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Fishery Bulletin 92(2). 1994 



ence winter (Fig. 13B) rather than 

 summer (Fig. 13D) mortality. The shift 

 to smaller individuals in populations 

 with summer mortality results in low- 

 ered reproductive effort. Hence, lost in- 

 dividuals are not replaced in the fall 

 and winter and the population declines. 



As mortality extends into the juve- 

 nile size classes, the difference in win- 

 ter and summer mortality should de- 

 crease and the seasonal shift in size- 

 frequency as a function of mortality 

 should disappear because a greater 

 fraction of the total mortality occurs in 

 individuals contributing relatively little 

 to the population's spawning potential. 

 This is confirmed by the model (Fig. 14, 

 Table 6). Interestingly, although 

 the seasonal variations in size-fre- 

 quency distributions are muted, 

 changes in size-frequency distri- 

 bution over the year are still 

 greater for populations that expe- 

 rience winter mortality. These 

 populations show a slight shift to 

 smaller size classes in the winter. 



To examine the effect of varying 

 food supply, we placed the spring 

 and fall blooms one month later in 

 the year (April/May and August/ 

 September) and then compared 

 the time development of oyster 

 populations suffering winter or 

 summer mortality with those pre- 

 viously described when the 

 blooms occurred one month ear- 

 lier (Tables 6 and 7). For popula- 

 tions experiencing winter mortal- 

 ity, delaying the spring and fall 

 blooms by one month (Fig. 2C) 

 does not significantly change the 

 simulated populations from those 

 obtained for the earlier blooms, 

 even when mortality extends to 

 the juvenile size classes (3 and larger). However, for 

 summer mortality, delaying the blooms by one 

 month dramatically improves the population's abil- 

 ity to sustain itself (Fig. 12 vs. Fig. 15; Table 5). 

 Moving the spring and fall blooms one month later 

 in the year produces 1) a strong spring spawning 

 pulse as well as the fall pulse and 2) a shift in the 

 population size-frequency distribution toward the 

 larger size classes, although yearly changes in the 

 size-frequency distribution are still characteristic of 

 summer mortality (Fig. 16 vs. 13). As a result, 



spawning effort increases under the delayed-bloom 

 condition as fall spawning extends beyond the sum- 

 mer season of mortality. Accordingly, variation in the 

 timing of food supply, under certain circumstances, 

 can be important in the success of an oyster popu- 

 lation, particularly in cases where adult mortality 

 is restricted to the summer months. 



Size limits for the fishery 



Three size limits have been used or considered as 

 the legal limit for market-size oysters: 2.5 in, 3.0 in, 



