Powell et al.: Modeling oyster populations 



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Size Class Size Class 



Figure 1 



A comparison of the final size-frequency distributions (day 

 2,160) in simulated Crassostrea virginica populations 

 exposed to a Galveston Bay temperature time series fol- 

 lowing 6 years of recruitment, growth, and mortality un- 

 der varying degrees of recruitment and mortality. In each 

 case, size classes 5 and larger were exposed to continu- 

 ous mortality at a yearly rate of (H) 50%, (A and G) 75%, 

 (B) 90%, (C and E) 99%, (D and F) 99.9%. Recruitment 

 was tenfold higher (or larval mortality tenfold less severe) 

 in E and F. Mortality rates extend down into size classes 

 3 and 4 in G and H. Further information in Figure 3 and 

 Table 2, cases 11, 12, 16-21. 



and 3.5 in. These correspond to size classes 5, 6, 

 and 7 in the model. The simulations used to test 

 the effect of these size limits were initialized with 

 a population size-frequency distribution having a 

 component in the larger size classes (Fig. 12). 

 With a yearly mortality rate of 99%, oyster popu- 

 lations increase when mortality is restricted to 

 size class 7 and larger (3.5 in) but decline rapidly 

 if mortality includes size classes 5 and 6 (2.5 in) 

 (Fig. 17). Hence, a change in the legal size limit 

 may have a substantial effect on the fishery and 

 on the oyster population as a whole. Of course, 

 the specific results would vary according to the 

 biomass-to-length conversion used. 



As the fishing season typically is confined to the 

 winter, we examined the effect of changing size 

 limits when mortality was restricted to the win- 

 ter or to the summer months (Fig. 17). Overall, 

 the same pattern persisted in both seasons. Popu- 

 lations declined more under the smaller size lim- 

 its. However, several significant differences are 

 also observed: 



1 Populations in which mortality was restricted 

 to the summer had a stronger spring spawn- 

 ing pulse; most spawning occurred in the mid- 

 summer and early fall in populations suffering 

 only winter mortality. 



2 Reproductive effort and population density was 

 consistently higher in populations suffering 

 winter mortality (Fig. 18, C, D, and E), density 

 by a factor of 2 to 4, reproduction by a factor 

 of 2 to 8; increased reproductive effort occurred 

 both because the number of adults increased 

 and because those adults spawned more with 

 the result that reproduction was more than 

 proportionately higher. 



3 The size-frequency distribution was shifted 

 toward the smaller size classes in populations 

 having winter mortality (Fig. 18, A and B) but 

 had little impact on the size-frequency distri- 

 bution with summer mortality. 



Overall, the number of market-size oysters 

 available at the end of the simulation was higher 

 at the larger size limits (Fig. 18F). As a result, a 

 greater potential yield was available to the fish- 

 ery at the larger size limits. One reason for the 

 higher yield available to the fishery at the larger 

 size limit (>3.5 in) is the shift in size-frequency 

 distribution toward larger size classes with adult 

 mortality. A second reason is the protection of a 

 larger portion of the reproductive population. 

 However, if unchecked, the continually growing 

 population in the last set of simulations, where 



