An unstable salinity regime is an important 

 ecological factor in the tidal rivers and streams 

 inhabited by C. tdrginica; diurnal, seasonal, and 

 annual fluctuations are the normal features of 

 such an environment. Their effect on C. virginica 

 depends on the range of fluctuations and the 

 suddenness of the changes. For instance, oysters 

 that were replanted in September from a low 

 salinity area of the upper Chesapeake Bay (10%o 

 to 12 7oo) to the high salinity water of Sinepuxent 

 Bay (32%o to 33 7oo) all perished within 3 to 4 

 weeks after planting. Examination of the new 

 grounds disclosed that the sudden change in 

 salinity during hot weather was the primary 

 cause of mortality. On the other hand, a sunilar 

 transfer made by the same grower succeeded in 

 October and November when the air and water 

 temperatures were much lower. 



The mean values of diurnal, seasonal, and 

 annual salinities are of little significance for 

 evaluating their effect on an oyster population. 

 The oyster can isolate itself from the outside 

 environment by closing its valves tightly and 

 survive adverse conditions, provided they do not 

 last indefinitely (see: ch. VIII). Since changes in 

 salinity are commonly associated with tempera- 

 ture changes, an attempt was made by Hedgpeth 

 (1953) to combine the two factors to express 

 what he calls "hydrographic climate." He plotted 

 ranges of means and extremes or monthly means of 

 salinity against temperature and obtained poly- 

 gons which provide a graphic representation of 

 the conditions existing in a given area for the 

 indicated period. The method appears to be 

 useful and may be profitably applied to oyster 

 research. 



Oysters inhabiting the parts of estuaries in 

 which salinity is below 10 %o are seriously af- 

 fected by fresh water and could be destroyed by 

 floods lasting for several weeks. Mobile Bay, 

 Ala., investigated in 1929, may be cited as an 

 example of this condition. Oysters in Mobile 

 Bay grew on reefs which extended from the upper 

 to the lower parts of the bay. The river discharge 

 into the bay normally resulted in a salinity 

 gradient from 5%o to 30%o. However, in the 36 

 years from 1893 to 1929 the two tributaries, tlie 

 Tombigbee and Selma Rivers, rose 27 times to 

 flood stage with the flood conditions lasting from- 

 4 to 31 days. The height of the rivers at flood 

 stage in Februarj' and April 1929 was 65.4 and 

 56.2 feet, respectively, and lasted for 32 days 



(GaltsofT, 1930) with the result that fresh water 

 prevailed over almost the entire bay and the 

 mortality at different parts of the bay varied from 

 100 percent in the upper parts to 85 and 54 percent 

 at the passes to Mississippi Sound. 



Oysters in Mississippi Sound often suffer from 

 long-continued low salinities. Mortality of oys- . 

 ters in the Sound occurs when the local precipita- 

 tion in the Sound area, in the Pearl River basin, 

 or at some more distant point in the Mississippi 

 River basin occurs more or less simultaneously 

 and lowers the salinity to a harmful level (Butler, 

 1949b, 1952). 



Freshets sometimes kill oysters in the James 

 River, Va. During a 6-week period from May 1 to 

 June 15, 1958, many of the native oysters died, 

 and as many as 90 percent perished on some 

 grounds where salinity did not become suitable 

 until July 1 (Andrews, Haven, and Quayle, 1959). 

 In a test made at the Virginia Fisheries Labora- 

 tory at Gloucester Point, oysters held in trays in 

 low salmity areas were "conditioned" to a low 

 physiological state (absence of heart beat and 

 ciliary motion and loss of mantle sensitivity). 

 The investigation lead to the conclusion that 

 oysters conditioned slowly at low temperatures 

 and low salinities can endure a prolonged situation 

 of unsuitable environment. Andrews, Haven, 

 and Quayle infer that "the mechanism of condi- 

 tioning appears to be a type of narcotization," 

 an interesting idea which, however, needs veri- 

 fication. 



The first symptoms displayed by an oyster 

 affected by water of lowered salinity are partial 

 or complete contraction of the adductor muscle 

 and slowing or cessation of water current through 

 the gills. With the drop counting technique 

 described in chapter IX it can be shown that the 

 ciUary activity of the gifl epithelium immediately 

 decreases when it comes in contact wdth water of 

 lowered salinity. The effect may be brief or 

 prolonged, depending on the degree of change 

 from the salinity level to which the oyster had 

 been adapted. When the salinity change is 

 about 10°/oo and continues for several hours, 

 both the rate of water transport and the time the 

 oyster remains open are decreased, and under 

 extreme conditions the feeding and respiration 

 cease. Experimental studies on the adaptation 

 of oysters to salinity changes were first made by 

 Hopkins (1936) on C. gigas of the Pacific coast. 

 He recorded the changes in the opening and closing 



FACTORS AFFECTING OYSTER POPULATIONS 



405 



