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. 



