VARYING THE TIMING OF OYSTER TRANSPLANT - IMPLICATIONS FOR MANAGEMENTFROM SIMULATION STUDIES

The transplanting of oysters from one ground to another is a common pr actice in the oyster industry. In Delaware Bay, for instance, oysters are typically transplanted from upper-bay low-salinity seed beds onto lower-bay leased grounds for growth and conditioning before market. Th e higher salinity on the leased grounds, however, also favours higher losses to predation and disease. A coupled oyster-Perkinsus marinus-pr edator model was used to investigate how varying the timing of transpl ant affects the ultimate yield of Eastern oysters, Crassostrea virgini ca, in Delaware Bay. Simulations were run in which oysters were moved from seed beds to leased grounds in November, January, March, April an d May. The number of market-size (greater than or equal to 76 mm) adul ts available for harvest in the following July to November was compare d for populations undergoing mortality from predation (crabs, oyster d rills) and/or disease (Perkinsus marinus). In all simulations, the abu ndance of market-size oysters declined between July and November. Howe ver, transplanting oysters in November resulted in the largest yield o f market-size oysters for all harvest times; transplanting in May resu lted in the smallest yield. The autumn transplant allows oysters to be nefit from the larger spring phytoplankton bloom over the leased groun ds in the lower estuary. The effect of varying the season of transplan t was most noticeable if oysters were harvested early (July or August) . In all simulations, transplanting resulted in a higher abundance of market-size oysters than direct harvest from the seed beds. Direct har vest would rarely be advantageous if the cost of transplant were insig nificant and the relative rates of mortality were as stipulated. Howev er, a May transplant is only moderately better than a direct harvest a nd the economic benefits of either option are likely to be determined by the cost of transplanting and the mortality associated with the pro cess. In the same vein, the decision as to when to harvest relies on b alancing the increased price obtained for oysters in the autumn with t he increased loss owing to predation and disease. Awaiting an autumn h arvest is clearly much riskier if the principal source of mortality is disease rather than predation, because disease mortality is concentra ted on the market-size oysters and is greatest in the autumn.