368 



Fishery Bulletin 92(2). 1994 



Figure 1 7 



Comparison of the time development of 

 simulated Crassostrea virginica popula- 

 tions exposed to mortality in three differ- 

 ing size classes: (A and B), sizes 5 and 

 larger; (C and D) sizes 6 and larger; (E and 

 F) sizes 7 and larger. Cases A, C, and E 

 show the time development under condi- 

 tions where mortality was restricted to the 

 summer. Cases B, D, and F show the time 

 development under conditions where mor- 

 tality was restricted to the winter. Further 

 information in Figure 3 and Table 2, cases 

 34-39. 



been blamed on overfishing. Although 

 no adequate data are available, one 

 suspects that the fishery may be a 

 principle source of mortality in the 

 winter, but not in the summer when 

 the various other agents of mortality, 

 such as diseases and predators, are 

 active. 



Oyster populations are more resis- 

 tant to winter mortality than to sum- 

 mer mortality. The increased likelihood 

 of an intense population decline during 

 the summer observed throughout the 

 oyster's latitudinal range is a product 

 of the basic physiology of the oyster. 

 Simulated oyster populations were 

 most resistant to population declines 

 when mortality was restricted to the 

 winter months under nearly all condi- 

 tions of recruitment, size-class specific 

 mortality and food supply; they were 

 never less resistant. The simulations 

 suggest that oyster populations can 

 withstand substantially higher rates of 

 mortality in the winter than in the 

 summer and, under conditions where 

 fishing is the primary cause of mortal- 

 ity, populations should be managed 

 more conservatively during the sum- 

 mer season. 



A latitudinal gradient in stability 

 exists in oyster populations. Population 

 declines without short-term recovery 

 are more likely at higher latitudes. The 

 simulations suggest that populations 

 should be more and more sensitive to natural agents 

 of mortality and to management decisions at ever 

 increasing latitudes. In effect, populations in the 

 Gulf of Mexico, by their physiology, can withstand 

 the vagaries of nature and the mistakes of man 



Galveston Bay 



100 - 



= 10! 



f^ 



w^i 



 . V <7f .. \ . ,.. < ■■,. 



 \ .,.. J . 



J\ 



i«i 



ft ' 1 

 i i 

 i i 



■•!•■.■■■* 



- 1500 



1000 



i 



-soo 



100 - 



fl 



I ! 



n 



, fTYr> ,,,,- fTr? . >> . 



^ 



3000 

 2000 

 1000 



30 36 42 



Julian Month 



much easier than populations on the Mid-Atlantic 

 and northeast coasts of the United States. The evi- 

 dence suggests the need for more conservative oys- 

 ter management at higher latitudes. In effect, the 

 Gulf of Mexico populations and the northeastern 



