Winterkill and Summcrkill 55 



Strong year classes of white crappies during the summer months where 

 the only evidence of their death was the fact that they suddenly dis- 

 appeared from wing net catches and never again reappeared.^-' •'^' 

 Crappies often are in their poorest condition in the summer, and manv 

 apparently fail to recover. 



Summerkills comparable in extent to kills occurring under ice in winter 

 sometimes take place in shallow weed-filled lakes during the hot, still 

 nights of July and August. All of the summerkills that I have observed 

 (that did not involve pollution from outside sources) occurred after 

 periods of several days during which skies were cloudy or partly cloudy, 

 temperatures ranged in the 80's and 90's both day and night, and winds 

 were calm or nearly calm. Under these weather conditions, the dissolved 

 oxygen that may be abundant in a weed-choked lake during the daytime 

 may disappear entirely during the calm hot nights with the resulting 

 wholesale death of fishes. Usually, some fishes survive summer oxygen 

 shortages, and these may be seen gasping for air at the lake surface as 

 die first light of the approaching dawn makes them visible. Often a quiet 

 period lasting several days and nights may be broken off by a violent 

 storm which restores the oxygen supply, lowers the water temperature, 

 and stops any further death of fishes. Probably, high water temperatures, 

 darkness, and rapid organic decay in shallow weed-filled lakes combine 

 forces to produce summerkills. 



Another type of summerkill of fishes is caused by the decay of "blooms" 

 of toxic algae (usually bluegreens). These toxic algae are concentrated 

 by winds, ^^ or they develop from the stimulus of nitrates and phosphates 

 originating from organic pollutants. Death of fishes may be caused by 

 oxygen deficiencies, by toxic substances released from decaying blue- 

 green algae, or both.-^ The death of domestic stock, forced to drink die 

 alga-filled water, has been attributed to these toxic substances. 



High or low oxygen tensions produced by unusual circumstances some- 

 times will cause the death of fishes. During April, 1940, a loss of fish was 

 observed at the south end of Lake Waubesa (Wisconsin), and in the 

 Yahara River below this lake.^-^ At that time an algal bloom of Chlamij- 

 domonas was concentrated in the south end of the lake and produced 

 oxygen to a level of 30-32 ppm at the lake surface. The death of fishes 

 was attributed to the presence of gas emboli in the gill capillaries which 

 blocked blood circulation. Black crappies, bluegills, northern pike, wall- 

 eyes, white suckers, and carp were killed. 



In October, 1936, a heavy mortality of fish was reported for the Yahara 

 River below Lake Kegonsa ( Wisconsin ).-i These fish died from an oxygen 

 deficiency caused by the decay of an almost pure culture of Aphani- 

 zomenon flos-aqiiae. The fish were crowding close to shore and were 

 gasping at the surface until they finally expired. The bluegreen alga, A. 



