LAKE MANAGEMENT REPORTS 



5. Winterkill of Fishes in an Illinois Lake 



George VI. Bennett 

 Aquati c Biologi st 



The death of fishes resulting from unfavorable conditions under Ice Is a common occurrence 

 In several north central states (OreenbanK 1945). In Illinois, It occurs with some frequency In the 

 north and central sections when winters are severe and the Ice on lakes and ponds Is covered with 

 snow for periods longer than about 15 days. 



The supply of oxygen for the respiration of fish under Ice Is often dependent upon the 

 ability of free-floating microscopic plants (phytoplankton) to carry on photosynthetlc activity. 

 When light that normally penetrates Ice Is excluded by a blanket of snow, photosynthesis stops, and 

 such dissolved oxygen as Is present In the water Is used up by the decay of organic matter and the 

 respiration of aquatic organisms. 



According to Greenbank (1945): "Changes In oxygen concentrations, even from week to week, 

 were definitely correlated with changes in the depth of snow on the Ice. Measurements of light In- 

 tensities showed that only a small amount of light penetrates through even a few inches of snow. 

 Unquestionably a foot or more of dry snow transmits too little light to actuate photosynthesis. On 

 the other hand. Ice, even though it Is cloudy, permits the penetration of considerable light." 



Most of the losses of fish occur during winters when snows are heavy and lie uranelted for 

 prolonged periods. In these cases, fish die and sink to the lake bottom, to appear when the ice 

 breaks up and their carcasses decay sufficiently to give them buoyancy. 



In other cases, fish have been observed to die during the period immediately following the 

 breakup of the Ice. These fish, weakened by adverse conditions under Ice, are subject to the attacks 

 of aquatic fungi (Sa^rolegnlales), which reach their maximum abundance In the warming waters of early 

 spring. The mycella of the fungi grow upon the weakened fish, producing a toxin that causes death. 

 Dying in the fish population usually stops when the water warms up to 60 or 65 degrees F. 



More Illinois reports of winterkill and post-thaw deaths of fishes from fungi were received 

 in the spring of 1945 than in any other year since 1938. Hiring the winter of 1944-45 there were two 

 periods of several weeks In duration when Ice-covered ponds in the region of central Illinois were 

 observed to be blanketed with snow. This situation was probably more or less general over central 

 and northern Illinois. 



In almost every case in which fish are killed by adverse conditions under ice, only a part 

 of the fish population dies; in no case reported In Illinois is there evidence of a complete kill. 

 However, loss of part of a fish population may constitute a greater disaster from the standpoint of 

 future fishing than death of the entire population. 



