24 NORTHERN FISHES 



Lake Itasca, on the contrary, only a few bass are caught each year, but 

 they are 12 inches long when 3 1/2 years old. A similar situation is 

 found among the blucgills in these two lakes. The bluegills in Itasca 

 grow one and a half times as fast as those in Long Lake. 



In 1939 in North Lindstrom Lake, Chisago County, the crappies were 

 so abundant that an angler could easily catch his limit in a short time, 

 but these crappies were not over 6 inches long and weighed only about 

 73 per cent as much as crappies of the same length in other lakes. The 

 growth rate for crappies in this lake was slower than that of most other 

 lakes in the state. The bluegills were also small, weighing only 67 per 

 cent as much as bluegills of the same length in other lakes and showing 

 very slow growth. Apparently the population density in this lake had 

 become so great that growth was extremely slow. Crappies, sunfishes, 

 perch, bass, northern pike, and other species often show large popula- 

 tions of stunted fishes and all the ones examined have shown very slow 

 growth rates. 



There is another piece of evidence that population density is an 

 important element in controlling the rate of growth. It has often been 

 noted that when a species is successfully introduced into a new habitat 

 its growth is unusually good, largely because of the small number of 

 individuals present and the lack of competition within the species (Eddy 

 and Carlander, 1939) . Walleyes grow more rapidly in the northeastern 

 part of Minnesota, where they have been recently introduced, than 

 they do elsewhere in the state, undoubtedly because of the lack of com- 

 petition within the species in these lakes. 



Creel censuses on many Minnesota lakes have thrown further light 

 on this subject. With the aid of the Civilian Conservation Corps, creel 

 censuses were conducted throughout the summer of 1940 on six lakes in 

 northern Minnesota. Every fish caught was measured, and scales were 

 taken to determine the growth rate. The species studied were largely 

 walleyes, northern pike, largemouth bass, and crappies. 



In many of the lakes the catch was small, numbering from 500 to 3000 

 fishes for the season and averaging from 1 to 5 pounds per acre. How- 

 ever, in one lake studied the catch was very heavy, numbering over 

 12,000 fishes for the summer and averaging nearly 20 pounds per acre. 

 The rate of catch per man-hour was about the same in all lakes. The 

 growth rate of the fishes was equal to or slightly above the average 

 for that region in all the lakes except one that was heavily fished. In this 

 lake the growth rate was decidedly lower than the average for the region. 



From these studies it seems that population density is a factor of 

 great importance in modifying the growth rates of fishes. The produc- 

 tivity of a lake seems to determine the total number of pounds of fish, 

 but the population density seems to control the growth rate of these 

 fishes. 



Studies show that the fishes of easily accessible lakes usually have 



