26 NORTHERN FISHES 



larger individuals of its own kind. Cannibalism is natural among game 

 fishes. The abundant lower age groups must be thinned out to allow 

 normal growth, and the natural agents are the large predacious fishes. 



Each year game fishes produce countless millions of eggs in Minne- 

 sota lakes. Thousands of these eggs hatch into fry. The question is 

 what happens to these fry. Preliminary studies on sunfishes and 

 crappies have shown that only a very small number of fry. ranging up 

 to 3 per cent, survive to become fingerlings, or one-year-old fishes. 

 From 10 to 75 per cent of the fingerlings survive to become two year 

 olds, and from 10 to 40 per cent of the two year olds survive at the end 

 of the third year, at which time most of these fishes reach what might 

 be called adult size. The results of these studies clearly indicate that a 

 large number of fry must be produced annually, since only a very small 

 fraction will survive to become adults. In an analysis of perch popula- 

 tions one sample of 15,000 perch taken from Ottertail Lake in 1939 

 contained only a few fish over one year of age. Assuming that the 

 present ratio of age classes indicated the survival of past years, only 

 2.8 per cent survived to the second summer, 2.0 per cent survived to 

 the third summer, and 1 per cent reached the fourth summer. 



In Lake Winnibigoshish thousands of adult walleyes have been tagged 

 annually since 1937. Although not entirely of the same age these tagged 

 samples may be assumed to be a cross section of the adult population. 

 The percentage of survival may be determined by the returns from 

 fishermen's catches in subsequent years. In 1937, 3000 walleyes were 

 tagged. In that season approximately 13 per cent were caught by fisher- 

 men; 5 per cent were caught in 1938; 3.5 per cent were caught in 1939; 

 and 1 per cent were caught in 1940. These diminishing returns are 

 indicative of the rate at which the 1937 adult population decreased. 

 These catches account for a total of only 23 per cent of the original 

 population. What has happened to the other 77 per cent? There has 

 been no serious epidemic of dead fishes, so it is reasonable to assume 

 that natural mortality has gradually accounted for these losses. Popu- 

 lation samples tagged in subsequent years show a similar decline. 



In experimental work where all factors were favorable for growth, 

 walleyes and both largemouth and smallmouth bass have been reared 

 to a weight of 1 pound in one year, whereas it takes about three years 

 for them to reach this weight in an average lake. It is possible for fish 

 to grow at this rate in lakes, but not under the natural conditions exist- 

 ing in the average Minnesota lake, where the population is highly 

 mixed in both sizes and kinds, ofl'ering an extremely complex system 

 of competition. 



This elastic growth rate of fishes would have great possibilities if it 

 could be utilized under natural conditions. No doubt it is responsible for 

 the rapid growth frequently noted in fishes newly introduced into suit- 



