Surber. — Biological Surveys in Minnesota. 237 



that fish, but by far the greater bulk of it has been smothered by 

 sand and clay. The lake at its normal level is connected with 

 large slough-like ponds at this time filled with aquatic plants, 

 and as these ponds have always been connected with it, it would 

 seem very improbable that the carp would discriminate in their 

 favor if it alone were responsible for the destruction of the vege- 

 tation. As these ponds, with their heavy growth of vegetation, 

 are literally alive with the best natural food for young fishes, and 

 the lake itself almost barren of like food, the most important 

 problem connected with its restoration is the construction of a 

 dam at its natural outlet sufficiently high to give the fish access 

 to the ponds. 



A problem of an entirely different character is presented in 

 the case of Cannon Lake, in Rice County, through which Cannon 

 River passes. Here we find if we construct a dam to raise the 

 lake to a level for proper winter storage, we cut off all access to 

 the natural spawning grounds of the fish in a small lake below. 

 Both this small lake and the main lake are level-controlled by a 

 power dam in operation for many years at a short distance below 

 the smaller lake. Under normal weather conditions this power 

 dam maintains a fair level in the two lakes, but under the sub- 

 normal weather of the past summer the dam was inadequate to 

 control the levels, even though water was consumed for a very 

 short period weekly, and in spite of the efforts of the owners of 

 the power dam to conserve water, it subsided far below normal. 

 This seems to be the inevitable result of too much drainage on 

 the headwaters of the river itself, but fortunately occurs only 

 during seasons of long-sustained drought. 



SUMMARY. 



The lessons derived from these investigations show us : 



1. That certain streams, happily in the minority, are unsuitable 

 for restocking, and have been dropped from the list. 



2. The substitution of brown trout in stocking certain waters 



produces excellent results where repeated failures have been 

 made with brook trout. 



3. An absence of proper food for young fish, mostly semi-micro- 

 scopic food, accounts for repeated failures in restocking de- 

 pleted streams and lakes. 



4. That much of the so-called destruction of aquatic plants by 

 the carp is not borne out by the facts. 



