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. 



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 



