IMPROVEMENT OF LAKES AND STREAMS 37 



Minnesota and some other northern states cannot follow this plan 

 because of the destructiveness of carp in northern lakes, long proved by 

 their devastation of vegetation, feeding grounds, and the spawning beds 

 of other fishes. In this waj^ the carp have already eliminated most other 

 fishes from some southern Minnesota lakes. The only sure way of ex- 

 terminating the carp would be to kill every fish in a body of water with 

 dynamite or with some powerful poisonous chemical. This procedure 

 is expensive and does not insure against the reintroduction of carp, 

 which may occur through the use of young carp as bait in a fisherman's 

 pail. The present method in general use is to control them by seining, 

 by trapping on their favorite spawning beds, and by screening inlets 

 where they may be gaining access from other waters. They are exceed- 

 ingly prolific, which adds to the diflficulty of controlling them. 



Proper vegetation in a lake is important for successful maintenance 

 of fishes. Vegetation not only shelters smaller fishes but is the basis 

 for the food of all fishes. It is an important agent in oxygen production. 

 Many lakes have their shore waters cleaned of all types of vegetation, 

 particularly in front of cottages and around bathing beaches. These 

 clean sand bottoms may be ideal for bathing but they are deserts as 

 far as fish food and fry production are concerned. 



Some lakes may have too much vegetation. The vegetation may 

 become so thick that it chokes the open waters and prevents free 

 movement of the fishes. It may result also in an accumulation of decay- 

 ing plant waste during the winter, which uses up the oxygen and causes 

 winter-kills. In some lakes conditions may be improved by restoring and 

 increasing areas of vegetation; in others, by removing vegetation and 

 restricting its growth. 



Low and fluctuating water levels are one of the most serious obstacles 

 to fish production in many of the northern lakes. Some lakes have had 

 their levels reduced by drainage; others have had their summer water 

 supply reduced by the removal of surrounding forests, which had acted 

 as reservoirs; and many are subject to great fluctuations in their levels 

 because of power dams. 



In many cases lowered water levels mean that the volume of water 

 has been so reduced that moderately shallow lakes are in danger of 

 winter oxygen depletion. Some lakes lose their fertile marginal bottoms, 

 which are replaced by less productive muds of the deeper bottom 

 deposits. Lakes subject to frequent fluctuations caused by power dams 

 are always in danger of having important spawning beds left high and 

 dry at a critical time. They are also subject to frequent destruction of 

 their most important food production areas. Under such conditions no 

 lake can produce its maximum yield of fish. In order to maintain the 

 maximum fish production every effort should be made to keep lake 

 levels normal and to prevent detrimental fluctuations. 



Streams differ from lakes mainly in the conditions caused by the 



