Temperature 



Reference has been made to the effects of water temperature 

 on the oyster's ability to exist In an unfavorable environment. At 

 temperatures below 5°C, (40°F,) , the oyster becomes physiologically 

 inactive. Its food and oxygen requirements are almost zero as long 

 as the temperature remains that low. As a result, an oyster may exist 

 in fresh water at low temperatures for a long time. As soon as water 

 temperatures rise above approximately 5°C., the metabolic activity 

 of the oyster increases and it is necessary for it to obtain food 

 and oxygen. If there is not enough food available in the water, or 

 if the oyster will not circulate water because of an unfavorable 

 condition, the oyster utilizes stored food in its tissues, and when 

 this is exhausted, the oyster dies. 



During the period December to May, water temperatures in Miss- 

 issippi Sound were continuously above 5°C. in 1949 and most of the 

 time exceeded 10°C. (SO^F.), These temperatures were abnormally 

 high. By the middle of March, temperatures of 20°C, (78°F.) were 

 common. At this temperature level, the oysters become very active 

 in the formation of eggs and sperm, and presumably require additional 

 food supplies, ^n this period, the low salinity of the water either 

 prevented the oysters from feeding or removed the natural supply of 

 food. The ill effects of fresh water on the oyster were intensified 

 by the mild winter ten5)eratures which the area experienced in 1948- 

 1949. 



Salinity 



The ability of the oyster to adapt itself to fluctuations in 

 the salinity of its environment is well developed. Since these 

 mollusks flourish naturally in estuaries they must be capable of 

 withstanding not only the increased salinities which occur during 

 times of drought but also the periodic freshening of the water 

 caused by annuel freshets and by flash floods from locali2.ed 

 increased precipitation. The oyster's resistance to changing 

 conditions is partially a result of the fact that it can close 

 its shells and seal itself off from an unfavorable environment. 

 This, however, is a temporary escape and sooner or later, depend- 

 ing on the temperature, it must reopen its shells. Observations 

 on oysters in many localities and over a period of many years have 

 shown that when salinities decline below 5 parts per thousand (ppm) 



18 



