366 



Fishery Bulletin 92(2), 1994 



Galveston Bay 



12 18 24 30 36 42 48 54 60 66 72 

 Julian Month 



12000 



n 8000 



TO 



D 



■o 



> 



■o 



IV 



a 



12 3 4 5 

 Julian Year 



1 23456789 10 

 Size Class 



Figure 1 5 



Simulated time development and population distribution of a Galveston Bay 

 Crassostrea virginica population exposed to summer mortality at a yearly 

 rate of 99.9% restricted to size classes 5 and larger and in which the food 

 time series contained blooms in April/May and September/October. Figure 

 12 contains comparable results in which the food time series contained two 

 blooms one month earlier. For additional information, see Figure 3 and Table 

 2, case 41. 



classes in the winter, hence losses are minimized. 

 Juveniles grow rapidly to adulthood in the spring 

 and spawn in the summer. As a result, reproductive 

 effort is higher and population stability is enhanced 

 when mortality is restricted to the winter. 



One of the interesting observations from the simu- 

 lations is the consistent difference in the seasonal 

 shifts in size-frequency distribution exhibited by 

 populations suffering adult summer or winter mor- 

 tality. Populations impacted most significantly by 

 summer mortality had relatively stable size-fre- 

 quency distributions over the year. Winter mortal- 

 ity produced strong seasonal shifts in the size-fre- 

 quency distribution. The results suggest that sea- 

 sonal shifts in size-frequency distributions might 

 provide a useful measure of the relative importance 

 of summer and winter mortality and of adult mor- 



tality in oyster populations. For example, the sea- 

 sonal cycle in market-sized individuals on some 

 Galveston Bay reefs (e.g. Figure 2.1 in Quast et al., 

 1988) is similar to the seasonal shifts observed in 

 simulated populations in which mortality was re- 

 stricted to the winter months, suggesting that the 

 fishery might be an important source of mortality 

 in these populations. 



Latitudinal gradient in stability 



Although not conclusive, the literature reviewed 

 earlier suggests a latitudinal gradient may exist in 

 oyster population stability. Populations at higher 

 latitudes may be more susceptible to population 

 crashes. The Galveston Bay and Chesapeake Bay 

 simulations support this possibility. Simulated popu- 



