Fish Populations 



The fertility of a body of water and certain other conditions de- 

 scribed in the preceding chapter largely control the poundage and to a 

 certain extent hmit the species of fishes produced. However, the size 

 and the rate of growth of the fishes are determined by other factors. 



One of the outstanding problems of the fish populations in many 

 lakes is the unsatisfactory size of the dominating species. The enor- 

 mous number of stunted fishes filling so many lakes is a result of the 

 density of the population in proportion to the carrying capacity of the 

 lake. 



Swingle and Smith (1940) showed that a given fish pond would 

 yield a definite number of pounds of fish; that if the population den- 

 sity was low the growth rate was rapid. Kawajiri (1928) showed simi- 

 lar results with rainbow trout. The total available food is evidently the 

 hmiting factor. 



Under natural conditions the effect of population density upon the 

 growth rates of fishes is difficult to determine. A number of attempts 

 have been made to determine the population densities in various Min- 

 nesota lakes, but they have met with little success. The effect of popu- 

 lation densities upon growth rates must be largely determined from 

 scattered examples. 



In 1938 studies were made of the fish populations of a series of Min- 

 nesota lakes in Ramsey County. The population densities were deter- 

 mined by seining large areas and calculating the number of fishes per 

 acre. Data were secured on the bluegill populations in six lakes. These 

 lakes were all of the same general type, comparatively fertile hard- 

 water lakes in glacial drift basins ranging in total dissolved solids from 

 140 to 270 parts per million. Three lakes showed low populations, 25.34 

 to 48.63 fishes per acre, and rapid growth, the fish taking 2.82 to 3.48 

 years to reach a length of 7 inches; three showed high population den- 

 sities, 80 to 450 fish per acre, and slow growth, the fish taking 4.29 to 

 4.76 years to reach the same length. The separation into two groups 

 was definite, clearly indicating the correlation between population 

 density and growth. 



These studies explain why fishermen may catch large numbers of 

 small fishes in one lake and small numbers of large fishes in nearby 

 lakes. In Long, or Lost, Lake in Clearwater County the bass population 

 is so dense that a fisherman may get his legal limit of largemouth bass 

 within an hour, but they are small, ranging from 7 to 9 inches in length. 

 In this lake the growth is so slow that the bass are calculated not to 

 reach 12 inches in length until they are 5 1/2 years old. In nearby 



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