Improvement of Lakes and Streams 



Most of the problems in fish conservation are concerned with the 

 factors that cause poor fishing. Because of the ever-increasing pressure 

 by fishermen, maintenance of good fishing conditions becomes im- 

 perative, and the improvement of waters for better fishing is one of the 

 fundamental phases of fish management. Lake improvement consists 

 of such activities as improving the spawning conditions, the carrying 

 capacity, and the conditions for meeting the specific needs of the 

 fishes for which the lake is managed. 



As we have seen, food production is one of the most important fac- 

 tors in determining the amount of fish a lake will produce, and attempts 

 have been made to increase the natural fertility. It is possible to in- 

 crease the fertility of many lakes by adding commercial fertilizer or 

 even ordinary barnyard manure. The proper use of these substances 

 increases the productivity of the lake in food and consequently in 

 pounds of fish. In Europe the fertilizing of fish ponds has been practiced 

 for centuries. Wiebe (1931) and others have shown that certain com- 

 mercial fertilizers will increase the yield in rearing ponds. This is now 

 a standard practice in pond operations. A few Minnesota lakes have 

 been fertilized, but no check has been made to determine the results. 



Many northern lakes are too far from the source of fertilizers to 

 justify the expense of fertilizing. Moreover, in Minnesota the extent 

 of some lakes that could otherwise be profitably fertilized is so great 

 that the expense is prohibitive. However, there is some consolation in 

 the fact that increased fertility would increase the winter consumption 

 of oxygen, and in a region of severe winters too much plant life in a 

 shallow lake would be undesirable. 



In some waters the food supply may be increased by introducing 

 forage fishes that do not compete with those already there and so do 

 not disturb the balance. Such a procedure has been followed in some 

 of the northern trout lakes where tullibee were absent. Lake Superior 

 ciscoes, which feed on material that was not being greatly utilized by 

 the trout, have been introduced to provide additional food for the 

 trout. Although the ciscoes were introduced only a few years ago, 

 they have multiplied to such an extent in some lakes that complaints 

 have been made that the trout are so well fed that they will not bite. 

 This step is only the first one, but it seems to indicate the successful 

 results of this procedure. The next step will be to increase the trout 

 population, and eventually trout fishing should be greatly improved. 



A good many remedies have been suggested to prevent the serious 

 problem of winter-kills, which occur when the water in a lake does not 



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