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 



