32 



NORTHERN FISHES 



depleting the necessary breeding stock. In some Minnesota lakes 

 anglers take as much as twenty pounds per acre annually, while in 

 others, including many of the northern lakes, the annual catch is less 

 than three pounds per acre. Obviously, it is important that this yield 

 limit be determined, and it will probably be necessary to limit the 

 amount of fish taken from a lake to conform with it. 



Several factors within the fish population may prevent a lake from 

 producing the maximum of the species to which it is best suited. Sur- 



Minnesota Conservation Department crew inakiiii: a ]i>li census of a trout 



stream by the shocking method. The fishes are temporarily stunned by 



electricity passing from the electrodes held by each of the outside men. 



veys should include a study of the existing conditions of the fish popu- 

 lation, not only to determine the species present but also to determine 

 the predominating age groups and the growth rate of the important 

 species. The ratio of the forage fishes to the predacious fishes must be 

 worked out, since forage fishes are a basic food supply for predacious 

 fishes. The ratio between the two-year, three-j'car, four-year, and 

 other age groups of the predominating fishes must be determined. 



Some lakes are overpopulated with small fishes and have too few big 

 fishes. In many fertile lakes that support a heavy fish population, 

 heavy selective fishing keeps the larger individuals of the game-fish 

 population reduced to a low level. However, population balances are 

 difficult to regulate, because such regulation involves restricted fishing. 



If fishermen could be induced to remove small game fishes in propor- 

 tion to the larger game fishes they catch, the competition within the 



