and with other forces, constitute the dynamics of any fish population in any body of water. These forces may result in cycles of abundance of certain fishes, or tliey may assure that one species eventually becomes dominant and stays so, until some unusual or catastrophic event occurs. Thompson ' '^ reported a population study of the fish of Lake Senachwine (Illinois) where very abundant year classes of black crappies not only controlled the survival of their own voung for the next four spawning seasons, but they also controlled the survi\al of voung of most other species of fish in this lake. During the fourth year of their dominance, the natural death rate of this year class of crappies was high. When tlie next spawning period arrived, the 5-year-old crappies were no longer numerous enough to dominate the fish population, but there were enough of them to produce a new dominant year class of crappies. Starrett and McNeil,*^^ while studying the fish population of Chautauqua Lake (Illinois), which in some ways is similar to that of Lake Senachwine, found that the relative abundance of several species of fishes fluctuated over periods of several years, but that no one )'ear class of any species dominated the fish population as did the black crappie broods in Lake Senachwine. In Chautauqua Lake, the 1948 year class of white crappies was much larger than any other year class of that species produced in any year from 1949 to the present (1961), but large vear classes of other species were produced in some \'ears. Only occasionally are predatory fishes confronted b\ a shortage of prey fishes; when this does occur, it is often the result of pressure from a dominant year class of the predatory species. Such a situation occurred with largemouth bass in Ridge Lake in 1941 and 1912.^^ The 1951 angler's catch of walleyes in Clear Lake (Iowa) was unusuallv high, apparently Balance 125 because of a failure in the perch crop in 1949 and 1950 which in turn was believed to have been related to a shortage of aquatic vegetation during those years.-^ In this same lake a very large yield of northern pike occurred in 1954, as a result of the transference in 1953 of some 17,000 young pike 10 to 16 inches long into Clear Lake from Ventura Marsh lying adjacent to the lake. This amounted to 5 pike per acre, or by the spring of 1954, to 7 pounds of pike per acre— a rather abrupt increase of at least 10 per cent in the predator population of the lake. Tlie pike caught were thin and later in the summer some were found dead along the shore.-^ Still another type of interplay of reproduction, competition, and preda- tion results in a progressive increase in one or two species of fishes until they become so numerous as to exceed the normal food resources for these species in their habitat. These abundant species spill over their habitat niches into those of other less aggressive species and crowd them suf- ficiently for food and space to prevent the survival of adequate young to maintain a level of population of the less aggressive fishes. It follows that the latter species eventually may be represented by a few old fish, and they may disappear entirely. This type of population change is non- reversible and is characteristic of fish populations subjected to limited or ineffectual predation. Only catastrophic changes in the habitat will modify the overpopulated and stunted condition of the dominant fishes. No known instances of over-use of habitat resources, followed by population col- lapses, such as are cited by Errington -^ for overpopulations of deer, muskrats, and some other mammals, have been reported for fishes, al- though diseases or parasites sometimes wipe out or severely reduce over- populations of fishes that are characteristic of hatchery ponds before fish distribution is begun. BALANCE Balance is a term used by some biologists to describe natural fluctu- ations of animal populations around a constant numerical level. Other biologists have expressed the opinion that the term is inappropriate ^^ because balance refers to a state of equipoise and is synonymous with equilibrium. Nicholson ^^ believes that "balance refers to the state of a system capable of effective compensatory reaction to the disturbing forces which operate upon it, such reaction maintaining the system in being." Others, for example Swingle,"^"^ ''^ use the term balance to define fish populations that yield satisfactory crops of harvestable fish in relation to the basic fertilities of the bodies of water containing these fish. According to Swingle, fish in a balanced population ( 1 ) must reproduce periodically, (2) must produce a sustained yield (presumably by angling), and (3) 126 Reproduction, Competition, and Predation must contain a combination of species including at least one piscivorous species. Unbalanced populations are those unable to produce succeeding crops of harvestable fish. This concept of balance ''^ is somewhat different from that of biologists who have applied this term previously. It visualizes a simple predator- prey relationship between carnivorous fishes (piscivorous) and omnivo- rous ones (prey species) in which the prey species make the maximum use of the food resources to produce adults of harvestable sizes and small fish to serve as food for the carnivorous fishes. The carnivorous fishes produce young to maintain stocks of adults for fishing and control the potential overproduction of stocks of both omnivorous and piscivorous fishes. In practice, this relationship of predator fish to prey fish may maintain itself for a number of years, but eventually it will change to become overbalanced, usually in favor of the prey fish, and human inter- vention will be required to restore the original relationship. This is not the "balance" of Nicholson because this system in itself is not capable of compensating for changes that may take place through natural variation of reproductive and survival rates, unless one is willing to include the management activities of man as part of the system. The sustained yield requirement of "balance" should be based on fish of sizes large enough to interest anglers. The smaller the minimum useful size set by biologists the larger will be the number of ponds that are acceptable ( "in balance" ) . Harvestable-sized fish according to Swingle "^^ are given by weights in the following table. The approximate lengths of these fish have been interpolated from these weights. Minimum Estimated weight, length, pounds inches Bluegills and other sunfish 0.10 5.0 Crappies 0.26 7.5 Largemouth bass 0.40 9.5 Bullheads 0.30 8.0 Gizzard shad 0.50 11.0 Channel cats 0.50 12.0 Gar 1.00 16.0 BufFalo 1.00 12.0 Carp 1.00 14.0 Youthful fishermen are likely to accept fish of any size; adult experi- enced fishermen are more conservative, possibly because they have to process the fish and perhaps have to eat them once they are cooked. Although one may eat smelt of relatively small sizes, because their bones are fine and become soft with cooking, the same cannot be said for small crappies, bluegills, and other sunfish. Bluegills or sunfish of 0.10 pound Balance 127 and crappies of 0.26 pound are listed as being liarvestable. Converted to inches, these weights would represent lengths of 5.0 inches and 7.5 inches, respectively, for bluegills and crappies; for most parts of this country these minimum lengths for harvestable fish should be increased to at least 6.0 inches (0.18 to 0.20 pound) for bluegills and other sunfish and 8.0 inches for crappies (0.30 to 0.35 pound). Neither gizzard shad nor gars are usually considered harvestable in a practical sense, and buffalo cannot be harvested by hook-and-line except by accidental snagging. Disagreement on minimum harvestable (useful) sizes for bluegills and other sunfish by only one inch ( 5 inches to 6 inches ) might make a great deal of difference in designating a population of fishes as desirable or undesirable (e.g., balanced or unbalanced). The use of the term "balance" in referring to fish populations that produce satisfactory yields is untenable because: (1) Balance has already been defined in biological terminology, so that the term should not be applied with specific reference to pond fish populations. (2) The simple predator-prey relationship which is the basis for "balance" in fish populations is an oversimplification of what actually is taking place.^^^ ^^ Fishery biologists should be no more willing to accept such a relationship tlian are game biologists to accept a fox-rabbit "balance" or a prairie dog-coyote ratio. (3) Selected species, numbers, and sizes of fishes released in an arti- ficial lake habitat represent the ultimate in artificial ecosystems and can hardly be expected to show any great stability or "effective compensatory reaction to the disturbing forces which operate upon it."^^ Therefore, "balance" is quite without meaning when applied to such a population. LITERATURE 1. Allen, K. R., Am. Fish. Soc. Trans., 71, 275-283 (1942). 2. Ball, R. C, Jour. Wildl. Mgt., 16(3), 266-269 (1952). 3. Ball, R. C, and Ford, J. R., Mich. St. Coll. Ag. Exp. Sta. Bull, 35(3), 384-391 (1953). 4. Ball, R. C, and Tait, H. D., Mich. St. Coll. Ag. Exp. Sta. Tech. Bull, 231, 1-25 (1952). 5. Ball, R. C, and Tanner, H. A., Mich. St. Coll Ag. Exp. Sta. Tech. Bull, 223, 1-32 (1951). 6. Barney, R. L., and Canfield, H. L., Fins, Feathers and Fur, 30, 3-7 (1922) . 7. Bennett, G. W., ///. Nat. Hist. Surv. Bull, 23(3), 373-406 (1945). 8. Bennett, G. W., Ill Nat. Hist. Surv. Bull, 24(3), 377-412 (1948). 9. Bennett, G. W., Am. Fish. Soc. Trans., 80, 231-239 (1951). 10. Bennett, G. W., Jour. Wildl Mgt., 16(3), 249-253 (1952). 11. Bennett, G. W., Ill Nat. Hist. Surv. Bull, 26(2), 217-276 (1954). 128 Reproduction, Competition, and Predation 12. Bennett, G. W., and Childers, W. F., Jour. Wildl Mat., 21(4), 414-424 (1957). 13. Bennett, G. W., Thompson, D. H., and Parr, S. A., Ill Nat. Hist. Surv. Biol. Notes, 14, 1-24 (1940). 14. Breder, C. M., Jr., Zoologica, 21(1), 1-48 (1936). 15. Brown, W. H., Am. Fish. Soc. Trans., 80, 210-217 (1951). 16. Brown, W. H., Prog. Fish-Cult., 14(2), 79-80 (1952). 17. Brown, C. J. D., and Thoreson, N. A., Jour. Wildl. Mgt., 16(3), 275-278 (1952). 18. Carbine, W. F., N. A. Wildl. Conf. Trans., 4, 275-287 (1939). 19. Carbine, W. F., Am. Fish. Soc. Trans., 71, 149-164 (1942). 20. Carlander, K. D., Am. Fish. Soc. Trans., 87, 34-38 (1958). 21. Carlander, K. D., Whitney, R. R., Speaker, E. B., and Madden, K., Am. Fish. Soc. Trans., 89(3), 249-254 (1960). 22. Clark, M., Jour. Wildl. Mgt., 16(3), 262-266 (1952). 23. Cooper, G. P., N. A. Wildl. Conf. Trans., 13, 188-193 (1948). 24. Childers, W. F., and Bennett, G. W., ///. Nat. Hist. Surv. Biol. Notes, 46, 1-15 (1961). 25. Durham, L., "A Study of the Largemouth Bass-Bluegill Population of Martin's Pond, McLean Co., Illinois," Master's Thesis, unpub., Univ. of 111. Library, 1-47, 1949. 26. Dyche, L. L., Kan. Dept. Fish and Game Bull, 1, 1-208 (1914). 27. Errington, P. L., Science, 124(3216), 304-307 (1956). 28. Eschmeyer, R. W., Jour. Tenn. Acad. ScL, 19(1), 31-41 (1944). 29. Eschmeyer, P., Mich. Dept. Cons. Inst, for Fish. Res. Bull, 3, 1-99 (1950) . 30. Hall, J. F., Proc. 12th Ann. Conf. Southeast Assoc. Game & Fish Comm., 116,91-116 (1958). 31. Hansen, D. F., Ill Nat. Hist. Surv. Bull, 25(4), 211-265 (1951). 32. Hansen, D. F., Bennett, G. W., Webb, R. J., and Lewis, J. M., ///. Nat. Hist. Surv. Bull, 27(5), 345-390 (1960). 33. Harrison, A. C, Inland Fish. Rept., Union of S. Africa, 1-19 (1940). 34. Heard, W. R., and Curd, M. R., Proc. Okla. Acad. ScL, 39, 197-200 (1959). 35. Hess, A. D., and Swartz, A., N. A. Wildl Conf. Trans., 5, 162-164 (1940). 36. Hubbs, C. L., Systematic Zool, 4, 1-20 (1955). 37. Hubbs, C. L., and Hubbs, L. C, Pap. Mich. Acad. Sci., Arts b- Letts., 13, 291-301 (1933). 38. James, M. F., Jour. Morph., 71(1), 63-92 (1946). 39. James, M. C, Meehean, O. L., and Douglas, E. J., U.S.F.W.S. Cons. Bull, 35, 1-22 (1944). 40. Jenkins, R. M., Proc. Okla. Acad. Sci., 36, 70-76 (1955). 41. Jenkins, R. M., Proc. Okla. Acad. Sci., 38, 157-172 (1958). 42. Jenkins, R. M., "Reservoir Management— Progress and Challenge," Sport Fish. Inst., Washington, D.C., 1-22, 1961. 43. Johannes, R. E., and Larkin, P. A., Jour. F. R. Bd., Canada, 18, 203-220 (1961). 44. Johnson, L. D., Wise. Cons. Dept. Tech. Bull, 17, 1-54 (1958). 45. Johnson, R. E., Conv. Int. Assoc, of Game, Fish ir Cons. Comms. Proceed., 38,35-42 (1949). 46. Kawamoto, N. Y., Prog. Fish-Cult., 23(2), 70-75 (1961). 47. Krumholz, L. A., N. A. Wildl Conf. Trans., 15, 251-270 (1950a). Literature 129 48. Krumholz, L. A., Am. Fish. Soc. Trons., 79, 112-123 (1950b). 49. Krumholz, L. A., Jour. WikU. Mot., 16(3), 254-257 (1952). 50. Kutkuhn, J. H., Pwc. Iowa Acad. ScL, 65, 571-579 (1958). 51. Lagler, K. F., and DeRoth, G. C, Pap. Mich. Acad. Sci., Arts ir Letts., 38, 241-250 (1953). 52. Larimore, R. W., 111. Nat. Hist. Surv. Bull., 27(1), 1-83 (1957). 53. Leonard, J. W., Am. Fish. Soc. Trans., 71, 219-227 (1942). 54. Luce, W. M., ///. Acad. Sci. Trans., 30(2), 309-310 (1937). 55. Marzolf, R. C, Jour. Wildl. Mgt., 21(1), 22-28 (1957). 56. McCaig, R. S., Mullan, J. W., and Dodge, C. O., Frog. Fish-Cult, 22(1), 15-23 (1960). 57. McCarraher, D. B., Prog. Fish-Cult., 21(4), 188-189 (1959). 58. Mraz, D., and Cooper, E. L., Jour. Wildl. Mgt., 21(1), 66-69 (1957). 59. Nicholson, A. J., Australian Jour, of Zool, 2(1), 9-65 (1954). 60. Oehmcke, A. A., Johnson, L., Klingbiel, J., and Wistrom, C, Wis. Cons. Dept. Pub., 225, 1-12 (1958). 61. Petriarche, M. H., and Ball, R. C, Mich. St. Coll. Ag. Exp. Sta. Tech. Btdl, 207, 1-35 (1949). 62. Rawson, D. S., and Ruttan, R. A., Jour. Wildl. Mgt., 16(3), 283-288 (1952). 63. Ricker, W. E., Am. Fish. Soc. Trans., 75, 84-96 (1948). 64. Rose, S. M., Science, 129, 1026 (1959). 65. Saila, S. B., Jour. Wildl. Mgt., 16(3), 279-282 (1952). 66. Smith, W. A., Kirkwood, J. B., and Hall, J. F., Ky. Dept. Fish ^ Wildl. Res. Fish. Bull, 16, 1-42 (1955). 67. Smith, E. V., and Swingle, H. S., Am. Fish. Soc. Trans., 72, 63-67 (1943). 68. Solomon, M. E., Jour. Anim. Ecol, 18, 1-35 (1949). 69. Starrett, W. C, and McNeil, P. L., Jr., III. Nat. Hist. Surv. Biol. Notes, 30, 1-31 (1952). 70. Stroud, R. H., Prog. Fish-Cult., 17(2), 51-62 (1955). 71. Surber, E. W., Am. Fish. Soc. Trans., 77, 141-151 (1949). 72. Swingle, H. S., Ala. Exp. Sta. Bull, 254, 1-23 (1942). 73. Swingle, H. S., N. A. Wildl Conf. Trans., 10, 299-308 (1945). 74. Swingle, H. S., Ala. Agric. Exp. Sta. Bull, 274, 1-77 (1950). 75. Swingle, H. S., Eighth Pac. Sci. Cong. Proceed., IIIA, 865-871 (1956). 76. Swingle, H. S., Prather, E. E., and Lawrence, J. M., Ala. Poly. Tech. Inst. Ag. Exp. Sta., 113, 1-15 (1953). 77. Swingle, H. S., and Smith, E. V., Ala. Poly. Inst. Ag. Exp. Sta. Bull, 254, 1-30 (1947). 78. Thompson, D. H., Ill Nat. Hist. Surv. Bull, 20(5), 492-494 (1935). 79. Thompson, D. H., "A Symposium on Hydrobiology," pp. 206-217, Univ. of Wis. Press, Madison, Wis., 1941. 80. Vincent, R. E., Am. Fish. Soc. Trans., 89, 35-52 (1960). 81. Viosca, P., Jr., Am. Fish. Soc. Trans., 73, 274-283 (1945). 82. Yashouv, A., Bamidgeh, 10(4), 90-95 (1958). 6 Theories and Techniques of Management objectives of management are to produce and maintain a fish popu- lation that will supply a satisfactory sustained return to those with the authority to take an annual crop. Few populations are handled with suf- ficient intensity to keep them producing at peak level, although many provide a fairly adequate sustained yield. Probably, private and public demand for angling would be satisfied if all available waters offered a moderate sustained yield. However, in many regions unproductive ponds and lakes (those that supply little or no fish) predominate. This is par- ticularly true of small artificial lakes and reservoirs located near centers of population. Most unproductive lakes or reservoirs contain "problem" fish popula- tions. Obviously, management effort should first be directed to restoring reasonable production to these bodies of water; the application of in- tensive fish management can come later. There are two methods of handling a "problem" population in a pond or lake. One is to eliminate the population entirely and start anew with fish from an outside source; the other is to change the problem population, either by direct action upon it or through indirect action, brought about by modifying the fishes' environment. Both of these approaches are in common use. However, before deciding on a management procedure, a rather careful diagnosis, requiring one or several methods of sampling the population, must be made. Following is a dicussion of the uses of fish samples and some common methods of taking them. FISH SAMPLING The fisherman or fisheries manager can rarely see beneath the water sufficiently to identify and count the fishes in a lake or pond. Conse- 130 Fish Sampling 131 qiiently, to determine the numbers and species present, he must resort to Hve-trapping techniques. It is seldom necessary to kill these specimens, whether or not their number is large enough to have any significance in relation to the remaining population. Reasons for Sampling Fish Populations There are several justifiable reasons for sampling fish populations. An adequate sample allows an appraisal of the components of a population, and exposes those segments of it, having sizes and numbers satisfactory for angling. As described in another chapter, the main causes of poor fishing are (1) overpopulation and stunting of desirable species, and (2) an overabundance of undesirable species with a concurrent shortage of acceptable ones. Hence, once the causes of poor fishing have been ex- posed, it is possible to plan a method of improving the population. For example, an excessive number of stunted crappies can be thinned out by partial poisoning; however, if there are overabundant and stunted bull- heads as well, complete elimination and restocking may be necessary. Obviously, a sampling method that will expose only the crappies does not provide a satisfactory diagnosis. It is frequently necessary to demonstrate to fishermen and owners that lakes are not "fished out." This is commonly called for in waters close to urban centers where species exposed to heavy fishing pressures may know a frog from a frog "popper" and a worm on a hook from a free one. These "fished-out" lakes are often filled with "wise" fish and fishermen will keep trying to catch them (at the same time receiving health and aesthetic benefits) if the fishery biologist can demonstrate that desirable species are present. Regular annual sampling should be done on important impoundments not only to record changes in the relative abundance of species, but also in their length-frequency distribution and their condition from year to year. Fish taken with various sampling techniques should be measured and weighed individually, and scale samples obtained where there is an indication of stunting or of exceptionally rapid growth. These data allow an annual appraisal of the status of all important species. When this information is integrated for several successive years, it shows unmis- takable trends that may call for certain management practices. Table 6.1 shows a hypothetical length-frequency distribution for blue- gills in an imaginary lake. In 1955, there were adequate numbers of large bluegills belonging to the 1953 year class ( determined from scales ) . These fish showed an average Index of Condition (C) of 8.0 or higher which demonstrated that the fish were relatively fat. In collections of 1955 and 1956 no excessively large year class more recent than 1954 was present although this year class was fairly well represented. However, the collec- 132 Theories and Techniques of Management tions of 1957 showed very large numbers of 1956 year-class fish that, after two growing seasons, were only about 3.0 inches in length. This would indicate a dangerous situation that might mark the beginning of overpopulation and stunting of bluegills. Further evidence of popula- tion pressure in 1957 was found among the 1954 and 1953 year-class blue- gills which had Indexes of Condition of 7.0 and 7.4 respectively, indicating that the larger fish were thinner in 1957 than in previous years. Table 6.1 Length-frequency distribution of bluegills IN clear lake from SEPTEMBER COLLECTIONS TAKEN WITH WIRE TRAP NETS. Total length in inches 1955 1956 1957 1958 3.0 13 4 256 18 3.5 27 3 140^ \ 36 4.0 33^^ \^63 27 216 1956 year class 4.5 12 72 47 84 5.0 15 18^ -^45 62 5.5 62 17 52-^ 14 6.0 128^ 26 11 "^ 30 1954 year class 6.5 47 "~"102_^ 10 3 7.0 12 13 ^^47^^ 4 7.5 1 2 9 ^^~^10 1953 year class 8.0 5 2 1 1 8.5 3 1 1 9.0 1 1 By September of 1958, the 1956 year class was severely stunted. After three growing seasons ( summers ) they averaged only 4.0 inches in length and were thin with abnormally large eyes (an indication of stunting). Bluegills of 6 inches or larger were still fairly common, but were very thin with Indexes of Condition ranging from 6.5 to 6.9. This bluegill population needed to be drastically reduced, particularly those fish be- longing to the 1956 year class. Furthermore, since the 1957 sampling dis- closed an abnormally large 1956 year class, measures should have been taken then to reduce its size. Sampling Methods Many types of gear have been used for sampling populations. Most of these are selective for one or more kinds of fish, and may give a faulty impression of the relative abundance of species— both those too easily caught and those not taken in proportion to their numbers. Table 6.2 gives a rough appraisal of the efficiency of several kinds of sampling methods in relation to a number of kinds of warm-water fishes. Fish Sampling 133 9G These represent the combined experiences of Starrett and Barnickol and the members of the Ilhnois Natural History Survey staff who have fished with these methods for many years. Gill Nets. These nets are made with hnen or nylon thread, fine enough so that fish, not seeing them will become gillcd or entangled (Figure 6.1). Gill nets are tied to give bar measurements (one side of a square mesh) ranging from 1 to 4 inches; sometimes special sampling nets are made by splicing 50-foot sections of increasing mesh sizes from 1 inch up to 3 or 4 inches. Gill nets can be set at various depths from surface to bottom. They are selective for pelagic fish such as herring and trout and are seldom used in shallow lakes. Table 6.2 An appraisal of the efficiency of several fish sampling methods commonly used in artificial lakes, as related to common warm-water fishes (in part from starrett and barnickol ^^) . Method of Sampling Trammel Wing nets Spot Boat Kind of fish nets Trap nets Seines poisoning shocking Angling Largemoiith bass poor poor fair good fair good Smallmoiith bass poor fair fair good fair good Simfish 1 good good good good good good Grapples good excellent good good fair poor Carp good good good good fair poor Gizzard shad fair good good good good — Gar fish good fair good good poor poor Bullheads fair good poor fair poor good Channel catfish poor good good good poor poor ^ Bluegills, green sunfish, red-ear sunfish, etc. Trammel Nets. A light gill net of small mesh is hung with plenty of extra webbing between two walls of netting consisting of very large mesh of heavy twine. A fish hitting the light net carries a pocket of this net through an opening in the larger net and so becomes trapped. Trammel nets are supplied with floats and weights; they are set across and/or floated in a current (in a river) or set around a school of fish (in a lake). These nets are selective for fish that can be frightened into hitting the net. They are commonly used for commercial fish— carp, buffalo, and catfish. Seines. These are pieces of webbing of various mesh sizes and lengths held upright in the water by floats and weights and pulled through the water to encircle fish. They are somewhat less selective than most odier types of gear. Seines can be used only where the water depth is less than the depth of the seine and where the bottom is clear of snags. When con- fined within a small area, certain fishes such as largemouth bass will jump 134 Theories and Techniques of Management over the top of a seine unless the cork hne is held up above the surface of the water.^^2 Other kinds of fishes attempt to go under the lead line at the bottom so that if the seine becomes snagged or rolls up, they may escape from it. Large seine hauls repeated annually on a specific lake may be used for predicting standing crops of fish in pounds per acre; isolated seine hauls are of little value in this respect."^^ Figure 6.1. TVA biologists use gill nets for sampling fish in large deep reservoirs. Hoop Nets, Wing Nets, and Trap Nets. A hoop net consists of a cylinder of webbing supported by hoops, open at one end and closed at the other. Inside are two funnels, one just inside the open end of the cylinder and the other midway between the open and closed ends. Hoop nets are set in rivers with the tail upstream and the open end downstream. The cur- rent keeps the hoops separated and the net stretched. Fish move into the net easily through the funnel openings, but have some difficulty in finding their way out again.^^ Usually in swimming around inside of the net after passing through the first funnel, some wander through the second funnel and are then inside the closed end of the cylinder called the pot. Fish are removed from the pot by releasing a drawstring after the net or pot has been lifted into a boat. Wing nets are modified hoop nets with short wings of webbing attached to the hoop at the open end of the net. They are used in quiet water where the wings guide fish into the net opening, and are held in a stretched Fish Sampling 135 position by stakes or weights."*^ Wing nets are sometimes fished with a long lead net set upright between the wings at the net opening. This lead net acts as a "drift" fence and fish following it soon find themselves in the wing net. Hansen ^^ found considerable variation in the catch of wing nets at various times of the year and under varying physical and chemical con- ditions associated with water— temperature, turbidity, dissolved oxygen, and carbon dioxide. Trap nets are usually modified wing nets with wings arranged in loops to direct fish toward the opening of the net no matter which way they attempt to go, and with a lead net attached inside a forebay so that fish following the lead net to its proximal end are already inside of the front of the trap. Mesh covering these nets is composed of nylon or cotton webbing squares varying in size from M inch to 4 inches. Hoop diameters range from 2/2 to 6 feet. Trap nets of larger sizes are not used for sampling. "Hoop nets" made of hardware cloth are more useful for fishing in ponds and small lakes than are hoop nets or wing nets made of string because the wire nets are not subject to muskrat damage. The nets are constructed of /2-inch hardware cloth and consist of a cylinder of wire, 2 feet in diameter and 4 or 5 feet long, with a single funnel leading into the open end and wire mesh across the closed end. Fish are removed through a small door covering an opening on one side, or through the open end of the cylinder after the funnel has been lifted out. Thompkins and Bridges ^^^ found that low doses of copper sulfate (0.15 ppm) in soft water irritated the fish and caused them to move about, thereby increasing the catch of wing nets set in the area of treatment. Some fish may be attracted into a net by bait ^' -^ or by the darkness of the water inside of it. However, other fish, that avoid nets of small mesh, will enter those of larger mesh because their interiors are scarcely darker than the surrounding water. Certain kinds of fish such as largemouth bass will seldom enter hoop nets, wing nets, or trap nets in clear water, whether the mesh size is large or small.^^ Because these nets are attractive to certain kinds of fish and are avoided by others, the nets are extremely selective and samples of fish taken by them will not be representative of the fish population from which they were taken. Minnow Seine Sampling. Minnow seines are often used to catch the young of various kinds of fish in order to gather information on the number of species of fish present in a body of water and to determine spawning success (relative abundance of young) of the several kinds of fish present. Usually when a dominant year class of one species of fish has been spawned, it will show up almost at once in early summer minnow seine hauls. 136 Theories and Techniques of Management Anyone who has made an intensive study of a fish population inhabiting a pond or lake will discover after a few years of sampling of young that the total numbers and relative abundance of these young may vary greatly from year to year. Also, the young of a given species may appear to be very abundant in the early summer when the young are small, but later, if the rate of survival was low, they may have become relatively scarce. Minnow-Seine Method of Pond Analysis. According to the minnow- seine method of testing ponds containing largemouth bass and blue- gills,^^- the condition of the fish population ("balance") may be judged on the basis of the success of reproduction of bass and bluegills for the current year and the past survival of bluegill spawn beyond the first year (3-, 4-, and 5-inch length groups of bluegills). The method is based on the hypothesis that with an overabundant stunted population of blue- gills, the bass (and sometimes bluegills, too) will be unable to produce enough to assure their appearance among fish caught in a reasonable number of minnow-seine hauls. On the other hand, with an overabundant stunted population of bass, there will be no intermediate-sized bluegills, and a scarcity or absence of small bluegills and perhaps small bass. These assumptions are valid if interference in spawning has not come from water too cold, turbid, or saline, with a pH too high or low, and if there is no great rise or drawdown of water levels at the wrong time.^^^ In 1950, an airing of conflicting ideas on minnow-seine sampling oc- curred when Dr. Gustav A. Swanson, editor of the Journal of Wildlife Management, published some pro and con opinions of it.^ Long-term intensive studies of populations, in which minnow-seine collections were interpreted by the minnow-seine hypothesis, often failed to accurately define the type of population present. If these studies could not demon- state the consistent validity of the method, one may doubt the value of less intensive investigations, regardless of the number of ponds sampled and catches subjected to the test formula. As stated in 1950,^ the author has found no published information ( an adequate series of experiments in which minnow-seine analyses were followed by draining or poisoning censuses of the adult fish populations ) to prove the value of the minnow- seine method. Tests of the method in lowa,-^ through use of a larger seine and age analyses of fish, demonstrated errors in interpretation of results from minnow-seine collections. However, shoreline seining with a fine-mesh seine to catch the smaller fish in a body of water can furnish a great deal of information about a fish population. Some acceptable values are as follows: (1) In previously unsampled waters it will give a partial, and in some in- stances, a complete list of species inhabiting these waters. (2) The collection of the young of bass, walleyes, northern pike, or other game fish not only indicates their presence in the water but also their Fish Sampling 137 ability to reproduce under current conditions. However, their absence from waters containing adults does not necessarily mean that these poj^ulations cannot reproduce and are "on their way out." (3) The production of dominant year classes of fish may be recorded first through minnow seining. (4) Some indication of overpopulation and stunting may be gained from minnow seining, although a larger seine is much more useful for this. Spot Poisoning. One of the more satisfactory methods of obtaining an unbiased sample of the fish population of a large lake is to poison a bay or lagoon connected with the larger body of water by a narrow channel. If the bay is not too small or too shallow, it will probably contain a population fairly comparable (in composition) to that of the larger lake,^^ particularly if the fish are warm-water species and the bay is treated with the chemical during the night when they are moving about in the shallower parts of the lake. A seine or block-off net ^' deep enough to reach from the top to the bottom of the bay is set across the channel connecting the bay with the main lake. Fish disturbed by the chemical treatment or frightened by the noises of boats are prevented from escaping by this seine. Following its placement, a canvas strip approximately its same depth can be staked across the channel adjacent to the seine. This canvas strip prevents the circulation of rotenone-treated water from the bay into the main part of the lake. The bay is then treated with derris or cube powder, 5 per cent rotenone, or emulsifiable rotenone with a dosage of sufficient strength to kill all of the fish trapped in the bay. These poisoned fish are picked up, counted, measured, and weighed, as in a regular census. Following the rotenone treatment the seine and canvas strip are left in place until the rotenone has disappeared from the water, so that no fish are killed outside of the bay. Spot poisoning may be done in open water by treating the circumfer- ence of an arbitrary one-acre circle and then applying the rotenone inward throughout its area. However, work in open water is not as satisfactory as in an isolated bay because in the former instance the treated water may move downwind out of the original circle, causing fish affected by the rotenone to disperse beyond the original area of treatment. In any case, it is well to pick a time when wind velocity is at a minimum to prevent, as much as possible, the mixing of treated and untreated water. As the behavior of fishes is influenced by seasons, several spot treat- ments of the same bay, made several weeks or months apart, will give a better population sample than a single spot poisoning. Boat Shocking. In 1949, an AC row boat shocker was developed for the purpose of sampling fish in lakes. ^^ This apparatus is useful for obtain- ing quickly a fish sample from lakes and ponds that are sufficiently 138 Theories and Techniques of Management shallow and have a water hardness of 25 or more parts per million (Figure 6.2). If the hardness is less than 25 ppm, the effective field of the electrodes is too small for the shocker to function efficiently. A boat shocker is often much more effective in taking fish at night, when fish are in the shallows, than in the daytime. A lighting system presents no problem because lights may be powered from the generator.'^^ Figure 6.2. An electric fish shocker mounted on an aluminum work boat. Biologist standing in front of boat controls a 220-volt generator, keeps elec- trodes in position, and picks up stunned fish. Biologist in rear runs outboard motor and cares for stunned fish which are placed in the tank amidship. The boat shocker is selective in that it may stun fish attempting to hide in vegetation or on the shallow bottom; whereas fish swimming ahead of the advancing edge of the electrical field may escape unless they are cornered at the end of a bay or channel and forced to swim through the field. In general, bass tend to swim ahead of the shocker boat while odier smaller centrarchids often try to hide in vegetation. Catfish and bullheads are seldom taken with an alternating current shocker because they are stunned on the lake or pond bottom where they are difficult to see and Fish Sampling 139 pick up. Certain kinds of fishes attempt to escape the shocker by diving into brush and into pockets at the base of rocks, stumps, and logs lying in the water, making these ideal collecting locations. Most fishes revive within a period of 30 seconds to 2 minutes. Occasionally a fish is killed by direct contact with an electrode. The shocker is not only used for sampling but also for collecting fish with "full" stomachs (food-habit studies) and for taking live specimens for stocking other waters. For some reason, many la)Tnen have an idea that the electric shocker can be used to clear lakes of undesirable fishes. When they discover that the fish stunned by the shocker represent only a sample, they are often disappointed. Both direct and alternating current are used on boat shockers. For collecting most kinds of fishes in shallow ponds, alternating current ap- pears to be more eflFective. However, some biologists prefer a pulsating direct current to give a combination of electrotaxis and forced swimming.-^ Tests made in a webbing enclosure in a shallow lake (Minnesota) in- dicated that about 240 interruptions per minute was most effective for catching fish.^^ Angling. Fishing with certain kinds of gear (fly rod, spinning rod, etc.) and certain types of artificial or natural baits may be highly selective for certain kinds and sizes of fish. For this reason, angling is sometimes very important as a method of sampling. Largemouth bass are usually taken more readily on hook-and-line than by any known type of net or trap. Several years ago I attempted to catch largemouth bass in a lake, at a time when it contained almost no fish other than bass of about 7.5 inches total length, by using 1-inch mesh wing nets with 60-foot lead nets. Six nets were set and raised daily on six consecutive days. The catch of all nets for the six-day period (36 net-days) was 6 of these small bass; on the last day that the nets were set I caught 47 bass on fly rod "poppers" in less than three hours. The ability to avoid nets and seines is shared also by smallmouth bass, although they are somewhat more vulnerable than are largemouths. For sampling smallmouths, a fly rod and artificial "popper" may serve effi- ciently. For example, biologists captured 192 smallmouth bass (6 to 11 inches ) in 22 hours of fishing at the rate of 8.7 per hour.^- The fish were used to restock a renovated lake. They probably could not have been taken from the source lake ( a deep quarry lake ) at this rate by any other method. Hook-and-line fishing may be useful for sampling specific fishes such as male bluegills guarding nests, or for taking fishes that inhabit a certain weed bed or lie beneath a log. Many kinds of fishes become trap-wise as well as hook-wise, so that most types of fishing gear become less efficient with intensive use. 140 Theories and Techniques of Management MANAGEMENT TECHNIQUES Once sufficient sampling of a fish population has indicated that man- agement is necessary, one should investigate the known techniques and decide which are applicable. Often several methods seem justifiable and one or more must be selected on the basis of expediency. Complete Fish Population Removal Complete removal of a population is usually desirable when a lake or pond becomes contaminated with species of no value for angling or fish production. Such fishes as buffalo, suckers (of several kinds), gizzard shad, and sometimes stunted black bullheads may have limited sport fishing value. These species often crowd out more desirable game and pan fishes. Even if these undesirable fishes are present in small numbers, they are always a potential danger to the production of a high sustained yield of more desirable species because of their capacity for producing tremendous numbers of young at a single spawning and their ability to modify their environment ( by stirring silt ) in their search for food. These fishes, and some others unlisted, are completely under control only when they are absent.^- Population Removal by Draining. All artificial ponds and lakes should be built with drain outlets of sufficient size to allow their basins to be drained within a period of 3 to 10 days. If a lake with a drain becomes contaminated with undesirable species or must be drained for any other purposes (such as the recovery of stolen goods), a Wolf -type weir ( Figure 6.3 ) can be placed across the outlet, the live fish separated from the water, and the valuable fish saved alive for restocking.^ ^" A Wolf -type weir is more satisfactory than any other type of screen because the water falls through the bottom of the wire-mesh weir instead of flowing through a perpendicular screen. The fish either are left exposed on the wire mesh of the weir bottom or they flop across the bottom screen into a holding box. This is the only type of screen that can handle a large flow of water without frequent shutoffs for cleaning the screen. A Wolf-type weir can be constructed below almost any outlet that will give 6 inches to 2 or more feet of working space below the level of the outflowing water. If it is necessary to catch very small fish or plankton organisms, such a weir may be covered with copper window screening or MS-904 Saran Screen.^^ Usually it is not desirable to use mesh of smaller than one-fourth to three- eighths of an inch. Before draining, it is necessary to make some arrangements, either temporary or semi-permanent, for storing desirable fishes. The surface spillways of some artificial lakes may terminate in stilling basins of suf- Management Techniques 141 ficient size and depth to hold fish. Such an arrangement was used at Ridge Lake where, in cool weather, all of the larger fish from 18 acres of water could be held for several days in a concrete stilling basin 70 feet wide, and 30 feet long, with a maximum depth of 4 feet when the basin was pumped fulL^^ Where no holding basin is available near the outlet, arrangements should be made to hold the fish in portable tanks of metal or canvas or in nearby ponds, a count being kept of the fish moved Figure 6.3. Small Wolf-type weir built across the concrete flume below drain valve for 2.5-acre pond. This weir will handle a comparatively large flow of water and allow capture of the fish alive because water drops through the bottom screen as well as flowing through the sides. to these ponds. Later, when a small amount of water has become im- pounded in the drained lake basin, fish in the tanks may be returned or, those released in the ponds may be recollected by seining. The cool days of early spring and late fall are best for lake draining operations, because fish can be handled at these times with a minimum of loss. Most lakes and ponds will not drain completely, and it is usually neces- sary to treat the water remaining in pockets or channels in the basin with some chemical to kill the small fish that may remain in this water and escape to the lake as the basin refills. For this purpose one can use H.T.H. 142 Theories and Techniques of Management powder ( calcium hypochlorite ) to give several parts per million * of free chlorine, or rotenone ( as 5 per cent cube powder ) or emulsifiable rotenone (2 to 5 per cent) to give 1 ppm or more. It is desirable to use a fish poison that disappears rapidly so that fish can be restocked widiin a few days. After the fish and water have been removed from a lake basin and the water pockets and channels treated, the outlet valve can be closed so that water will collect in the basin. However, the basin may be allowed to dry for several months before reimpoundment is begun, if the fish to be restocked can be held for this length of time. Population Removal by Rotenone Treatment, The use of rotenone con- taining plants as an aid in catching fishes is common to the native in- habitants of many widely separated tropical and subtropical countries. Leonard ''^ and Krumholz ^^ described the catching of fish by natives of Australia, Oceania, and southern Asia by the use of tuba, the local name for a substance (rotenone) originating from plants (Genus Derris) native to those regions. In tropical South America, the same substance, extracted from plants belonging to several genera such as Lonchocarpus and Trephosia, is known as cube, timbo, barbasco, and by other names, de- pending upon the plant source and locality. Both Dr. Leonard and Dr. Krumholz recount descriptions of tuba fishing parties from the writings of early explorers in Sumatra, Sarawak, and Brazil. The insecticidal properties of rotenone are well known and for many years there has been a large importation of rotenone-bearing plants into the United States. Probably Professor Eigenmann was the first to use native fishing methods with rotenone for collecting specimens of fishes in South America. Dr. Carl L. Hubbs used powdered derris root with 5 per cent rotenone content for collecting fish in Guatemala in 1934. Rotenone was first used in fisheries management in the United States in 1934 when Milton B. Trautman at the suggestion of Dr. Hubbs at- tempted to eliminate goldfish from two small ponds on the W. O. Briggs estate near Birmingham, Michigan. The attempt was not entirely success- ful because the dosage was too light. In September, 1934, Michigan fisheries biologists attempted to eliminate a population of stunted yellow perch from a 4.3-acre lake in Otsego County, Michigan.-^ After 1937 the technique of killing fish witli rotenone spread rapidly to other states. In 1938, biologists with the Illinois Natural History Survey censused 6 ponds using rotenone treatment.^ Leonard's laboratory studies of the toxicity of rotenone to fishes in- * Parts per million is promulgated on a weight basis, i.e., one pound of a chemical added to a million pounds of water equals one part per million. This is not too difficult to visualize if one will remember that an acre of fresh water (43,560 square feet), one foot deep, will weigh about 2.7 million pounds. Thus, 2.7 pounds of a chemical applied to one acre of water, one foot deep ( one acre-foot ) , will give a dosage of one part per milHon (ppm). Management Techniques 143 dicated that a concentration of 0.5 ppm of derris powder with 5 per cent rotenone content should be lethal to all kinds of fishes. Also, a 14-degree elevation of water temperature from 60°F to 74°F decreased the reaction time to equilibrium loss and death of the fish by one-half, and rotenone was found to be somewhat more toxic in acid than in alkaline water. Biologists soon discovered that derris and cube mixtures with water were slow in penetrating the deeper waters of thermally stratified lakes. For this reason it was physically possible to kill warm-water fishes such as yellow perch, rock bass, and largemouth bass inhabiting the upper warmer layers of water without killing many brook or rainbow trout,-^- 07, iii jj^_ habiting the colder strata below (Figure 6.4). Hayes and Livingstone^"^ combined the technique with the stocking of brook trout in a Nova Scotia lake and were able to increase the trout yield by 230 per cent. Wide experience with rotenone-bearing compounds including the newer emulsions formulated by several chemical manufacturing companies has shown that it is risky to depend upon a dosage of material containing 5 per cent rotenone of less than 1 ppm to give a complete kill of fish.^^ In Illinois we have used a standard dosage of 3 pounds of 5 per cent rotenone-bearing material or 3 pints of emulsifiable rotenone, 5 per cent, per acre-foot of water, a dosage somewhat larger than 1 ppm. The extra chemical takes care of: (1) inaccuracies of lake volume estimates, (2) fishes showing high resistance to rotenone, ( 3 ) water of high organic con- tent and/or alkalinity, and (4) unevenness of spreading. It is better to use too much rotenone than too little when all of the population must be killed (Figure 6.5) because if a few fish survive, both the cost of the rotenone and the treatment time of the crew have been lost, and the lake must be retreated. In every case special methods of application should be used to carry the rotenone into deeper parts of a lake. One of the simplest is to in- troduce the rotenone mixture or emulsion dirough a weighted hose con- nected to a tank supported in the boat a foot or two above the water level. The movement of the boat ( driven by an outboard motor ) and the action of gravity forces the liquid into the deeper waters. The surface and edges of the pond or lake can be treated by the use of any type of small-power sprayer apparatus, and this same equipment can be used to pump the liquid into deep water. A hand sprayer is sometimes eflFective for covering edges of a small pond; for treating open surface water, the material can be poured over the side of the boat in advance of an out- board motor. Bilge-pump attachments available widi some makes of outboard motors have been used for spreading emulsifiable rotenone. Powdered derris or cube suspended in water might clog a bilge-pump attachment. Care must be exercised to spread the fish-killing material as evenly 144 Theories and Techniques of Management a; Q 53 50 Temperature F Figure 6.4. Selective poisoning of warm-water fish in trout ponds in Mas- sachusetts by the use of rotenone. Tests indicated that the rotenone did not penetrate below the zone of rapid-temperature transition at 30 to 35 feet. Trout that remained at depths below about 35 feet were unaffected, while most of the warm-water fish above 30 feet were killed. [From Thompkins, W. A., and Mullan, J. W., Prog. Fish-Cult., 20 ( 3 ) ( 1958 ) ] Management Techniques 145 as possible througliout the lake from surface to bottom. This can be done by following a grid pattern which divides the lake surface into parallel and crossing lines of treated strips (Figure 6.6). A fish has little chance to escape the treated strips in the grid. Treatments for complete kills of fish should be made during seasons when the surface water of the pond or lake is 70°F or higher, because, as cited above, the efficiency of rotenone is much reduced in cold water. *lte * . I- c3 fl r3 as •-5 )r ^^ oj ^ ^ ^ il- — rt 2 cj t^ Q ^ c/5 s -ti a.-r^ O^ fl " O .»x in CS -M 2 ^ O 3 -• . ^ o C/3 • fl W5 S ^ o (u K tr, o 5 2; - 'S ja *- ^^ O CD > « ^ o o S "> « ^ ^ -2 ^ .52 2 ca ■•* rt Q C ^ O -£ > ^ e« 'S a ^ O r. u o 9 ^ ^ S"ti Oh Jh ^ O •*-• c 57" o "^ 00 o S-l W2 >^ 1^ 0) 4-. « o g 2 o « , Q « o fl, •£ >; . s o s 4-) o ^^^ i ^ C/2 fl ^ X 4-* 3 •"^ Jr? H ^ ^ 3 TO 4- « TO <^ « g O S .^ S .0 13 o ^ c/5 s5 en . *^ TO 3 S !^ y 4> N rt si 55 "^^ /I ^ TO o 3 5 •« fl> 3 bJO TO O O N O in C 5-1 O 3 ^ TO TO " .„ ^ j^ " o flj r« CS >~^ S CO O c/5 TO ■ • i-i 3 '^ 3 r ■: 3 3 CJ P ^ 3 '^ 1- . '^ 0:3 « « o 3 uh . ^ Dh"*^ •-^ V3 tl Sh 4-* 3 ^ O TO CI4 146 Management Techniques 147 seining and boat shocking and reduces die amount (and cost) of rotenone needed to give a complete kill of undesirable fish. It may also pull the water out of cattail marshes and shallow bays overgrown with aquatic vegetation and pond weeds, and thereby eliminate the danger that a few small fish might survive in these areas. Fish are more sensitive to rotenone than are most other aquatic organ- isms except entamostraca.^^' "^^ The length of time that rotenone-treated water will remain toxic to fishes depends largely upon water temperature; at 70° to 80°F waters can be restocked with fish within 4 to 5 days after treatment. Lakes treated during cold weather may remain toxic for much longer periods— as much as 30 days. A color test has been developed for measuring the amount of rotenone in water. "^^ Potassium permanganate ( KMn04 ) or chlorine ( CI2 ) will quickly oxidize rotenone and disappear from treated water.^^' ^^ Prevost, Lanouette, and Grenier ^^ have demonstrated that the toxicity of some preparations decreases after an initial group of animals has been in it for a time. Thus, die disappearance of toxicity in a derris-powder suspension after 48 hours reported by Leonard '^- is not entirely an effect of time but is certainly related to the fact that the first two sets of fishes killed in the preparation had caused the toxicity to drop below the lethal point. This may in part account for the fact that most preparations appear to be less toxic out-of-doors than in laboratory aquaria. Selective Poisoning. Recently a search has begun for selective poisons toxic to certain kinds of fish but not to others. The U. S. Fish and Wildlife Service began this search while looking for a chemical to kill larval sea lampreys, and after testing more than 4000 chemicals, they found an effective lamprey poison, as well as other chemicals that were toxic to certain other species.- As yet, selective poisoning of fishes is still in ex- perimental stages. DDT and Other Insecticides. The release and widespread use of DDT after World War II caused apprehension among conservationists and numerous studies were made of the effects of small and large scale ap- phcations of DDT on fish and wildhfe.-^' ^2, 53. 54, 74, los. 109 ^s with many other chemicals, DDT was more toxic to fishes in laboratory tests than in field tests. On land it was often applied in dosages calculated in pounds or fractions of a pound per acre. These same dosages, based on surface area, were tested on shallow ponds containing miscellaneous fishes. In one series of experiments where DDT in the emulsifier Triton X-100 was fol- lowed by a treatment of rotenone applied at 1 ppm to kill all remaining live fish, a dosage of DDT at the rate of 1 pound per acre was found to kill all fish. A similar application at the rate of one-half pound per acre killed bass, crappies, bluegills, and some carp, but enough carp survived to repopulate the pond had they been allowed to spawn. 148 Theories and Techniques of Management Since the appearance of DDT, many other chlorinated hydrocarbon insecticides have been developed. Some are more toxic to fish than DDT and others are less toxic. The development of organic phosphorus in- secticides soon followed the chlorinated hydrocarbons. Many of the organic phosphorus compounds were more dangerous to handle than the chlorinated hydrocarbons but applied to waters containing fish they were somewhat less toxic. Table 6.3 shows the amounts of 10 chlorinated hydro- Table 6.3 Comparative toxicity of chlorinated hydrocarbon and or- ganic PHOSPHOROUS insecticides TO FATHEAD MINNOWS IN HARD WATER AT 25°C. (from HENDERSON, PICKERING, AND TARZWELL ^8) Chlorinated Hydrocarbon Organic Phosp] lorous 96 hr. TLm 96 hr. TLm Insecticide ppm (mg/1) active Insecticide ppm (mg/1) active Endrin 0.0013 EPN 0.25 Toxaphene 0.0051 Para-oxon 0.25 Dieldrin 0.016 TEPP 1.00 Aldrin 0.028 Parathion 1.60 DDT 0.034 Chlorothion 3.20 Methoxychlor Heptachlor Lindane 0.035 0.056 0.056 Systox Methyl parathion Malathion 4.20 7.50 12.50 Chlordane 0.069 Dipterex 51.00 BHC 2.000 OMPA 135.00 carbon and 10 organic phosphorus insecticides in ppm required to give a 50 per cent mortality (median tolerance limit, TLm) in 96 hours. ^^ Of these chlorinated hydrocarbons, Toxaphene which holds second place to Endrin in its toxicity to fish has been used as a fish poison to clear all fish from lakes. Both Endrin and Thioden have been tested as fish poisons by Canadian biologists.^" Toxaphene. According to information furnished by Mr. Lynn H. Hutchens (U. S. Fish and Wildlife Service) in 1953, Messrs. Jack Hemp- hill and Jack Killian of the Arizona Game and Fish Commission first used toxaphene for killing fish. They used a dust containing 40 per cent toxaphene, and the cost of treating a lake was found to be only 15 per cent of the cost of treatment with derris or cube powder containing 5 per cent rotenone. In this treatment of Becker Lake (Arizona) with toxaphene, several horses were deliberately allowed to drink the water and no losses resulted. No dead deer, raccoons, or other wild animals have been observed around such lakes. There is apparently no danger in human consumption of fish so destroyed.^^ Management Techniques 149 Under alkaline conditions toxaphene is said to break down into hydro- chloric acid and water. Shallow ponds lose their toxicity more rapidly than deep, and the former treated before the fall overturn of one year might be read\' for restocking the next spring or early summer. In contrast, 8 alkaline, relatively deep lakes in British Columbia remained toxic for more than 9 months.-'" Alkalinit), the action of microorganisms, and turbidity ^^ are important in the rate of detoxification of toxaphene. ^^' '^" A variety of dosages of toxaphene have been tested for killing fish, ranging from 0.1 ppm to 5 ppb * (.005 ppm). Usually a dosage of not less than .05 ppm of emulsifiable toxaphene in hard water applied when the temperature is in the 70° to 80°F range is to be recommended. Toxaphene is more toxic to small fish than large ones, and to black, yellow, and brown bullheads than to channel catfish, which show a great deal of resistance to it.^*^ Thus it may be used at a dosage of 5 ppb ( .005 ppm) as a poison for small fish or at a somewhat higher dosage as a selective poison for bullheads. Toxaphene is roughly 3 times as toxic to fish as rotenone and can be used at concentrations one third as great."*-^ Toxaphene seems to have little effect on phytoplankton, and zoo- plankton usually reappears within 3 or 4 weeks after treatment.^^ Most invertebrates seem to be quite sensitive to toxaphene and bottom fauna may be killed (except for oHgochetes -^) by a dosage of 0.1 ppm.^^ This might result from the tendency of toxaphene to collect at the bottom (specific gravity 1.6). Among the invertebrate bottom organisms, dragon- fly nymphs were the earliest to reappear after treatment. Chironomidae were absent for more than 9 months.-"'- ^^' '*^' ^^' '^' ^"' ^^ Most aquatic biologists have hesitated to use the newer insecticides for killing fishes because of the residual toxicity of these materials, and the unpredictable length of time required after treatment before a lake or pond can be restocked. Sodium Cyanide. Sodium cyanide is useful as a fish poison in ponds and small lakes, primarily because the poisoned fish may be revived by placing them in fresh water if the fish are removed while still active. Ponds dosed at the rate of 1 ppm sodium cyanide become nontoxic to fish in about 4 days. Fish to be revived are usually collected within the first hour or two after treatment.^'' As cyanides can be fatal to humans, this method of fish poisoning should be done only by competent technicians. Sodium cyanide, once applied to to the pond, offers little danger to wild or domestic animals. At 1 ppm, it has no noticeable effect on tadpoles, frogs, snakes, turtles or aquatic insects. Sodium Sulfite. Sodium sulfite at a dosage of 168 ppm had been used experimentally to salvage fish in a small pond. The sulfite reduced the * ppb = parts per billion. 150 Theories and Techniques of Management dissolved oxygen and forced the fish to gulp air at the surface. Fish re- covered fully when placed in fresh water if they were collected when still gulping air. This method is practical only in small ponds because of the cost of the sulfite ( 10 cents per pound ) .^^^ FISH POPULATION ADJUSTMENT As mentioned above, there are a number of ways that a low-producing population may be adjusted to achieve a higher yield without eliminating and replacing the entire population. These methods are applicable when: (1) A population consists of desii-able fishes, but with some species stunted and others becoming scarce due to excessive competition. (2) There is high demand for one or two species and low demand for one or more other species inhabiting the same water. (3) Eliminating the indigenous population and starting over with a new one is impossible or impractical. Use of Nets and Seines In small ponds, wire traps or wing nets are used to reduce excessive populations of crappies, bluegills, and other sunfishes and to permit an increase in the largemouth bass. Wing nets employed in Fork Lake con- trolled bluegills and allowed the development of a very large bass popula- tion. The very same type of selective cropping may be done with intensive seining provided the pond or lake basin lends itself to the making of a productive seine haul, and the seine and crew are available. ^^^ The main drawback to either of these methods is that they both entail a great deal of work, and some rather expensive equipment. Also, relatively few lakes or ponds are well adapted to cropping with wing nets or seines, and the average pond or lake owner does not have access to this equipment. Partial Poisoning Soon after the use of rotenone to poison an entire population became widespread fisheries biologists noted its differential toxicity to various species and sizes of fishes. This led to the development of the selective or partial poisoning technique with rotenone, designed to kill certain parts of a population without seriously damaging the remainder of it. This technique for removing warm-water fishes from trout lakes has been described on page 143. In 1945 and 1946, I applied a shoreline treatment of rotenone to Park Pond of the South Pollywog Association holdings of a stripmine pond area in east central Illinois, in order to reduce an excessive population of gizzard shad and small sunfish. Later, when Dr. R. Weldon Larimore was Fish Population Adjustment 151 studying the growth of warmouth in Park Pond,^^ he found that the warmouth had made unusually rapid growth (114 and 128 per cent of expected annual length increment) during 1945 and 1946. This he could not explain until it was discovered that the years of good warmouth growth corresponded to years of population thinning through partial poisoning. Experience in partial poisoning operations has shown that gizzard shad are killed with lighter dosages of rotenone than almost any other warm- water fish.^* In general most of the centrarchids (sunfishes) are moder- ately sensitive to rotenone, but smaller individuals of a species are gener- ally more susceptible than larger ones. For this reason and because young fishes of many species inhabit the warm, quiet, shallow waters near the shore on bright summer days, a shoreline rotenoning operation can be used to kill numerous fish too small to interest anglers. ^^^ Timing in Partial Poisoning. The timing of a partial treatment is im- portant because the final effect may vary, depending upon whether the operation is done in the spring, mid-summer, or early fall. Sup- pose, for example, that a lake contained an excessive number of small bass and one wished to thin this population to allow for an expansion of a relatively small population of bluegills. A partial poisoning operation in May or June, but after the bass had spawned, would reduce the severity of predation by young bass on newly-hatched bluegills (the spawning season for bluegills lasts from late May to mid-September in the latitude of central Indiana and Illinois) and would allow a greater survival of these young in June, July, and August. Many of these bluegills might grow fast enough to exceed the size of easy predation before the next year class of bass was produced the following spring. In another instance, a lake might contain a large population of stunted bluegills and a few large bass unable to reproduce successfully because of predation on bass eggs and fry by hoards of hungry bluegills. In such a situation partial poisoning should be performed either ( 1 ) immediately before the bass spawning season in the spring or (2) at the end of the bluegill spawning season, in September or early October. If the operation were done between these specific times, the food and space gained by the removal of a portion of the excess of bluegills would be taken up almost immediately by new hatches of young bluegills. However, population reduction by partial poisoning, just prior to the bass spawning season ( and the bluegill spawning season as well ) , would curtail bluegill preda- tion on the bass eggs and fry, resulting in a proportionate increase in the survival of young bass. Similarly, if partial poisoning were performed in early fall after the bluegills had stopped spawning, the space gained at the expense of a part of the population would not be filled, either through 152 Theories and Techniques of Management the production of new bluegills or growth o£ those escaping poisoning. Much of this space would still be available when the bass spawned the following spring, insuring improved survival of young bass. In the in- stances cited above, timing is of utmost importance if the desired results are to be forthcoming. The greatest weakness of the partial poisoning technique is that with- out supplementary information on the standing crop of fish, it is im- possible to gauge accurately the extent of the operation in terms of the per cent of a fish population removed.^^ Usually the operation is too con- servative for optimum results, and a repetition may be necessary. In some instances it is useful and practical to do a partial poisoning operation just prior to the spawning season each year for as long as 5 successive years. Each treatment insures the production and survival of a year class of bass for that year and before the end of 5 years the bass population should be approaching the maximum that the water will support. Swingle, Prather, and Lawrence ^^^ recommend the stocking of 150 to 200 fingerling bass per acre following a summer marginal poisoning operation. Such a stocking might check the success of sunfish reproduction which could be expected to fill up the space created by the poisoning with a new year class of small bluegills, green sunfish or other kinds of sunfish present. As mentioned above, this restocking is unnecessary if the poisoning operation can be done in spring before the bass have spawned or in the fall near the end of the fish-growing season. Shoreline vs. Sectional Treatment. In partial poisoning operations one may poison completely a bay or an arm of a lake, using a dosage of rotenone of sufficient strength to kill all fish. In such a case it may be practical to separate the rest of the lake from the treated bay by blocking the opening with a canvas strip, long and deep enough to reach across the mouth of the bay (Figure 6.7). This strip can be hung on a wire supported by posts driven into the lake bottom. Beckman ^ demonstrated an increase in the growth rate of rock bass after he had poisoned the fish in half of a lake having a natural con- striction near the center. Unless the arm or bay to be treated represents one half or more of the total lake surface area, such a fish poisoning operation may be insufficient to reduce a stunted fish population. Swingle, Prather, and Lawrence ^^^ do not favor sectional poisoning because more desirable fish are killed by this method than by marginal poisoning. This may or may not be a valid argument. Sectional poisoning removes fishes in proportion to the relative abundance of kinds and sizes in a pond or lake and, therefore, is a useful method of cropping. It is probably the only technique short of complete poisoning that is effective in reducing stunted populations of bullheads. It is often practical to combine marginal poisoning with sectional Fish Population Adjustment 153 poisoning. In such a combined operation the section of lake receiving complete treatment should represent 20 to 80 per cent of the total lake area (Figure 6.8). In calculating the dosage of rotenone to be used, the part of the lake to receive complete treatment should be dosed at 1 ppm— the remainder being given a marginal treatment with an amoimt required to treat it at the rate of 0.25 ppm. This marginal amount is sprayed within Figure 6.7. Canvas "fence" used in partial poisoning of a lake. Water to right of canvas was treated with sufficient rotenone to kill all of its fish. Several fish in the "live box" on the left side of the canvas were unaffected. a 20-foot strip parallel to the shore and completely encircling the lake exclusive of the section receiving the heavier dosage ( Figure 6.8 ) . When a sectional poisoning is combined with marginal treatment, the fish collected from the sectional part may be considered a representative sample of the entire population of the lake. From the kinds and sizes of fish in this sample it may be possible to estimate the extent of artificial cropping in the current treatment and what further measures may be required to thin out the more severely stunted components of the pop- ulation. In planning the partial poisoning it is well to calculate the dosage, order >i f^ CD O -^ 4-> C^ ^ >-^ 0) <4— 1 s O ^ t H :r! o 0) <4H be i ^ fli H o S ?-l a; CD > s o *^ •s ® rt o 4-» •£ o o? O ^ o o ^ S 2 .SCO fl ■*-• >- S fl ^ 4^ V3 « P « 5/2 I-" 154 Fish Population Adjustment 155 the chemical, and plan the mechanical aspects of the operation well in advance. If this has been accomplished, the operation can be performed on short notice— at a time when wind and weather conditions are fa- vorable. Artificial Fluctuation of Water Levels In Chapter 4, I described certain experiments that demonstrated that the total weight of a population was related to the size of the water area it inhabits. Thus, if a body of water devoid of fish is stocked with a few sexually mature individuals, these fish will reproduce and they and their offspring will add flesh until their total weight approaches the poundage of fish that the water area wall support. This process may require one or more growing seasons, but eventually the poundage of fish will level off at some figure related to the size of the habitat and its food-producing capacity (natural fertility). This poundage adjustment may be downward if more pounds of fish were stocked originally than the lake would support. Various levels of population density favor certain species, that is, some are better able to compete for food and space than others. Under extreme competition some become dominant and others, if exposed to this com- petition for several years, may entirely disappear from a population. These species that do poorly when subjected to severe competition may actually become very abundant if stocked with other fishes in a body of water with plenty of space. Suppose then, that instead of adjusting the population by netting, seining, or partial poisoning, we subject it to extreme crowding for weeks or months through the release of much of the total volume of water ( Figure 6.9 ) . When the fish are crowded during warm weather, the entire population is under stress. Smaller and weaker fishes of many species starve or are killed through food competition, strife, or predation; the total poundage of the population is adjusted downward to conform with the smaller habitat and reduced food supply. The species, which as adults or fingerlings are best able to withstand crowding, will remain dominant although they may have stopped growing entirely. Then the habitat is rapidly expanded by the addition of new water! The exposed lake bottom is reflooded and there is suddenly plenty of space and more food. All fish that have survived the period of crowding begin to grow rapidly. Under conditions of unlimited food and space, certain species that were adversely affected by crowding, produce large broods of young. If these young are piscivorous they may actually check the expansion of some of the species formerly so successful. Thus, there has been a sudden shift in dominance brought about by a drastic change in the habitat.^^ If this change is man-made, we have only taken our cue 156 Theories and Techniques of Management Figure 6.9. Aerial view of fall drawdown shows lake area reduced by more than 50 per cent. from nature where water levels are rarely stable, but are usually in a state of fluctuation, often mild, but sometimes very severe during floods. Flood-plain lakes in the valleys of rivers are subject to the rivers' fluctua- tions and, because these lakes are relatively shallow, these fluctuations may cause extreme changes in the surface areas of these lakes. Prior to 1920, changes in water levels similar to those described above Fish Population Adjustment 157 commonly occurred as a natural cycle in many of the flood-plain lakes along the Illinois River in Illinois. Professor Stephen A. Forbes ^~ de- scribed tlie changes in water levels in the Havana region where in late summer the lakes, which extended over thousands of acres in spring when the river was high, covered only hundreds of acres; and many con- necting channels were so low that it was often difficult to move a boat from the river into these lakes. According to the average gauge readings at Havana, water levels were usually highest in spring, gradually dimin- ishing throughout the summer until they reached a low point in early fall. Levels usually rose in fall and winter but floods seldom occurred before spring. There were notable exceptions to this cycle, and floods have occurred in summer, fall and winter. After World War I, most of the bottom land lakes of the Illinois River valley were surrounded by earthen levees and pumped dry, and the lake basins were used for farming. The few lakes that were left or reconverted after agricultural use were more or less stabilized through the construc- tion of levees and spillways that kept the river water out unless it rose above the spillways' crests and held the lake water in when the river was lower than the crests. During the pre-leveeing period of wide fluctuations of lake levels and areas, the lakes in the Havana region were famous for their fishing, par- ticularly for their largemouth bass fishing. Presidents of the United States have fished there; fishing trains brought anglers from distances beyond the range of the horse and wagon. Records show that it was not considered unusual for fishermen to catch 100 bass in a day. There are still plenty of fish in the Illinois River and adjacent undrained bottomland lakes,^^ but the populations are composed largely of crappies, bluegills, yellow bass, sheepshead, buffalo, carp, bullheads, and channel catfish, and where vegetation is abundant yellow perch may be common. Although largemouth bass are sometimes caught by bass fishing experts, the average angler does not go to the Illinois River for bass except in a few special locations. Interest in the effects of fluctuating water levels upon fishes was stimu- lated by the late Dr. R. W. Eschmeyer and his colleagues,^^' ^^' ^-' ^^ through their investigations of TVA waters. In 1947, Dr. Eschmeyer stated that several permanent-level pools on TVA impoundments had provided poorer fishing than other reservoirs subjected to wide fluctuations of water levels. He suggested that "the winter drawdown apparently limits the abundance of rough fish (by limiting their food) without serious injury to the game fish population." Drawdowns on TVA lakes followed no definite schedule, but most of the drop in level occurred in winter follow- ing needs for power. The sudden lowering of the water level of a lake with the accompanying 158 Theories and Techniques of Management reduction in water volume and surface area afiFects all parts of an aquatic habitat and all components of the animal and plant communities that inhabit the water. Ejfects Upon the Exposed Lake Bottom. According to Neess ^° the bot- tom of a lake or pond is divided into regions, "an upper, loose well aerated, and often highly colloidal layer of decomposed organic material, plant debris . . . and a lower anaerobic zone, differing widely in composi- tion from place to place and often containing a large proportion of mineral matter." These soil layers have the ability to direct certain processes in the pond because the mineral composition of water is largely a reflection of the mineral composition of the soils of the pond bottom and the surrounding basin; also the colloidal fraction of the bottom materials consisting of humic substances, ferric gels, and clay is capable of absorbing certain soluble nutrient elements and governing their later distribution. In a pond or lake where there is a shortage of oxygen near the bottom, decomposition of organic matter is slow and the products are reduced to incompletely oxidized compounds such as hydrogen sulfide, methane, and short-chain fatty acids. When the water is drawn off of a lake bottom and the bottom is allowed to dry out and crack open, an abundance of oxygen becomes available, the processes of decomposition are stepped up, and the pH of the bottom soils is raised. Under these conditions there may be a release of certain fertilizing substances from organic colloidal systems, making available greater quantities of potassium and phosphate. In Euro- pean pond culture it was once considered important to grow a crop plant or a legume on the exposed pond bottom. Later the need for this practice was questioned -' although the crop furnished income to the pond owner when the pond was not producing fish. Whether a lake or pond bottom exposed by a drawdown will develop a vegetative cover depends upon the length of time the bottom lies ex- posed and the season of the year when the drawdown is made. A winter or early spring drawdown, which is prolonged by drought or purposely extended throughout the following plant growing season, will insure a luxuriant growth of terrestrial weeds on the exposed lake bottom. These weeds will reflect the fertility of the exposed lake bottom by their height and the density of the stand. Drawdowns made in July and August will be followed by some germination of seeds and growth of terrestrial plants, but drawdowns made as late as early September in the north are not followed by growth of terrestrial vegetation in the basin. Whether or not plants grow upon the exposed bottom seems to be unimportant; of primary significance is the exposure of the bottom to rapid and complete oxidation. Effects Upon Rooted Aquatic Vegetation. Most forms of submersed rooted aquatic plants are not greatly affected by a drawdown, e.g., ex- Fish Population Adjustment 159 posure of such relevant portions of the bottom may not free them from this vegetation if water levels are normal by the next growing season, although it may be somewhat more sparse and scattered. The drawdown is not an effective method of controlling rooted aquatic plants. In some instances it may be responsible for increasing the extent of beds of rooted aquatics, because plants may gain a root hold in parts of a lake when the water level is down, that ordinarily are too deep for them. For example, in Allerton Lake near Monticello, Illinois, a September drawdown of six feet (maximum depth of lake 14 feet) was maintained throughout a long warm fall (1955). During this period, curly-leaved pond weed, Potamogefon crispiis, gained a start in parts of the lake where the water was seven to eleven feet in depth when the lake was full. Then, as the lake refilled over winter and spring, this pond weed continued to grow so that in the summer of 1956 it reached the surface in all areas seven to eleven feet deep. This created a severe nuisance for boating and swimming and, when the lake was drawn down again in the fall of 1956, the drawdown had a minimum effect upon small green sunfish, red-ear sunfish, and bluegills, because they were protected from bass predation by the dense mats of vegetation in the deeper waters. A fish census made by draining the lake completely a month after the drawdown, exposed excessive populations of small green and red-ear sunfish and demonstrated the importance of pulling the water out of beds of vegetation if a draw- down is to be effective in ridding a lake of small sunfish. Effects Upon Invertebrates. When water is released from the basin of an artificial lake through an outlet valve, all motile aquatic animals are either stranded or forced to move down with the water. Animals that escape being stranded are concentrated and exposed to new environmental conditions. Such weak swimmers as many kinds of entomostraca, rotifers, and small insects, particularly those that are littoral, are stranded as the water recedes. Larger aquatic insect larvae, such as dragonfly and mayfly nymphs, may attempt to crawl along with the receding water, but most of them eventually are stranded and die or are eaten by birds or other vertebrates. Some crayfish may be stranded, but most of them burrow into the lake bottom or move down with the receding waters. In draining Ridge Lake,!^ it was not unusual to find 200 to 300 pounds of crayfish in the stilling pit below the gate valve in the outlet tunnel, after all the water had escaped from tlie basin. These crayfish came through the outlet gate with the water during the time the lake was being drained. Effects Upon Fishes and Other Vertebrates. The receding water not only strands small invertebrates but many small fishes as well, particularly in the littoral zone where sticks, debris, and mats of rooted vegetation trap these small fishes in temporary water pockets which soon dry up. Certain kinds of small fishes are stranded more often than others. For 160 Theories and Techniques of Management example, small green sunfish are stranded more often than are small blue- gills and small bluegills more often than small red-ear sunfish. Green sunfish are commonly found in shallow water along the lake edges, as are bluegills. The red-ear prefers deeper water and shows a tendency to move away from the water's edge as the lake level moves down. Neither young largemouth nor young smallmouth bass are ordinarily stranded with receding lake levels, although both may be trapped by dense mats of vegetation. Few large fishes are stranded unless they become trapped in shallow basins on the lake bottom and die later when the water in which they are trapped dries up. Small fishes that are not stranded and move down the lake basin with the water are forced from the protection of rooted vegetation and shallow- water debris into the open water of the lake where they are subject to predation from larger fishes, bullfrogs, and fish-eating reptiles, birds, and mammals. These forces and the mechanical stranding of small fish materially reduce the populations of smaller fishes without greatly reduc- ing numbers of the larger ones. The result is a selective culling action which is more specific for sunfish than for bass, and which may not be extensive enough to be beneficial unless the drawdown: (1) reduces the lake surface area by more than 50 per cent (Figure 6.9) and (2) forces the fish from the protection of beds of aquatic plants. The selective culling action resulting in a reduction of sunfish may set the stage for high survival of bass at the next bass-spawning season. Thus, fall drawdowns in several successive years may result in such a numerical buildup of bass that they will be of smaller average size than under more stable water levels (Figure 6.10). Heavy predation on the small fish during a fall drawdown may con- tinue as long as their numbers are concentrated and the water remains warm enough for rapid digestion. When the lake cools below 55°F, diges- tion is greatly slowed and the rate of predation diminishes accordingly. Although small fish concentrated by a drawdown are vulnerable to predation by many aquatic animals, it seems probable that piscivorous fish account for the death of more small fishes than all other predators together. As yet, no one has been able to evaluate the element of time in relation to the culling of small fish following a drawdown, but it is reason- able to assume that small fish losses, while heaviest at first, may continue with reduced intensity over a period of several weeks or months. Flat areas in the bottoms of reservoirs suitable for making seine hauls are sometimes cleared of stumps and debris before the reservoirs are filled. Then, later, when the reservoirs are drawn down, seines may be used to harvest concentrations of carp, buffalo, and other commercial fish, thereby giving an additional assist in the process of population improve- ment. Fish Population Adjustment 161 Types of Drawdowns. From the standpoint of angling there is some evidence to suggest that prolonged droughts affect fish populations favor- ably. In droughts extending over several years, lakes may gradually de- crease in area. This gradual decrease must cause adjustment in the fish population. However, it is difficult to see how a slow drawdown would have a selective effect in eliminating excessive small fish. 13-1 DC DC Spring Spring 1947 1949 DC Spring 1951 DC Spring 1953 DC Spring 1956 DC Fall 1959 FD FD FD FD FD Stable Water Levels BASS r-' '52 '53 SUMMERS 10 ■15 ■25 -7.0 -6.5 -6.0 > < w t- r a 2 PI -5.0 Figure 6.10. Changes in average sizes of largemouth bass and bluegills caught by fishermen at Ridge Lake (Illinois) under several types of manage- ment: 1945-1950 biennial draining and culling of small fish; 1951-1955, fall drawdowns with draining censuses in spring of 1953 and spring of 1956; 1956 to fall of 1959, stable water levels. DC = draining census; FD = fall draw- down. Fall drawdowns increased the number of bass but reduced their average size. Under stable water levels following the drawdown period, the number of bluegills expanded more than 500 per cent and their average size decreased. After 1957, the average size of the bass increased and their number declined. Annual cycles of water levels, such as were described above for the Illinois River and adjacent bottom-land lakes, can be shown to have a pronounced effect on the fish population. Reservoirs with the greatest water-area fluctuations contained the largest per cents (by weight) of predatory species, which included many of our game species. ^^^' ^^^ Large man-made and controlled reservoirs have various types of annual cycles of water fluctuation; these cycles may be only remotely related to cycles of rainfall and runoff. Man-made cycles may vary from one lake to the 162 Theories and Techniques of Management next, depending upon purposes of water release. If the reservoir is designed to control floods, water will be expelled between floods or prior to anticipated high runoff so that the lake may be partially empty for the storage of excess runoff water. However, if its purpose is to supply water for navigation, the drawdowns occur during the drier parts of the year. In most parts of North America, dry periods correspond to late summer and early fall. However, in some cases, where there is an annual cycle of need for power, the water may be used to generate it, and the drawdown may conform to no schedule, or follow one bearing no relationship to rainfall and runoff in the watershed of the reservoir. More experimental work must be done on drawdowns to allow biologists to predict the exact effects of these operations upon fish populations. Nearly all of the experimental work has been on drawdowns made at the end of the summer fishing season. As yet, no one can say whether a draw- down made in mid-summer would be more beneficial than one made in early September. Where fishing is an important use for a "waterfowl" lake, there has been severe conflict between fishermen and those concerned with water- fowl, relative to the time that a drawdown is made. Waterfowl managers usually favor a mid-summer drawdown to allow the planting of millet or other quick-maturing grain on exposed mud flats. Fishermen may wish to prolong the summer fishing period as long as possible because, once the lake is drawn down, it may be difficult or impossible to move boats across the exposed mud flats. Moreover, the basin of the lake that is left will probably slope into deep water so gradually that fishing from the bank, once the lake has been drawn down, may be impossible. The duck enthusiast will insist that the lake must be lowered in suf- ficient time to absolutely insure a grain crop. The interests of these two groups could be compatible except when the lake bottom is flat and there is too little water left for the fish to survive. In states where spring fishing is permitted, a drawdown in July will have given the fishermen a season of three or four months. They may then well afford to concede to the desires of those who would plant millet or some other duck food crop. Fishermen not only benefit from oxidation of the bottom, but also from the mechanical action of the roots of the grain plants growing in the lake bottom and the lake fertilization resulting from the decay of plant stems and duck excrement in the lake basin. Lake Fertilization The fertilization of ponds and lakes for increased production of fish has its origin in antiquity, and for centuries it has been common practice in Europe and parts of Asia to fertilize carp ponds. "^^ In the United States Fish Population Adjustment 163 the production of fish in ponds for commercial sale is limited. Most ponds and artificial lakes are for sport fishing. The primary exponents of pond fertilization for the improvement of sport fishing are located in the southeastern United States where soils are often infertile. Swingle and Smith ^^^' ^"^ stated that fertilized ponds in Alabama support 4 to 5 times as great a weight of fish as unfertilized ones; and consequently, the former give mucli better fishing. They recom- mended the use of 100 pounds of 6-8-4 ( N-P-K ) and 10 pounds of nitrate of soda per acre of pond for each application, with a seasonal schedule of 8 to 14 treatments, beginning March or April and extending to September or October. In Alabama such a fertilization program produces a "bloom" of plankton algae that prevents the developm.ent of filamentous algae and shades out any rooted submersed aquatic vegetation. It will not control water-lilies, lotus, or spatterdock; but these may be killed by removing the leaves several times during the summer. ^^^ The first leaf cutting should be made early in June and later ones every three weeks until no new leaves appear. According to Swingle,^^^ the algae produced by inorganic fertilization were largely genera of the Chlorophvceae: Scenedesmus, Ankistrodesnius, Chlorella, Staiirastriim, Pandorina, Cosmarium, Chlamydomonas, Nan- nochloris, Pediastrum, Coelastrum and others. Euglenophyceae were also abundant and occasionally dominant. Dinophyceae were often present but usually not in large numbers. The bluegreen algae, Coelosphaerium and Microcystis, occasionally became abundant for limited periods. Varia- tions in kinds of algae were observed in various types of ponds. Swingle recognized the competition between plankton algae and fila- mentous algae for dominance. He stated that either 6-8-4 or cottonseed meal applied in clear ponds in cold weather will stimulate the growth of filamentous algae on the bottom which will rise to the surface and shade out plankton algae. However, if these substances are applied when the water is "warm," plankton algae will be produced. Most organic material encourages the growth of filamentous algae unless it colors the water and thereby shades the bottom. There is no question but that the application of balanced inorganic fertihzers will increase the production of fish in a pond or lake by in- creasing the phytoplankton and, in turn, the aquatic animals at various trophic levels between the phytoplankton and fishes.'^ Not only do potassium, phosphorus, and nitrogen function as fertilizer materials in an aquatic environment, but some other elements such as manganese may produce chemical changes that release inorganic ferti- lizers from insoluble chemical compounds in the substrate of a pond or lake,^" giving an end eflFect similar to that obtained by direct fertilization. 164 Theories and Techniques of Management As yet the fertilization of waters is far from an exact science; however, some of the information now available is given below. Manganese. Hasler and Einsele ^^ described the use of manganese to release phosphates from iron. Thus, manganese dioxide (Mn02), which is not a fertilizer, may release phosphate (PO4) from an insoluble bond with iron, so that the eflFect is the same as though phosphate were added. Lime. Many authors stress the importance of lime in pond fertilization where there is a natural shortage of calcium.-"' ^^' ^^' ^^' ^^^' ^^^' ^-^ In waters containing less than 10 ppm the addition of lime may be followed by a large increase in fish production. Lime is believed to have many eflfects, particularly on the bottom mud where it changes the colloidal and adsorptive properties and creates an alkaline environment, which is more suitable than an acid environment for bacteria and fungi. Thus, it increases, indirectly, the rate of decay. It is believed to have several pos- sible chemical actions, such as the precipitation of iron compounds, and may counteract the poisonous properties of sodium, potassium, and magnesium ions. The calcium in lime may displace other fertilizing substances from organic colloidal systems, making available K+, and ~P04.^^' ^^* The calcium in stripmine waters may be responsible for the establishment of a strong buffer system that keeps the high sulfate ( ~S04 ) from being toxic to fish and other aquatic organisms. In Europeon fish ponds, enough lime is added to the drained pond basin to give a slight alkaline reaction and a crumbly mud structure. When unslaked lime is used on a drained pond basin, it is believed to have a toxic effect on aquatic organisms and fish parasites. In soft water, the addition of lime may be followed by an increase in carbon dioxide storage in the form of bicarbonate. Swingle ^^^ believed that calcium competes with the algae for the free carbon dioxide, but Nielsen ^^' ^^ demonstrated that aquatic plants used bicarbonate ( HCO3 ) directly in photosynthesis, up to one half of the amount present. Bi- carbonate was used more slowly than free carbon dioxide because the latter diffuses about 8 or 9 times as fast as bicarbonate. Calcium may be applied in the form of "quick lime" (CaO) or as agricultural limestone. It should never be applied at the same time as phosphate, and "quick lime" should be applied 2-3 weeks before fish are stocked. Potassium. Ponds with sandy bottom soils are often poor in potassium and respond markedly when this element is added. Usually it is difficult to measure the effects of adding potassium. These effects may be direct if there is a potassium scarcity or indirect if the addition of potassium displaces hydrogen from soil colloids, forming dilute acids in which phosphorus becomes soluble, i.e., potassium may indirectly make phos- Fish Population Adjustment 165 phorus available. German fish ciilturists usually mix potassium and pliosphate fertilizers and apply them together.'*^ Phosphorus. Phosphorus is the most important fertilizing element in lakes and ponds, but may be easily lost through combination with an excess of calcium to form tricalcium phosphate (Ca3(P04)2).^'^ As carbon dioxide increases, the precipitated salt may be converted to the more soluble di- and monocalcium phosphates. ^^ As mentioned in connection with manganese above, iron may unite with phosphate to form an in- soluble precipitate. Phosphorus also may combine physically with micells of ferric hydroxide or be absorbed directly on organic soil colloids on the pond bottom. For these reasons, phosphorus added to a lake or pond quickly goes out of solution, but still may be available on the pond bottom. It therefore follows that phosphorus applied at one time in some quantity may become available in small but useful amounts over a long period of time. European workers recommend about 17 kilograms per hectare (15.2 lb/acre) of phosphorus (applied as superphosphate) as an optimum dose.^^ Experiments in this country do not seem to substantiate this amount as optimum. Nitrogen. Nitrogen is more often used in this country than in Europe as a fertilizing material. Some algae are able to fix nitrogen from the atmosphere if phosphorus is available,-^' ^^' ^*^ particularly the bluegreen algae, Anahaena and Nostoc.^^ However, nitrogen in fertilizers gives a quick source of this element to the algae. Other Functions of Fertilizers. There are some uses of fertilizer other than those of increasing phytoplankton. Swingle ^^^ mentions Irwin's work on the use of inorganic fertilizer to cause clay particles to settle out of muddy ponds ( see Chapter 3 ) . Ball ^ believed that the addition of fertilizer to the entire shoal area of North Twin Lake ( Cheboygan County, Michigan) stimulated the growth of filamentous algae on the bottom which appeared to interfere with the nest building of sunfishes. Although no controlled experiments have been projected to date, it seems likely that undissolved salts of commercial fertilizers falling into nests of centrarchids containing developing eggs or yolk sac fry would cause the embryos to die. Commercial fertilizer is usually broadcast in shallow water over an area that corresponds roughly with that selected by bluegills and other sunfishes for nesting. If a fertilization schedule called for an application of fertilizer to the shoal waters of a pond at two- week intervals from early spring to September or October, it is probable that many centrarchid embryos would be killed. This might give sub- stantial assistance in keeping the bluegills or other sunfishes from becom- ing overly abundant. One of the techniques of pond management often suggested is the 166 Theories and Techniques of Management systematic destruction of sunfish nests. Less efiFort would be required to drop a small handful of chemical salts into a nest from a boat than to mechanically destroy the nest, and the former method might be more eflFective. Jackson ^^ used sodium hydroxide pellets for this purpose with good success. If fertilizer should prove useful in poisoning sunfish em- bryos, pond fertilization would be serving a dual purpose. Dangers from the Use of Fertilizers. The application of inorganic ferti- lizers to ponds and lakes for increasing fish production ^^^' ^^^ has not been well accepted in parts of the United States outside of the southeast. The objections to pond and lake fertilization are many, and it seems apparent that results have been variable and quite unpredictable.^^ --' ^-' ^^ In the northernmost states, the suffocation of fishes under ice is com- mon during severe winters with heavy snowfall. Fertilization of ponds and lakes in this region increases the danger of winterkill. ^^' ^^^ Ball and Tanner ^ stated that the addition of fertilizer to one of their experimental ponds was the indirect cause of winterkill, because the fertilizer stim- ulated the algae which later decomposed under the ice. In all parts of the country there is the ever-present danger of "summer- kill" of fishes, where calm hot weather along with an abundance of plank- ton algae may result in nocturnal oxygen depletion in lakes and ponds. ^^ This occurrence is not uncommon in organically rich lakes which are not fertilized. Swingle and Smith ^^^ advise against applying fertilizer when rooted aquatics are decomposing. They cite an instance when an applica- tion of fertilizer was broadcast over decomposing masses of Najas, with the result that oxygen was depleted and bass and other fishes died. In Michigan ponds, the use of fertilizer could not be depended upon to control higher aquatic plants,"* and produced filamentous algae even if not applied until after the water had warmed in the spring.^' ^^ This agreed with findings in Wisconsin ^^ and in the hard water ponds of West Virginia.i^^ The nuisance values of algae stimulated by inorganic fertilizers are stressed by several authors. Ball and Tanner^ state: "Tlie appearance of the lake and its use for swimming, boating, and other recreational pur- poses were adversely affected by the fertilizer. The matted green scum formed by the filamentous algae around the shore and festooning the marginal vegetation was very unsightly and was a hindrance to fishermen, both in the use of their boats and by the fouling of their baits. The odor of the decaying algae was very unpleasant." Patriarche and Ball ^^ warn about the unsightly condition that occurs when a growth of filamentous algae follows fertilization. Hansen, et al.,^^ describe a bloom of Rhizo- chlonium sp. in Lauderdale pond (Illinois) which covered from 25 to 75 per cent of the surface and stopped fishing except where the alga was absent. Fish Population Adjustment 167 There has been a tendency on the part of some aquatic biologists to over-simpHfy the problem of pond fertilization and to consider results obtained under some conditions to be universally meaningful. ^^^ Actually die problem of fertilization of waters is so complex that it is difficult to duplicate results from one pond to another, to say nothing of duplicating results from one research station to another. There are dangers inherent in the fertilization of any eutrophic lake by any means.^^- ^^^ Hasler and Einsele ^^ cite the changes that have taken place in Lake Snake, Vilas County, Wisconsin; Pontiac Lake, Michigan; Lake Okoboji, Iowa; Sylvan Lake, Noble County, Indiana; and Stadlsee near Waldsee in Wiierttemberg, Germany. They also describe the pos- sibility that fertilization may upset an efficient natural food chain for one that is much less efficient. "For example, in a natural lake, a rich growth of Cyclotella, a small diatom, fulfills the ideal food requirement of Daphnia, but fertilization might encourage previously rare or non- existent algae which are not adapted at all well as food for Daphnia, while the desirable form, Cyclotella, is suppressed." Swingle and Smith ^^^ demonstrated that by applying inorganic ferti- lizer to ponds in the proper amount they could increase the standing crop of bluegills from 130 pounds per acre to between 300 and 500 pounds per acre. These results have not been demonstrated in Michigan,^^ in Indiana,^^ in Illinois ^- or in any other part of the United States outside of the southeastern states. In ponds in some of the least productive soil types in Illinois the addi- tion of recommended amounts of inorganic fertilizer increased the average standing crop of fish by about 1.22 times.^- The improvement in fishing was such that uninformed fishermen could not tell which ponds were fertilized and which were not; yet in terms of total yield, rate of catch, and average size, the fertilized ponds produced considerably better blue- gill fishing than did unfertilized control ponds. In contrast, the controls usually produced a higher yield of bass 10 inches or larger than did the fertilized ponds. The fertihzation of ponds and lakes cannot be recommended as a general fish management technique outside of the southeastern United States, because the results are too variable and uncertain. Once the fertility of small impoundments in productive soils has been built up, this fertility may manifest itself in luxuriant annual crops of filamentous algae, bluegreen algae, or rooted aquatic vegetation. There are already numer- ous examples of such ponds, most of which are quite productive of fish; but they are problem waters because a treatment to kill rooted vegetation will be followed by obnoxious blooms of algae which in turn may require chemical treatment. These lakes have reduced aesthetic values, and fishing and swimming are hmited by plant growths of one type or another. 168 Theories and Techniques of Management If fertilization appears desirable in starting new ponds of low natural fertility, the program should be stopped before undesirable plant growths are evident. New gravel-pit ponds, stone-quarry ponds or dug ponds having basins of raw clay are often poor fish-producing waters when first formed. The addition of several hundred pounds per acre of commercial fertilizer during the first year will improve fish production without creat- ing nuisance vegetation problems in later years. AQUATIC VEGETATION AND CONTROL MEASURES The vegetation that develops in an aquatic environment is as char- acteristic and specialized as that associated with any terrestrial habitat. How then may aquatic plants suddenly appear in a new artificial lake, which a few months before was a dry valley supporting only terrestrial grasses and shrubs? Since the valley was flooded, the terrestrial plants have disappeared and have been replaced by widespread floating mats of green "scum" composed of one or several varieties of filamentous algae. In shallow water are a few scattered plants of a fine-leaved pondweed {Fotamogeton sp.), a higher plant that grows almost entirely below the water surface and which cannot support its own leaves when it is lifted out of water. How did these plants manage to suddenly appear in a location separated from other standing water by miles of dry land? Undoubtedly, resting cells of various kinds of algae blow about on winds. Seeds of certain higher plants may be transported by special organs which allow them to become airborne (as in the feathered seeds of the cattail). Still other seeds which are covered with a very hard coat are eaten by aquatic birds and pass through their digestive tracts undigested, only to fall to the pond bottom and germinate in the next location visited by the bird. It is conceivable that seeds and parts of plants might become entangled with or stick to the muddy toes of aquatic birds and be carried for short distances in this manner.^- Some seeds float from one location to another through connecting water courses. Thus, during a flood period a pond located well upstream in a watershed might furnish floating seeds to a downstream impoundment. Aquatic plants get around in one way or another, and certain species are likely to appear before others. Among the large emergent plants the cattails usually appear first, perhaps because the seed-bearing "fuzz" of the cattail head is so readily carried by the wind. Of the submersed pondweeds, the fine-leaved varieties usually appear first, later to be followed by coarser-leaved varieties. Why this should be so is unknown, although there is evidence that the new habitat is more suitable for some species than others. This may be demonstrated by Aquatic Veg^etation and Control Measures 169 artificially introducing a variety of submersed and emergent aquatic plants into a newly-impounded water area. Usually only a few kinds will survive and these are the species that might be expected to move in naturally. Experience has shown that it is often a complete waste of money to purchase aquatic vegetation for plantings in new impoundments, par- ticularly if these species are not common in similar waters. Types of Aquatic Plants Aquatic vegetation exclusive of bacteria may be separated into several types : Algae (1) Plankton algae— free floating cells of single or colonial habit, forming characteristic groups, plates, short strands, or spheres with or without power of movement: Phacus, Scenedesmus, Microcystis, Pandorina. (2) Filamentous algae— usually forming strands or threads of cells which may grow on the pond bottom but often float to the surface forming scums or floating mats of hairlike strands: Spirogyra, Zygnema. (3) Algae that grow upward from the pond bottom in a plant form not unlike that of some of the higher plants: Nitella, Chara. Higher Plants (4) Floating aquatic plants— unattached and floating about on the surface: Water hyacinth (Echhornia), Watermeal (Wolffia) , Duckweed (Lejnna). (5) Submersed aquatic plants— mostly below the surface and supported by the water: Pondweeds {Potamogeton) , Coontail {Ceratophyllum) , Waterweed (Elodea) , Milfoil (Myriophylhnn) . (6) Emergent aquatic plants— mostly above the surface and self-support- ing: Cattails (Typha), Buhush (Scirpus), Arrowheads (Sagittaria) , Figure 6.11. (7) Woody plants and trees— not true aquatics but usually associated with water: Button bush (Cephalanthus) , Cypress (Taxodiuin) , Willows (Salix). These plants serve the same functions in an acpatic habitat as in a terrestrial one, i.e., some are sources of food for herbivorous animals, some represent substrata upon which certain animals live, still others serve as cover and a mechanical aid in escape from natural predators. Aquatic plants compete for space in an aquatic environment much as terrestrial plants do. However, the environment of the former is less stable than the terrestrial environment, and for this reason the plant communities are much less stable. This is particularly true of the algae which are short lived and sensitive to minute changes in the environment J 70 Theories and Technique ft of Management and of the submersed aquatie plants which may be shaded out by liigh levels of turbid water. Alf^ae as a Basic Load. Certain of the algae are believed to form basic foods for jjcrfjivorous animals in the aquatic environment, much as the grasses are basic foods for many of the herbivorous animals in a terrestrial liabitat. In this function some species of algae are much more valuable than others just as some grasses and grains on land are more valuable Figure 6.11. Dense stand of cattails ( hacki[;round ) with Arrowhead {Sagit- taria) in foreground. These are among the more common forms of nuisance emergent aquatic vegetation. than others as foods. Probably the plankton algae and bottom microflora in shallow water arc more readily utilized than the filamentous forms. Actually vcuy lillle is known of specific aquatic food chains and the relative values of various species of algae. Plant cells or plant debris serve as foods for certain species of aquatic animals from protozoa to fishes. However, most of the fishes important for angling are not herbivorous, or eat only limited amounts of plant material. This is true for bluegills which feed largely on insect larva and entomostraca but which at certain seasons apparently take algae and the leaves of some submersed aquatic weeds. It has not been determined Aquatic Vegetation and Control Measures 171 whether this vegetable matter is a selected food or simply stuffing, taken because other more desirable foods were not readily available. Dangerous Algae. Several species of bluegreen algae ( Cyanophycae ) produce toxic substances when they die and decay. These algae have been responsible for mammalian, avian, and fish deaths.''^ The genera in- volved in these deaths were Aphaiiizomenon, Anabaena, Nodiilaria, Coelosphaeriiim, and Glaeotrichia. These algae are particularly dangerous when they appear as "blooms" on lakes and ponds and are concentrated by wind action along the downwind lake margin. Domestic stock drinking Figure 6.12. Floating mats of filamentous algae are a nuisance to boaters and swimmers and make fishing nearly impossible. this concentration of water and bluegreen algal cells rapidly show signs of acute poisoning. The toxic substance produced by the cells will cause the death of animals when algal cells are themselves excluded, and will survive the equivalent of water treatment using alum coagulation, filtra- tion, and chlorination. However, as far as is known, no human deaths or outbreaks of human gastroenteritis have been positively traced to these algae, although unexplained outbreaks of gastroenteritis have been re- ported in the same areas where extensive algal blooms were present.^^ Nuisance Algae. Most filamentous algae are considered nuisance plants because they eventually rise to the water surface and float about as green "slime" or "scum" until they die and disintegrate (Figure 6.12). In this position they are obnoxious to swimmers, and foul motor blades, oars, and lines of boaters and fishermen. 172 Theories and Techniques of Management There are several genera of filamentous algae that grow luxuriantly on the pond surface to form a thick blanket that may almost completely cover the pond. Lawrence ^^ lists Pithophora as a nuisance form for this reason in the southeastern states, and Hansen el al.^- describe a pond in southern Illinois that was nearly always partly covered with a floating layer of Rhizoclonium. Hatchery personnel in northern states are some- times bothered with Htjdrodictyon, an alga in which the elongated cells are arranged in the form of a net with 6-sided mesh. Small fish become entangled in these algal nets and die because they are unable to escape. Some algae that grow on rocks and submerged concrete are dangerous to bathers and wading fishermen because they create slippery footing and often cause waders to fall. One type of Spirogijra with very coarse fila- ments is notoriously slippery, and I once saw a bather slip and sit down at the top of a steep, spirogyra-covered lake spillway and slide entirely to the bottom before he could stop. Needless to say, he repeated the act until the algae as well as the seat of his bathing suit was practically gone. Control of Algae. Algae are very sensitive to copper and for many years crude copper sulfate crystals dissolved in water and sprayed on algae or dragged in a sack behind a boat has been a standard method of algae control. In soft water, 1 ppm or less was toxic to algae, but when used in hard water the copper ions united with carbonate in the water to form an insoluble precipitate that was useless in killing algae. Thus, it was necessary to use a much stronger dosage ( 5 to 12 ppm ) in order to control algae. At dosages higher than about 12 ppm the copper became toxic to fish. Because the hardness of water varies a great deal it is difficult or impossible to define a dosage, and only trials will allow one to discover the amount needed for an effective treatment for a specific water. Copper citrate is sometimes used in algae control work. This copper compound is much more expensive than copper sulfate, but a dosage of 0.5 to 1 ppm is usually sufficient to kill algae. Copper citrate is also more toxic to fish than is copper sulfate. CMU [3-(p-chlorophenyl)-l, 1-dimethylurea] has been recommended as a deterrent to algal growth after an established bloom has been killed by other chemicals.^^' ''^ Loss OF Fish Production Through Rooted Vegetation There is some evidence that dense stands of submersed rooted aquatic plants may bind up nutrient materials throughout the growing season,^ so that they are not available for the production of phytoplankton and the organisms that feed upon phytoplankton. This, in turn, may be reflected upon the fish through an eventual reduction of their food supply. An apparent relationship between fish yields and increasing stands of Potamogeton foliosus and P. nodosus is shown in Table 6.4.^ The area of Aquatic Vegetation and Control Measures 173 open water in this pond was reduced to 51.2 per cent of the total surface area by a dense stand of P. foliosus and P. nodosus. Tlie fish yield was reduced to 58.1 per cent of the yield taken during the year when aquatic vegetation was hirgely absent, although during the year when the low yield of fish \\ as taken the net fishing intensity was increased 359 per cent and the angling intensity was increased 157 per cent. Swingle ^"^'^ in- vestigated a pond that became filled with a heavy growth of naiad, Najas guadalupensis. He concluded that the rank plant growths did not reduce the hook-and-line yield. Evidence from the study cited above indicated that the fish were actually supported in this pond at a lower poundage than they had been before the dense stand of vegetation developed. Table 6.4 Reduced yield of fish (in spite of increased fishing pressure) associated with the spread of dense stands of rooted poxd- WEEDS, Potamogeton foliosus and P. nodosus, in a pond in cen- tral ILLINOIS.'-^ Area of Open Water Not Filled Net-Fishing Angling Year Yield with Vegetation Intensity Intensity Per Cent Per Cent Per Cent of 1939 Per Cent of 1939 of 1939 Net- Net Man- of 1939 Pounds Yield Acres Area days Fishing hours Angling 1939 223.4 100.0 1.25 100.0 92 100.0 27.0 100.0 1940 200.2 89.6 0.95 76.0 182 197.8 36.3 134.4 1941 129.9 58.1 0.64 51.2 330 358.8 42.3 156.7 Algae and rooted vegetation are in competition for available plant nutrient materials and space in an aquatic habitat; when algae are abundant they shade rooted aquatic plants and bind up the plant nutrient materials within their cells. Similarly, rooted plants trap nutrients when they become abundant and hold them from use by algae until the higher plants die and decay and the nutrients are again released. Rooted vegeta- tion often is able to suppress the growth of algae and, because it is longer-lived and more stable than the algae, it tends to persist throughout the growing season. This vegetation may die down in the fall when the water becomes cold, but the release of nutrients in cold weather is of little use to the trophic cycles of the lake or pond. Sudden Plant Die-offs. Occasionally progressive plant "die-offs" occur in ponds and lakes. In two instances of plant die-offs that I have observed, the deaths began at specific locations and spread to include all of the rooted vegetation in a pond. One of these occurred in early August of 1941 at Fork Lake (Ilfinois).^ Here the vegetation involved was P. foliosus -t-j a o < >^ >% ^^ r^r^ r^ ^ ^ ^ r-| r-i 2 2 IS 2 bJO tuO bJO tuO bJO bO bJO bJD 3 P 3 P 3 3 P P ^ o o , o o O O o o 5r! '-' s-i >-l ^H ^H Sh Vh ^H )-c ]:< ^ o o 0) o o QJ O O o o Is -5-^ Ctf "-^ "^i ^ 0) 0) (-] ^ ^ (D (U (U -M 4-> (U (D ci bJD tJO C tuO tuO C bJO bO bC bO O .2 .rt O .'^ .'S o .2 .2 .2 •2 - 1—^ 1— ( t: o o -73 'o 'o ■^ 'o 03 <4h M-i 03 M-4 <-tH 03 Mh Wh 4=! 1+; spre wet wet spre wet wet spre wet wet •>-> o ^ 4-> ^,^ a >^ jii, CLi p^ o^ a, en C3 03 c/3 c3 ■"C! -M ;-( %i ;-, Sm ui u G f^ S C U M H $ CX <: o p ;^ M z o C/2 u CO u l-l pa < 03 O a r-[ u 'o a. a a. O p CD (M "13 > •i-H P o^ W o^^P^ ■^ --HH *o Q lO lO <3 T^ ^^ ^[^ cm" cm" of ^ a, 4-> willo P bJO 1-1 ^•2 a ^ w o i-l "-" Q lo lo Tt* "^ "^ CM cq" cm" .a a u ^ ■id 3 03 i~-, CM -^p^ Q lo in Tfl "^ "^ cq" cm" cm" Oc -a 03 o N .2 p 4-> o Oi o3 o p 'rt a Q <: Oi So en Cattails, Typha u cm ~^a > > ^ t^ f^ ,^ > > > > ;? ^ K* ^ O^ ^ _^ f~* o 'o 'o 'OJ .^ ^^ c3 ^ ^ ^ 'E. a. Oi a. Oi Oi cu C3 C3 C3 Eg a. Tj^ CO 0-r^ CO CM CO lO ■* (O a. O =:^ o 1-1 jO g O --^ — ^ ' ^^ ;_ Oo,-^ « rt -^H-^ S ^ r-| 4-) 1 '^ V-' (— ' O ^.^. c O G c-l oi W CO W G O 4-1 o o CT) Co Ho Si, ^ 2 o s Q. a C/2 a. D-(0 F— I 00 O g O 2 c o Co tJD s -—I lO O g Wc^ CD G O a. fe •4-1 bJO a. o o s > 2 O C _o 'a o ft O 12 "^ 1/3 li-i o o SE- CT'S SJ o ^ ^ C O "O .-sec ^ S o 3 O O ^ JO 60 •-' 1^ «« sis a, o Vi • *■* <-> a s* a> t/3 ^ o :i ^ «J ^ ^ c -a -^ O (U ,-- • -; tH — t:? o '^ c/3 *j T3 CS o ?> 1/3 O tj_, C3 e o G 2 1 S •o ^ .S 1) o ^ = G O ^ >^ I- c •^ o > .5 1-, o Uh g on 175 176 Theories and Techniques of Management which began to die in a small area of shallow water at the upper end of the pond and spread until all of the vegetation had died and disintegrated, and an algal bloom of Aphanizomenon flos-aquae had developed. A second example of vegetation die-offs occurred in Ridge Lake ( Illinois ) in 1946 when Mr. W. W. Fleming was studying plant-invertebrate relationships in dense stands of the pond weeds, P. joliosus, P. nodosiis, P. pectinatus, Najas flexilis, and Elodea canadensis. ^^ In 1948, this die-oflF began about July 8 and gradually eliminated the rooted vegetation until on July 23 nothing but open water could be found at his selected sampling stations which were previously in dense stands of rooted pond weeds. This die-off at Ridge Lake has occurred during most summers since 1946; usually when the last of the early summer vegetation is dying, a new second crop is developing in areas where the old crop died first. By early September, there is almost a complete replacement of vegetation in areas where it was present before the die-off, but the stand is some- what less dense than it was in the original stand that grew in late spring and early summer. It seems possible that this die-off is caused by some disease or parasite, but no causative organism has been isolated. Role of Aquatic Vegetation in Management Originally, aquatic biologists held the belief that beds of higher aquatic plants were an essential part of the aquatic environment, presumably be- cause they were almost always present in lakes and ponds. This concept was entirely discarded by Swingle and Smith ^^^ who recommended the use of inorganic fertilizers in ponds to stimulate the growth of "blooms" of phytoplankton to shade rooted aquatics and thereby cause them to die. These investigators demonstrated that the phytoplankton blooms stimu- lated a higher production of zooplankton which, in turn, raised the level of food for such omnivorous feeders as bluegills, and thereby increased the total fish production. At present, excessive amounts of either rooted aquatic vegetation or algae are considered undesirable in ponds and lakes used for fishing, boating, and bathing. Where there is no history of intentional fertilization, excessive vegetation may be indicative of mild or severe organic pollution from barn lots or septic tanks. One of the drawbacks to locating housing developments around small artificial lakes is that such developments often are not connected with sewage disposal systems; rather, each house is supplied with its own septic tank and tile field. If the house is close enough to the lake to benefit aesthetically from it, the tile field must of necessity be laid in land sloping toward the lake. Eventually effluents from these tile fields enter the lake and, because they carry phosphates and nitrates, they act as fertilizers which stimulate aquatic vegetation and create nuisance problems. Prospective home owners who contemplate the pur- Aquatic Vegetation and Control Measures 177 chase of lots for permanent homes on small lakes should insist on a sewage system which \\ ill carry all effluents away from the lake. Dense stands of vegetation, besides being a nuisance, offer too much protection to small fishes, and are sometimes directly responsible for overpoj^ulation and stunting. This is true not only for submersed vegeta- tion, but also for emergent forms such as cattails, bulrushes, arrowheads, water willow and pond lilies. For these reasons, where economically justifiable, excessive aquatic vegetation should be controlled. Control of Higher Aquatic Vegetation For more than 30 years, sodium arsenite was used for the control of submersed rooted aquatic vegetation, often with good results.^'^ The main objections to its use are that ( 1 ) it is a poison which may accumulate in a pond or lake; (2) it is dangerous to handle and apply; (3) it is not very effective in the control of certain water weeds, such as sago pondweed, Potamogeton pectinatiis, and curly-leaved pondweed, P. crispus. Recently many terrestrial herbicides have been tested for their potential usefulness in aquatic weed control. ^^ Not only must these herbicides kill aquatic plants, but they must also show low toxicity to fish and aquatic invertebrates. For example, CMU [3-(p-chlorophenyl)-l,l-dimethylurea], a terrestrial soil sterilant was found to control Najas in ponds when applied at a rate of 15 pounds per acre.^^ This material was nontoxic to fish and most aquatic organisms. Some of the more promising herbicides are given in Table 6.5.^^ How- ever, progress in this field is so rapid that it is probable that new and more efficient herbicides will soon replace some of these listed in Table 6.5. LITERATURE 1. Anon., Jour. Wildlf. Mgt., 14( 1 ), 85-88 ( 1950) . 2. Applegate, V. C, Howell, J. H., Hall, A. E., Jr., and Smith, M. A., U. S. Dept. of Int., Fish ir Wildl. Serv. Special Scientific Report, 207, 157 pp., (1957). 3. Ball, R. C, Am. Fish. Soc. Trans., 78, 146-155 (1950). 4. Ball, R. C, Jour. Wildlf. Mgt., 16(3), 266-269 (1952). 5. Ball, R. C, and Tanner, H. A., Mich. St. Coll. Tech. Bull, 223, 1-32 (1951). 6. Beall, H. B., and Wahl, R. W., Prog. Fish-Cult., 21(3), 138-142 (1959). 7. Beckman, W. C, Am. Fish. Soc. Trans., 70, 143-148 (1941). 8. Bennett, G. W., Ill Nat. Hist. Surv. Bull, 22(3), 357-376 (1943). 9. Bennett, G. W., Ill Nat. Hist. Surv. Bull, 24(3), 377-412 (1948). 10. Bennett, G. W., North Am. Wildlife Conf. Trans., 19, 259-270 (1954a). 11. Bennett, G. W., Ill Nat. Hist. Surv. Bull, 26(2), 217-276 (1954b). 12. Bennett, G. W., and Childers, W. F., Jour. Wildlf. Mgt., 21(4), 414-424 (1957). 178 Theories and Techniques of Management 13. Benson, N. G., and Conner, J. T., Prog. Fish-Cult., 18(2), 78-80 (1956). 14. Bowers, C. C, Prog. Fish-Cult, 17(3), 134-135 (1955). 15. Bridges, W. R., U.S.F.W.S. Spec. Sci. Kept, 253, 1-11 (1958). 16. Brown, C. J. D., and Ball, R C, Am. Fish. Soc. Trans., 72, 268-284 (1943). 17. Brown, C. J. D., and Thoreson, N. A., Jour. Wildlf. Mgt., 16(3), 275-278 (1952). 18. Buck, D. H., and Whitacre, M., Prog. Fish-Cidt., 22(3), 141-143 (1960) . 19. Burdick, G. E., Dean, H. J., and Harris, E. J., N. Y. Fish & Game Jour., 2(1), 36-67 (1955). 20. Burnet, A. M. R., New Zealand Jour, of Set, 2(1), 46-56 (1959). 21. Carlander, K. D., and Moorman, R. B., Prog. Fish-Cult., 19(2), 92-94 (1957). 22. Clark, Minor, Jour. Wildlf. Mgt., 16(3), 262-266 (1952). 23. Cobb, E. S., Jour, of Tenn. Acad, of Sci., 29(1), 45-54 (1954). 24. Gushing, C. E., Jr., and Olive, J. R., Am. Fish. Soc. Trans., 86, 294-301 (1957). 25. Davis, H. S., Prog. Fish-Cult., 50, 1-13 (1940). 26. De, P. K., Proc. Roy. Soc. London B, 127, 121-138 (1939). 27. Demoll, R., Handh. der Binnenfischerei Mitteleuropas, 4, 53-160 (1925). 28. Erickson, A. B., Puhl. Health Repts., 62, 1254-1262 (1947). 29. Eschmeyer, R. W., Pap. Mich. Acad. Sci. Arts & Letts., 22, 613-628 (1937). 30. Eschmeyer, R. W., Jour. Tenn. Acad. Sci., 17(1), 90-115 (1942). 31. Eschmeyer, R. W., and Jones A. M., N. A. Wildlf. Conf. Trans., 6, 222-240 (1941). 32. Eschmeyer, R. W., Manges, D. E., and Haslbauer, O. F., Jour. Tenn. Acad. Sci, 22(1), 45-56 (1947). 33. Eschmeyer, R. W., Stroud, R. H., and Jones, A. M., Jour Tenn. Acad. Sci., 19(1), 70-122 (1944). 34. Fassett, N. C, "A Manual of Aquatic Plants," pp. 1-405, with revision appendix by E. G. Ogden, Univ. of Wise. Press, Madison, Wis., 1957. 35. Fitzgerald, G. P., Trans. Wis. Acad. Sci., Arts & Letts., 46, 281-294 (1957). 36. Fogg, G. E., Brit. Jour. Exp. Biol, 19, 78-87 ( 1942) . 37. Forbes, S. A., Bienn. Rpt. III. Sta. Fish Comm., 1892-1894, 35-52 (1895). 38. Hall, J. F., Kij. Acad. Sci. Trans., 17(3-4), 140-147 (1956). 39. Hansen, D. F., ///. Acad. Sci. Trans., 37, 115-122 (1944). 40. Hansen, D. F., ///. Nat. Hist. Surv. Bull, 25(4), 211-265 (1951). 41. Hansen, D. F., ///. Acad. Sci. Trans., 46, 216-226 (1953). 42. Hansen, D. F., Bennett, G. W., Webb, R. J., and Lewis, J. M., ///. Nat. Hist. Surv. Bull, 27(5), 345-390 (1960). 43. Hasler, A. D., and Einsele, W. G., N. A. Wildlf. Conf. Trans., 13, 527- 555 (1948). 44. Hasler, A. D., and Jones, E., Ecology, 30(3), 359-364 (1949). 45. Hayes, F. R., and Livingstone, D. A., Jour. Fish. Res. Bd. Can., 12(4), 618-635 (1955). 46. Hemphill, J. E., Prog. Fish-Cult., 16(1), 41-42 (1954). 47. Henderson, C, Prog. Fish-Cult., 11(3), 157-159 (1949). 48. Henderson, C, Pickering, Q. H., and Tarzwell, C. M., Am. Fish. Soc. Trans., 88(1), 23-32 (1959). Literature 179 49. Hepher, B., Bamiclgeh, 10(1), 3 (1958a). 50. Hepher, B., Bamidgeh, 10(1), 4-18 (1958b). 51. Hiltibian, R. C, ///. Nat. Hist. Siirv. Mimeo Scries, A-5, 1-18 (1961). 52. Holimaii, C. H., and Surber, E. W., Am. Fish. Soc. Trans., 75, 48-58 (1948). 53. Hoffman, C. H., and Surber, E. W., Prog. Fish-Cult., 11(4), 203-211 (1949). 54. Hoffman, C. H., and Linduska, J. P., Sci. Monthhj, 69, 104-114, (1949). 55. Hooper, F. F., Trans. 2nd Sem. on Water Poll, U.S.P.H.S., 7 pp. (1959). 56. Hooper, F. F., and Grzenda, A. R., Am. Fish. Soc. Trans., 85, 180-190 (1957). 57. Huish, M. T., Proceed. Ann. Conf., S. Eastern Assn. Game 6- Fish Commrs., 11, 66-70 (1958). 58. Hvnes, H. B. N., "The Biology of Polluted Waters," pp. 255, University Press of Liverpool, Liverpool, England, 1960. 59. Ins^ram, W. M., and Prescott, G. W., Am. Midland Naturalist, 52(1), 75-87 (1954). 60. Jackson, C. F., N. H. Fish and Game Dept. Tech. Circ, 12, 1-16 (1956). 61. Jackson, C. F., N. H. Fish and Game Dept. Tech. Circ, 14, 1-28 (1957). 62. Jenkins, R. M., Okla. Acad. Sci. Proc, 37, 164-173 (1959). 63. King, J. E., Okla. Acad, of Sci. Proc, 35, 21-24 (1954). 64. Kiyoshi, G. F., and Hooper, F. F., Prog. Fish-Cult., 20(4), 189-190 (1958). 65. Krumholz, L. A., Jour. Wildlf. Mgt., 12(3), 305-317 (1948). 66. Krumholz, L. A., Jour. Wildlf. Mgt., 16(3), 254-257 (1952). 67. Lambou, V. W., Prog. Fish-Cult., 21(3), 143-144 (1959). 68. Larimore, R. W., III. Nat. Hist. Surv. Bull, 27(1), 1-83 (1957). 69. Larimore, R. W., Durham, L., and Bennett, G. W., Jour. Wildlf. Mgt., 14(3), 320-323 (1950). 70. Lawrence, J. M., Prog. Fish-Cult., 16(2), 83-86 (1954). 71. Lawrence, J. M., Prog. Fish-Cult., 18(1), 15-21 (1956). 72. Leonard, J. W., Am. Fish. Soc. Trans., 68, 269-280 (1939). 73. Lindgren, P. E., Fish. Bd. of Sweden, Iris, of Freshwater Res., 41, 172- 184 (1960). 74. Linduska, J. P., and Surber, E. W., U. S. Fish and Wildlf. Serv. Circ, 15, 1-19 (1948). 75. Loeb, H. A., IV. Y. Fish ir Game Jour., 4(1), 109-118 (1957). 76. Maloney, T. E., Am. Wat. Whs. Assn., 50, 416-422 (1958). 77. Mayhew, J., Proc of Iowa Acad, of Sci., 66, 513-517 (1959) . 78. Moody, H. L., Proc. Ann. Conf., S. Eastern Assn. Game ^'^ So . o3 Si « ^ .2 s is ^'^ o CIhPM ^ O C) o o o O o CO .04 c> - < o o o o 00 o o o o CO o (N o Ph Q c CM u^ ^ CO cs c> CO o d MAN-HOURS PER ACRE, IN THOUSANDS. 192 Fishing Mortality 193 maintain a catch rate of 0.50 pound per hour they would need to con- struct a lake of about 107 acres: (1) Figure 7.2: For an average catch rate of 0.50 pound per lioiu- the fishing pressure should be about 75 man-liours per acre per season. (2) 8000 ^ 75 = 106.6 acres. A lake of this size might cost $1000 to $1500 per acre or $110,000 to $160,000. Types of Fishing Pressure The total hours of fishing and the fishing schedule vary greatly from lake to lake. For example, a lake open to the public may be fished at a high rate during May and June and then chiefly on weekends during July, August, and early September. Where both boat and bank fishing are permitted, the daily pressure may be very high— as much as 50 man- hours per acre per day if the fishing is good. But at this level of fishing, the rate of catch will drop off very markedly in 4 days or less. At Ridge Lake (Illinois) where bank fishing was not permitted and only 7 boats were a\ ailable on 18 acres of water, the approximate rate of accumulation of fishing hours was 9 or 10 per acre per day. As the lake was open 5 days per week, we can estimate a fishing pressure of about 25 to 50 man-hours per week during the first week; later in the season the pressure was less.^ Many private lakes and farm ponds are fished in a leisurely manner: On one day three fishermen fish for a total of 12 hours, but the pond is not visited by fishermen again for several days or weeks. The accumulation of fishing hours is so slow that only 50 hours per acre are logged for an entire season. The same may be true for large reservoirs but for a dif- ferent reason: some artificial reservoirs are so large that the fishing pressure of all available fishermen builds up a seasonal pressure of only a few dozen hours per acre. As fishes react in different ways to various levels of fishing, the schedule and intensity of fishing affects the yield. The bass in Ridge Lake showed a much reduced catch rate after the morning fishing period of the opening day (Figure 7.3), and by the end of the third day, the rate had nearly reached a low point for the summer— after only about 25 hours of fishing pressure per acre.^ Creel censuses on three Kentucky lakes demonstrated that 70 per cent of all largemouth bass caught during the first week were taken in the first 30 hours of fishing. ^-^ Records of the largemouth bass catches from relatively infertile un- managed waters in Virginia indicated that about 19 trips per acre re- moved the "harvestable surplus" of these fish amounting to 3.6 bass per acre averaging approximately a pound each."^^ According to Martin ^^ there is an easily harvested segment of any bass population which can be readily taken at a high rate of catch by light fishing pressure. After 194 Fishing and Natural Mortality these fish have been removed, additional fishing pressure has Httle eflFect upon further harvest and the rate of catch dechnes rapidly. Several common warm-water fishes are rather seasonal in their biting habits and fishermen increase fishing pressure at these times because they know that their chances of catching fish are improved. For example, both white and black crappies bite best in the early spring before the lakes A.M. P.M. A.M. P.M. A.M. P.M. A.M. P.M. A.M. P.M. 1st Day 2nd Day 3d Day 4th Day 5th Day Figure 7.3. Decelerating rate of catch of largemouth bass at Ridge Lake (Illinois) during the first week of public fishing in each of the named years, and the average rate of catch for all of these years. The first 5 days of fishing usually showed an accumulated fishing pressure of less than 40 man-hours per acre. [From Bennett, G. W., III. Nat. Hist. Surv. Bull, 26 (2) ( 1954)] warm to temperatures in the 70°F range. Warmouths bite much better in late spring and early summer than in late summer. Also, some fish bite well under the ice in winter, while other common species are scarcely ever caught through ice fishing. The fishing intensity of ice fishermen in sections of northern United States where the ice is thick enough to support them may nearly equal that of the summer anglers, and exceeds the summer fishing in certain localized areas. Fishing intensity for certain species may be increased with changes in Fishing Mortality 195 weather conditions because fishermen know that rising waters, for ex- ample, stimulate certain species of fish to move about and feed. Most sight-feeding fishes become inactivated by rising waters because increased turbidity limits their vision. Returns from angling effort directed toward species of warm-water fishes other than largemouth bass indicate that intensive fishing pressure also depresses the rate of catch but not so rapidly as with the largemouth. Thus it is safe to state that a leisurely pattern of angling over a season is more conducive to satisfactory fishing than is an alternation of relatively intensive angling with periods of complete rest. Factors Related to Rate of Catch The exact relationship between the number of fish per acre or per acre-foot of water and the rate of catch is usually obscured by one or more factors, some of which have been discussed previously. Quite obvi- ously there must be some relationship between numbers of fish available and catch rate, but often the relationship is clear only when numbers of fish are reduced to a very low figure.^- Stroud ^^ studied the recovery of marked "salvaged" fishes released in Massachusetts lakes and ponds to gain information on the relationship between available fishes and angling returns. He could only estimate roughly the population density of the various marked species recovered. Thus, he calculated an over-all harvest of 9 per cent ( from 10 ponds ) for marked largemouth bass, with a somewhat higher return for smallmouth bass; whereas marked chain pickerel ranged from 15 per cent to 59 per cent. Recaptures of other warm-water pond fishes were usually between those for bass and pickerel, although in some instances a large per cent of the salvaged fishes did not survive. The presence of more pan fishes, such as bluegills, crappies, yellow perch or bullheads, per unit of water, could conceivably mean better fishing. One reason for poor fishing in unfertilized Alabama ponds given by Swingle and Smith ^^ is that the "water is too poor to support many legal-sized fish." However, Hansen et alr^ were unable to show that the rate of catch in fertilized ponds (which contained somewhat higher poundages of fish than the control ponds) was consistently better than in the control ponds. Lux and Smith ^^ attempted to discover which of a number of physical, chemical, and biological factors bore a relationship to seasonal changes in the angler's catch in a Minnesota lake. They concluded that as the avail- able food supply increased after the middle of June, the fishing became progressively poorer. This may explain a seasonal cycle, but cannot be used to explain trends extending over several seasons. Evidence shows that rate of growth of fishes and rate of biting are often 196 Fishing and Natural Mortality in direct relationship. In fact, anyone fishing a new impoundment will probably discover the excellent fishing typical of an expanding popula- tion.^^ Fishing during the early years of impoundment in most water- supply reservoirs is better than in later years, and most of these reservoirs go through a predictable cycle. ^' ^^ Exceptions seems to be those reser- voirs having large annual water-level fluctuations which prevent the development of "climax" fish populations. One possible explanation for the excellent fishing in new reservoirs is that the fish have an abundance of available food and are growing rapidly. They have the habit of feeding for long periods each day, and bite readily at almost any time. This situation may be contrasted with one where inter- and intraspecific competitions are moderately keen and food is more readily available at certain periods of the day than at others. Growth is slower and fish no longer have the opportunity of gorging themselves; instead they have developed feeding cycles related to periods of the day when certain foods are more readily available than at other times. During these periods the fish are caught quite readily by anglers, but the catch may show little relationship to the relative abundance of the fish. When fish are crowded and inter- and intraspecific competitions are very severe, the fish are thin and stunted and their growth may have practically stopped. Under these conditions, they bite very poorly ^^ and give the impression that few or no fish are present. One explanation for this behavior pattern is that these fish are living on a subsistence diet of small aquatic organisms and are not conditioned to utilize foods as large as most live or artificial baits. In summary, the evidence seems to favor the assumption that, within limits, there is a positive relationship between good fishing and rapid growth and an expanding population of fish and a negative relationship between good fishing and population density, although these relationships are not always clear. Role of Commercial Fishing in Sport-fish Management When angling becomes temporarily or permanently poor in waters where commercial fishermen are operating nets, the commercial operators are usually blamed for the poor angling, even though they are not taking the same species fished for by the anglers. Although commercial fishermen compete with anglers for such fishes as walleyes and lake trout in low-producing northern lakes, no valid evidence exists that commercial fishing in shallow warm-water lakes provides competition for the angler,''"' -^^ even when commercial fishermen are permitted to take all sizes and kinds of fish. An experiment involving the use of illegal-meshed commercial gear in a small pond clearly demonstrated the effects of intensive fishing with Fishing Mortality 197 fyke or wing nets for largemoiith bass and bluegills.'^ Six 1-inch-mesh wing nets with leads were set across a 1.38-acre pond in two gangs so as to completely block the pond at two points (Figure 7.4). These nets were fished for 96 to 149 hours each month, from March to November of each year for two and one-half years, and all fishes captured were removed, regardless of size or species. These nets held bass as small as 9 inches and bluegills as small as 5 inches. The catch consisted largely of bluegills, as the bass soon avoided the nets. Supplementary cropping was done by hook-and-line. At the end of the study period the bluegill population Figure 7.4. Outline map of Fork Lake (Illinois) showing the customary arrangement of wing nets and lead nets for cropping studies. No fish could swim for any great distance in this 1.38-acre pond without running into a lead net or wing net. Arrows mark openings of wing nets. [From Bennett, G. W., Ill Nat. Hist. Bull, 24(3) (1948)] amounted to about 67 pounds per acre and the bass to over 120 pounds per acre. The netting operation unquestionably was a major influence in the buildup of a very large population of bass. In some states commercial fishermen have been forced out of business except on the major rivers, and the state fisheries departments have had to assume the task of controlling rough fish. This is an expensive never- ending job, and when commercial fishing is outlawed, rough-fish removal must be paid for by the sport fishermen. However, in states where com- mercial fishing is legal, rough-fish removal is self-sustaining and the com- mercial man operates at a profit. Many studies show that removing large crops of coarse fish with com- mercial gear is beneficial or at least not harmful to sport fishing. This is universally true in large shallow lakes and reservoirs containing sub- stantial numbers of coarse fish which, because of low fishing pressure, are not cropped by hook-and-line.^- Here commercial fishing is about the 198 Fishing and Natural Mortality only means (except through natural predators) of reducing competition among species. Originally the scope of operation of the angler and commercial fisher- man overlapped: They both took any and all species on the basis of relative abundance and their ability to capture them. However, gradually the angler was able to restrict the commercial fisherman to "rough" species (carp, buffalos and catfish), while reserving for himself all of the "fine" fish ( crappies, bluegills, white bass, etc. ) and game fish ( largemouth bass, northern pike, pickerel and often walleye ) . These restrictions on the com- mercial fisherman did not benefit the angler, for the quality of the fishing became worse. In the 1930's an interesting relationship existed between anglers, com- mercial fishermen, and natural fish predators at Reelfoot Lake (14,500 acres, Tennessee). Here anglers made an average catch of 0.89 pound of fish per hour. Largemouth bass averaged 1.91 pound each, crappies 0.70 pound, sunfish 0.37 pound, and catfish 2.39 pounds. ^^ The total anglers' yield in 1936 was 22,124 pounds. At the same time, commercial fishermen were taking 529,093 pounds plus an additional estimated 95,000 pounds of small fish killed in netting operations.^^ Commercial fishermen were not restricted to any species, i.e., they were taking the same kinds of fishes as the sport fishermen. More obvious fish predators on Reelfoot Lake were 4000 egrets, 1500 cormorants, and 500 Ward's herons, plus smaller numbers of 7 other species of fish-eating birds. These birds were taking more than 400,000 pounds of fish of kinds and sizes related to their availability. The total of fish taken by birds, commercial fishermen, and anglers in 1937 was 1,046,133 pounds or about 72 pounds per acre. On the basis of rate of catch for angling and sizes of fish taken, the fishing in 1937 was excellent. However, soon after this, Tennessee began restricting commercial fish- ing. First, the state prohibited the commercial fishing of largemouth bass, then regulations on other species became increasingly restrictive until in 1955 commercial fishing was abolished.'"*^ During the period 1937 to 1958, as small fish were given more protection in Tennessee, the growth rate for bluegills and other centrarchids at Reelfoot Lake decreased. By 1953, anglers had increased almost 100-fold, yet they were catching only about 21 pounds of fish per acre.^^ However, had they been taking as many pounds per fisherman as in 1937, their catch would have approached 153 pounds per acre. The commercial fishing yield in 1953 was 21 pounds per acre instead of the 1937 yield of about 36.5 pounds per acre. No report is available on numbers of fish- eating birds on the lake in 1953, but since the number of fishermen in- creased 100-fold, it is doubtful that cormorants, egrets, and herons were Natural Mortality 199 as abundant as in 1937. It seems probable tliat the total yield of fish in 1953 was considerably less than in 1937 but more selective for larger fish. From the standpoint of the angler the relationship in 1937 between anglers, commercial fishermen, and fish-eating birds was near optimum, and the combined action of these cropping agencies was taking a reason- able annual fish crop. Thus the benefits of an expanding population were evident: fish had food and space to grow. Because the total annual mortality of fish in this lake was nearly optimum (72 plus pounds per acre), fishing remained good. Without the help of the commercial fisher- men, the fish-eating birds and other natural predators available in 1937 could not have kept up with the reproductive potential of the fishes. What happened to the population is fairly evident, for the number of fish caught per fisherman in 1953 remained about the same as in 1937,^^ yet the weight of fish taken was less than one-third that of the earlier year. NATURAL MORTALITY Natural mortality includes all causes of death of fish exclusive of pollu- tion, angling, and commercial fishing. Deaths may result from predation, injuries received through unsuccessful attempts at predation, competition for food and space resulting in fatal injury or starvation, disease or ex- cessively heavy infestations of parasites, catastrophes such as adverse weather conditions, floods, etc., as well as from senile degeneration or a combination of several of these factors. Causes of Natural Death Important causes for the deaths of fish change with a fish's age and size as related to its normal life span. In the embryo and early free-swimming stages when fish are very small, high mortality rates are probably caused by predation from aquatic insects and larger fish.^^ The approximate numbers of largemouth bass fry in enumerated schools at Ridge Lake (Illinois) were compared with the bass taken by lake draining at the end of the second succeeding growing season. It was estimated that the survival rate of schooling bass fry ranged from 1 in 29 to 1 in 195.^ These bass fry were exposed to very favorable conditions because many potential predators in the form of small bluegills, large predaceous aquatic insects, and crayfish had been removed from the lake prior to the bass spawning season. A survival ratio of schooling bass fry to yearlings of at least 15 to 1 was attained by the first year class of bass spawned in Ridge Lake after water was first impounded. Predation rates on embryos and fry of other nest-building centrarchids may be higher than 200 Fishing and Natural Mortality those of largemouth bass because the former produce larger numbers of eggs and probably o£Fer less protection to their young. If the young fishes of the larger species escape predation and can find sufficient food, they may survive the first growing season and be well on their way to adulthood. Many of the smaller species reach sexual maturity and spawn during the early part of the second summer of life. Most of these yearlings are small enough to be preyed upon by some kinds of fishes and nearly all of the predaceous amphibians, reptiles, birds, and mammals; however, they are beyond the size for predation by most aquatic insects. By the end of the second growing season, direct preda- tion may become a minor cause of death and other mortality causes may take over. Ricker ^^ investigating the natural mortality rate among the fishes of several Indiana lakes concluded that once the fish reached sizes larger than 5 inches, senility must account for most of the natural mortality. Ricker believed that senility in fish was active over a wide range of ages, relatively much wider than that of domestic animals and man. He based this assumption on his discovery that natural mortality in the bluegill was rather constant in fish from three to six or seven years, the approxi- mate maximum age of this species. Once a fish reaches a size beyond that of "easy" predation its death may result from a combination of several factors. A 5-inch green sunfish may fall prey to a 16-inch bass because a bacterial infection of the sun- fish's fins has caused it to swim in such an abnormal manner as to attract attention. The internal parasites of a minnow may reduce its swimming activity so greatly that it cannot avoid being captured by a crappie or a heron. Even senility may be followed by predation before the individual fish has time to die from organic degeneration.^^ When Do Fish Die? Fish may die at any time of year but it is probable that most of them expire during spring, summer, and fall ( one must discount deaths caused by suflFocation under ice resulting from unusual circumstances ) . However, Snieszko ^^ believes that winter conditions are responsible for reducing the resistance of fish to bacterial diseases in the spring and that reported deaths associated with rising water temperatures are due to a deficiency of antimicrobial components in the fish's blood; the studies of carp blood by Schaeperclaus ^^ and Plancic ^- substantiate this hypothesis. There is little question that many fish die in the spring, a large number of which appear to be diseased or infested with aquatic fungi ( Saproligniales ) . In the case of fungus infestations, it is believed that injuries acquired by fish might heal during other times of the year. In the spring, however, conditions are optimum for the growth of aquatic fungi, and they readily Natural Mortality 201 gain entrance through skin abrasions and produce a toxin that causes the death of the fish. Achhja sp., one of these fungi, is reported to attack healthy fish without breaks in the skin. Biologists active in state fisheries departments learn to anticipate a spring period each year when many phone calls and letters report deaths of fish in various state and private waters. Part of these originate directly from winterkill: the fish that have died over the winter decompose and float to the surface in spring soon after the ice goes out. Other spring reports of dead or dying fish can be assigned to disease or fungus in- festations. Usually, nothing can be done to stop the fish from dying and the situation must be left to run its course. In no case that I know of has a partial loss of fish had serious consequences, unless severe winterkill was the cause of death. When fish suffocate under the ice, a partial kill may have serious consequences to later fishing ( see Chapter 3 ) . Many fish die during the summer and fall. Usually these never appear at the surface or else float up in numbers too small to receive attention. The complete disappearance of a year class of crappies during summer is not unusual. In one instance where 1-inch mesh wing nets with leads were used at 30- to 40-day intervals over a period of years for catching crappies, a year class was followed from the time its members were first large enough to be caught until they suddenly and permanently disappeared, indicating a complete mortality for that year class. -^ Angling returns of marked crappies in Lake Chautauqua (Illinois) showed that fishermen were taking less than 5 per cent of the available large fish; thus, about 95 per cent of the large crappies were dying from old age. Most of these deaths apparently occurred during the warm months. ^^ Scavengers As a result of pollution, many fish die at about the same time, making available a large amount of carrion for such scavengers as survive in the lake. However, these remaining scavengers are unable to assimilate such an abundance of protein. As a result, the fish decay and float to the surface where they may be consumed by terrestrial scavengers and blowfly larvae. Underwater scavengers are probably not so eflicient as terrestrial carrion feeders. However, they do well enough to consume a seasonal quantity of carrion of as much as 100 pounds per acre in some waters, without allowing any of these fish to appear on the surface. Crayfish are important as underwater scavengers and will attack injured or disabled fish before they are dead."' Certain kinds of turtles also act as scavengers. Length of Life of Fishes Growth studies based on scale analyses furnish valuable information on the length of life of fishes. Most species do not live as long as is popularly 202 Fishing and Natural Mortality believed, but those inhabiting the cooler northern sections of the country live longer than the same ones in the warmer southern and central sections: Largemouth bass in northern Wisconsin may reach ages of 14 to 15 years; in central Illinois, 500 miles south, these fish seldom live longer than 10 to 11 years. As a rule, the species that attain the largest sizes live the longest, al- though in any single species, individuals that grow rapidly and gain exceptional sizes are usually short lived for their species. For example, when I was investigating the ages and growth rates of largemouth and smallmouth bass in Wisconsin, I found that bass of 5 pounds or larger were often not more than 5 to 7 years old. In contrast, the bass that showed 14 or 15 annuli on their scales (14+ to 15+ years) seldom exceeded 4 to 4.5 pounds in weight; none of the fishes of exceptional sizes for these species were slow-growing individuals. It was as if a fish were "wound up" like a mechanical toy when small, with the potential to run down rapidly or slowly depending upon its individual genetic make-up, the available food, and the forms of competition encountered. Table 7.1 gives approximate ages for some common fishes of interest to anglers. Probably most of the ones that reach ages within the range shown in Table 7.1 die of senility. Table 7.1 Approximate life spans of some sport fishes. Kind of Fish Re gions Life Span in Years Largemouth Bass North, Central, and South 14 to 16 9 to 12 Smallmouth Bass Same as largemouth Walleye North South 15 to 16 10 to 12 Northern Pike North 16 to 17 Muskellunge North 16 to 17 Crappie 4 to 7 Bluegill 5 to 8 White Bass 2 to 5 Problems of Measuring Natural Mortality It is usually quite impossible to observe much more than casual activities of the larger aquatic animals in the smallest and clearest ponds; therefore, direct observation is presently of little importance for obtaining Natural Mortality 203 information on numerical changes in a fish population. This does not minimize the value of underwater observations for many other purposes. A comprehensive measurement of fish mortality, however, requires either a comj)lete inventory of a fish population at specific intervals (which is usually impossible) or a mathematical approach, either where returns from marked fishes over several seasons are employed to estimate natural losses for the entire population during that time, or where num- bers of fish caught (separated into age classes) are used with data on eflFective eflFort, to estimate natural mortalities.'*^ A dependable creel census is essential for furnishing information on fishing mortality. If successive annual broods of young of a given kind of fish were nearly the same size, a comparison of the numerical sizes of all of the year classes present in a lake in any year would give accurate information on total annual mortality over the life span of that species. Thus a consideration of the relative abundance of successive year classes of a species in any mixed population may show total mortality, although it may show little more than length of life of that species in the lake in question. If fish could be marked in sufficient numbers, a measure of the returns from fishing for these marked fish over a period of several years would give an estimate of natural mortality. This method was developed by Ricker ^^ who investigated the mortality rates of bluegills in several Indiana lakes, by fin marking these fish prior to the opening of the fishing season (June 16) in two or more successive years. Catch records of marked fish over a period of two or more years furnished data on rate of exploitation. The relationship between the number of fish captured and marked the first year, and recaptured in the first and second years, allowed calculations of total mortality from which angling mortality could be subtracted to give nat