226 Fish Behavior and Angling 



On the afternoon of July 12, 1952 the water in this pond reached a 

 temperature of 38°C (100.4°F) and fish began to die without showing 

 symptoms of oxygen shortage. Not all of the fishes died and several days 

 later there were still live killifish, and gambusia (a southern species) as 

 well as bluegill, pumpkinseed, longear and green sunfish, all in apparent 

 good condition. Most of the mud minnows, chubsuckers, and several 

 species of minnows, i.e., golden shiner, blacknose shiner, blackchin shiner, 

 sand shiner, and bluntnose minnow were all dead, as were the brown and 

 yellow bullheads, stonecats, madtoms, and Iowa darters. The smaller 

 individuals of some species survived while the larger ones died; this was 

 true of the minnows, catfish and madtoms. 



The problem of lethal temperatures is very complicated because it is 

 dependent upon the previous acclimation of test animals, and is affected 

 by diet, length of photoperiod and probably other factors related to the 

 physiology of the fish. Thermal deaths may be complicated by oxygen and 

 carbon dioxide tensions, mineral content, and salinity of the water.^* 

 Therefore, when high temperature mortality occurs out of doors as 

 described by Bailey above, one can hardly explain why some fish died 

 and some survived. 



Temperature Acclimation 



Fish in pools that are drying up may die of high water temperatures, 

 or high or low dissolved oxygen tensions, although these deaths usually 

 go unnoticed. The temperature at which a fish dies (both high and low) 

 is affected by the temperature to which the fish was acclimated prior to 

 exposure. Brett ^^ lists upper and lower lethal temperatures for a number 

 of freshwater fishes when these fishes previously had been acclimated to 

 various temperatures. Lethal temperatures for three warm-water fishes 

 listed by Brett are given in Table 8.1. 



The changes in a fish's ability to tolerate low or high temperatures 

 resulting from a change in acclimation temperature, probably does not 

 have a great deal of survival value under natural conditions. It has been 

 shown by Brett and others that the rate of increase in ability to tolerate 

 higher temperatures is relatively rapid, requiring 24 hours at temperatures 

 above 20°C (68°F). Conversely the loss in this increased tolerance to high 

 temperatures, and a gain in resistance to low temperatures is much slower, 

 requiring as much as 20 days to approach complete acclimation in some 

 species. Thus, sudden drops in air temperatures that cause rapid and 

 abnormal cooling of natural waters might cause the death of fishes 

 acclimated to warm waters, while warm-air temperatures resulting in 

 rapid and abnormal warming of cold waters might have little effect be- 

 cause the fish could become acclimated to higher temperatures almost as 

 fast as the water temperature could increase. 



