20 NORTHERN FISHES 



out the winter. This lake was approximately 6 feet deep, with a rich 

 mud bottom, and contained thousands of bullheads. Late in November 

 it froze over solidly and the water was completely sealed from the air 

 by at least 12 inches of ice. Oxygen tests were made frequently through 

 holes cut in the ice, and the supply was found to be normal. Although 

 the ice became 20 inches thick, this condition continued through De- 

 cember and on into the second week in January. The plants were re- 

 ceiving enough sunlight through the ice to enable them to hberate 

 sufficient oxygen to maintain the oxygen content of the water at a 

 level equal to that of the summer. In the second week of January the 

 ice was covered with 6 inches of snow, and the sunlight could no longer 

 penetrate. Two days later oxygen )vas absent throughout the waters 

 of the lake. In the spring thousands of dead bullheads lined the shore 

 as the ice melted. Not one living fish could be found. 



In addition to oxygen depletion as the cause of winter-kills, other 

 conditions may arise and become important factors in killing fish. 

 Fish-kills have been noted under conditions of relatively low oxygen 

 in one lake, whereas in another lake of equally low oxygen content the 

 fishes survived, indicating that other factors, probably associated with 

 decomposition, such as hydrogen, sulphide or changes in carbon dioxide 

 tension, may play an important part in killing fishes. Furthermore, 

 there is no doubt that the physiological condition of the fishes may 

 determine their tolerance to these various factors. Of all these factors 

 oxygen depletion is undoubtedly the most important, and the chief 

 cause of most of our fish-kills. 



Temperature plays an important part in determining which areas of 

 a lake will be productive. Most animals and plants grow better and 

 more abundantly at high temperatures, although there are a few 

 fishes, such as members of the whitefish family and various species of 

 trout, which must have low temperatures. The temperature of the 

 water depends primarily on the depth. All the heat a lake receives in 

 the summer comes through the surface area exposed to the sunlight. 

 Two lakes of equal surface area, one deep and the other shallow, re- 

 ceive the same amount of heat, and so the deep lake never warms up 

 as much as the shallow one. Consequently we have cold-water lakes 

 and warm-water lakes, depending largely on the depth and surface 

 area. The deepest lake in the region covered by this book is Lake 

 Superior, which because of its great depth of over 1000 feet does not 

 receive enough heat units during the summer to raise its summer 

 temperature more than a few degrees over that of winter. Because 

 most food animals and plants grow faster at high temperatures than 

 at low ones, in the same length of time much more food will grow in 

 the warm water than in the cold. 



The major effect of temperature on most of our lakes is the estab- 



