MANAGEMENT OF WATERS 31 



Local sportsmen often want to have the rough fishes and bullheads 

 removed and replaced by game fishes. However, these lakes are not 

 very suitable for anj'^ of the game fishes, and only by continuous effort 

 can such fishes be maintained. Owing to the shallow depths and the 

 high oxygen consumption, winter-kills occur frequently. 



Because of the enormous number of lakes in Minnesota it is impossible 

 to survey every one. These types have been established for the purpose 

 of classifying the other lakes. Most Minnesota lakes, and to some extent 

 Wisconsin lakes, may be classified on the basis of a few factors, such as 

 the chemical condition of the water, the depth and area, and the type 

 of shore vegetation. A few lakes will not fit into the categories just 

 described, but these lakes are generally of intergradient types. In other 

 states local conditions may alter these categories, which must be 

 worked out for each region. 



Lake surveys determine the type of lake and the kind of fishes suitable 

 for it and also give some idea of the basic fertility of the lake. But most 

 of the lake surveys commonly used for Minnesota lakes do not determine 

 the number of fishes in the lake or the number that can be supported. 

 Determination of these figures is one of the great problems in fish man- 

 agement and is essential before an intelligent management plan can be 

 made. 



Of all aquatic animals fishes are the most difficult to study quan- 

 titatively. Fishes move about so readily that no collection obtained 

 over a definite area can be considered an absolute quantitative sample. 

 The only absolute measurement of fishes is by removal of every fish, for 

 which the only practical method would be poisoning. Obviously such 

 a measure would mean the end of the existing fish population and also 

 of further study. Sampling with seines and nets may give some idea 

 of the fish population, but the results cannot be considered more than 

 crude measurements. Most of the work done on the Minnesota fish 

 populations has of necessity been confined to sampling with seines and 

 various types of nets, giving at best only rough estimates. 



In a state that has as many lakes as Minnesota, surveys in which 

 the fishes in all the lakes are quantitatively measured are practically 

 impossible. Furthermore, fish populations change from year to year, 

 and so such a census would have to be repeated annually. However, an 

 ordinary lake survey, such as has been developed and used in Minne- 

 sota, determines the general fertility of the lake and thus gives some 

 sort of an index to the potential production of the kinds of fishes for 

 which the lake is suited. 



The total weight of fishes that can be removed from a lake without 

 disturbing the general production has never been worked out. Fish 

 managers believe, however, that there is a yield limit, just as there is a 

 limit to the amount of game that can be taken from land areas without 



