Winterkill and Summerkill 49 



dioxide tensions below die surface, to low carbon dioxide tension at the 

 surface. These fish soon became affected by the rapid changes in carbon 

 dioxide tension and died by the millions; larger fish rising to the surface 

 from greater depths also became incapacited by sudden changes in 

 carbon dioxide tension. However, it was significant that throughout the 

 period when fish were dying, there was ample oxygen to support fish at 

 all levels. 



Investigating biologists -^ conducted laboratory experiments to de- 

 termine the cause of death of Norris Lake fish. Rock bass were placed in 

 a hardware-cloth cage and lowered to the bottom of a water-filled 10- 

 gallon carboy. The same number of fish were released in the carboy 

 outside the cage. The carboy was left open so these latter fish could 

 come to the water surface and gulp air. The water in the carboy was 

 supplied with carbon dioxide to produce a COo tension above normal. 

 Fish that were free to come to the surface of the water died before those 

 that were held in the cage, thus indicating that rapid change in carbon 

 dioxide tension from high at the bottom of the carboy to low at the surface 

 affected the fish adversely. However, rock bass in the cage were able, by 

 adjusting the alkalinity of the blood, to counteract the ill effects of high 

 but constant carbon dioxide tension, and thus to extract oxygen as 

 efiBciently as if the carbon dioxide tension were low. This situation held 

 as long as the carbon dioxide tension remained fairly constant; but when 

 the fish were forced to alternate between high and low tensions, they 

 soon lost their equilibrium and died. 



The combinations of circumstances which produce the biological 

 phenomena described above probably appear rather infrequently. More 

 common are fish deaths occurring under ice in winter and in very weedy 

 lakes during hot summer months. 



WINTERKILL AND SUMMERKILL 



The terms "winterkill" and "summerkill" are applied to sudden mortal- 

 ities of fishes which occur in winter and summer, usually as a direct 

 result of suffocation. Conditions that set the stage for a winterkill are, 

 however, very different from those which result in an oxygen deficiency 

 in a lake or pond during the summer. 



Winterkill 



In the north, winter ice forms a seal over lakes and ponds which pre- 

 vents the exchange of gases between the water-air interface. Moreover, 

 the penetration of light through ice is less than through clear water, and 

 the light may be blanketed out entirely by a layer of snow upon the ice. 

 When all photosynthetic activity is stopped because of insufficient light, 



