Powell et al.: Modeling oyster populations 



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4 5 6 7 

 Size Class 



4 5 6 7 

 Size Class 



Figure 16 



The seasonal changes in size-frequency distribution of the 

 Crassostrea virginica population depicted in Figure 15. Figure 13 

 gives comparable results for the comparable simulation depicted 

 in Figure 12. 



lations in Chesapeake Bay were more susceptible to 

 population crashes than those in Galveston Bay. 

 Simulated populations in Galveston Bay consis- 

 tently had higher population densities after 6 years. 

 Reproductive effort was higher because more of the 

 year occurred within the temperature range condu- 

 cive to spawning. Higher reproductive effort bal- 

 anced a larger rate of mortality; hence mortality 

 rates had to be substantially higher in Galveston 

 Bay to effect a population crash. Although not simu- 

 lated, recovery rates should have been faster as well. 

 Like the distinction between winter and summer 

 mortality, this latitudinal gradient in population 

 stability would appear to result from the basic physi- 

 ology of the oyster. The fundamental physiological 

 mechanisms associated with reproduction and the 

 division of net production into somatic and reproduc- 

 tive growth would appear to be responsible. 



Implications for fisheries management 



The methods for managing the C. virginica fishery 

 are generally limited to three somewhat intercon- 

 nected decisions: 1) what size limit should be set; 

 2) what season should be allowed; and 3) what popu- 

 lation density should trigger season closure? The 

 setting of size limits may depend on biological and 

 economic issues. Only biological issues will be con- 

 sidered here. Two aspects of oyster physiology are 

 most important in determining size limits. 



First, under conditions of crowding and at lower 

 latitudes, oysters fail to grow to large size. The 

 former is due to food-limiting conditions. The latter 

 is due to warmer temperatures resulting in the 

 shunting of net production into reproductive growth 

 (Hofmann et al., in press). A considerable body of 



data supports food limitation in oyster 

 populations, from aspects of spatial dis- 

 tribution (Powell et al., 1987), to reduced 

 growth in crowded locations (Osman et 

 al., 1989), and the observation of in- 

 creased growth coincident with high 

 mortality (Crosby et al., 1991). A latitu- 

 dinal gradient in size bespeaks of the 

 importance of temperature in determin- 

 ing the degree to which net production 

 is allocated to somatic growth (Hofmann 

 et al., in press). Both phenomena are 

 reproduced by the model. Clearly, in ei- 

 ther case, the setting of size limits as 

 currently done has the effect of artifi- 

 cially reducing yield. If economic consid- 

 erations warrant it, lower size limits 

 should be set in these populations. In 

 crowded conditions, adult mortality 

 might even increase adult size and yield. 

 Second, raising size limits increases population 

 density and, under certain conditions, the resulting 

 increase in reproductive effort can eventually result 

 in an increased number of market-size oysters at the 

 larger size limit. Such conditions are met in popu- 

 lations of relatively low density where oysters of 

 legal size are already abundant. Of importance is 

 the recognition that this condition occurs only in 

 populations suffering a relatively high degree of 

 mortality relative to the recruitment rate. Many 

 other agents of mortality, besides the fishery, are 

 important in oyster populations and these agents 

 generally do not respect legal size limits. The model 

 suggests that raising size limits will only be effec- 

 tive if the fishery is the predominant cause of mor- 

 tality in the population or if other agents of mortal- 

 ity are generally restricted to these same size 

 classes. If all adults are affected, then raising size 

 limits will be ineffective. 



Besides the setting of size limits, management 

 policy normally includes a restriction of the fishing 

 season. Fishing seasons on public grounds are gen- 

 erally restricted to the winter months. In some 

 cases, certain areas are set aside for a summer sea- 

 son as well. Natural mortality rates are high in 

 oyster populations, generally greater that 70% per 

 year (Mackin, 1959). Oyster populations in the Gulf 

 of Mexico withstand this degree of mortality with- 

 out long-term population declines. In this sense, the 

 populations are stable (other species are stable at 

 much higher mortality rates, e.g. Zonneveld [1991]). 

 Rates of recruitment are sufficient to balance mor- 

 tality over the long term. Nevertheless, population 

 declines do occur (Sindermann, 1968; and others ref- 

 erenced previously) and these have, on occasion, 



