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 



