106 ILLINOIS BIOLOGICAL MONOGRAPHS [106 



there is still a great abundance of cold water rich in oxygen, but every 

 meter; every centimeter that the oxygen level creeps upward, the amount 

 of cold, oxygen-abundant water is cut down, and the fish is forced into 

 water of increasing warmth. In the case of very hot summers, when the 

 thermocline ascends unusually high, the cisco is forced into water far 

 warmer than that which it ordinarily selects and to which it is therefore 

 best adapted. The result is often disastrous. Dead fish begin to appear 

 floating on the surface, fish perfect in every detail, unscarred, unpara- 

 sitized, uninjured. Dozens of the fish I have picked up just as they reached 

 the surface, to find them still alive though barely active. If the wind is 

 blowing the shores of certain lakes, particularly Pine, Okauchee and 

 Oconomowoc, become lined with dead fish. I was called to investigate 

 such an "epidemic" among the cisco of Pine lake in August 1917. On the 

 west (windward) shore I counted 72 dead fish in less than 100 feet of shore 

 line, and this was by no means exceptional. As the result of my survey 

 at that time I estimated that no less than 175,000 ciscos died within a 

 week in that lake alone. This is significant when one considers what I have 

 already said about the abundance of the fish in the lake. Similar catas- 

 trophes have occurred in Oconomowoc, La Bella, Okauchee and Silver 

 lakes, though in no other lakes have the numbers run so high. Okauchee 

 lake has come the closest to the record set by Pine and it ranks second 

 in the abundance of the species in its waters. It is significant to note, also, 

 that it is the larger fish which are most affected, suggesting a greater 

 adaptability, resistance and vitality on the part of the young fish. It is 

 further to be noted that the mortality is highest among the females which 

 at this season of the year already have a considerable mass of spawn in 

 the body, and this may explain the excess of males over females to which I 

 have already alluded. There is no remedy for these "epidemics": they are 

 caused by the fish being forced into the warm upper stratum of water by 

 the rise in the thermocline, and the fishes are simply not physiologically 

 adapted to warm water conditions. For the purpose of determining the 

 effect of temperature upon Leucichthys artedi, several fish were placed in 

 a large tank on February 3, 1924. The temperature of the water was 

 noted, as were the gas conditions, and then warm water from a boiler was 

 permitted to enter the tank slowly at one end. The results are shown in 

 figure 16. Here it will be seen that the fish avoid water above 17°C if 

 possible, but will stand a temperature of several degrees above this. 

 At 26°C they begin to float at the surface; this is very close to the tem- 

 perature encountered in nature after a prolonged, quiet, hot spell of a hot 

 summer. The pH readings throughout the experiment showed no indication 

 that the hydrogen-ion concentration was concerned with the reaction of 

 the fishes; the same was true for oxygen. Since the fish require more 

 oxygen with a rise in the temperature of the water, the amount varying 



