FISH POPULATIONS 25 



a lower growth rate than the fishes of those lakes that are not reached 

 so easily. Accessible lakes are usually in fertile, agricultural districts 

 and are relatively fertile, whereas inaccessible lakes are usually in 

 infertile, nonagricultural districts and are relatively infertile. When 

 the lake is subject to interference by man, the amount of selective fish- 

 ing rather than the fertility of the lake appears to determine the size 

 of the fishes. 



Although the evidence is not yet conclusive, it seems that these large 

 populations of undersized fishes are associated with heavy fishing. The 

 fisherman is selective and tends to remove the larger fishes. Under nor- 

 mal conditions most fishes produce many more fry than the lake can 

 support. These larger game fishes are the natural checks upon the small 

 fishes, even of their own kind. When the large fishes are removed, the 

 small fishes have no check and so survive in large numbers, becoming 

 crowded and stunted. 



The selectivity of the angler is largely responsible for many of the fish 

 population troubles, including the important perch problem. Perch are 

 not as popular with anglers in Minnesota as they are in some other 

 states. In many of the larger lakes, such as Ottertail and Mille Lacs, the 

 fishermen's catch consists predominantly of walleyes and contains very 

 few other fishes. In these lakes the fish population consists of almost 

 equal parts of walleyes and perch, with a very small proportion of other 

 fishes, even the northern pike. These proportions do not constitute a 

 highly desirable balance and can be attributed only to the heavy, one- 

 sided fishing for a single favorite. Such methods are bound to create un- 

 desirable population balances and must be corrected. 



Experimental work carried on in other states shows that the num- 

 ber of pounds of the particular kinds of fishes supported by a given 

 body of water is limited by its fertility and that more pounds of rough 

 than of game fishes can be supported per acre. Under a given set of 

 conditions the poundage remains the same, no matter what the num- 

 ber of fishes. Accordingly, if the fishes are numerous they are stunted, 

 or undersized; if there are few individuals, the poundage remaining 

 the same, then the fishes are large. 



David H. Thompson (1940) has found that he can increase the an- 

 nual production of fishes in small Illinois lakes or ponds by intensive 

 fishing, removing large numbers of fishes of all sizes and thus releasing 

 the population pressure so that the remaining younger fishes can grow 

 more rapidly. Apparently this principle operates effectively where the 

 fish population is not highly mixed in regard to species. The indiscrimi- 

 nate removal of all age groups by fishing is equivalent to the natural 

 control exercised by large predators. 



Fishes have enormous reproductive potentials. Ordinarily they pro- 

 duce more eggs and fry than the lake could possibly support if all the 

 fry grew to adults. Usually the worst enemies any game fish has are the 



