Prevalence of Perkinsus marinus in the Eastern Oyster, Crassostrea virginica in Relation to Tidal Placement in a Georgia Tidal Creek

This experiment was designed to evaluate the effects tidal zonation and bot· tom placement of the eastern oyster, Crassostrea virglnica, have on the prevalence and intensity of the oyster parasite, Perkinsus marinus, a suspected causative agent for sub· tidal oyster mortalities experienced In the coastal waters of Georgia. Twelve grow-out bags (1 m x 0.5 m; 12.7 mm mesh), each containing 200 oysters, were placed in Jointer Creek, Georgia in March 1992. Bags (3 replicates each) were placed intertidally and subtidally on the creek bottom and off-bottom. Ten oysters per bag were removed monthly for twelve months beginning March 1992, and were inspected for prevalence and intensity of Perkinsus marinus, using the thioglycollate method. Oyster mortality and shell length data were also evaluated. Neither prevalence (p = 0.3505) nor intensity levels (p = 0.2993) of Perkinsus marinus in oysters were significantly different among the treatments. Although there were no significant differences in prevalence or intensity of the pathogen among treatments, the intertidal off-bottom treatment had the lowest values most frequently. Perkinsus marinus was present in all replicates every month. Prevalence and intensity of infection followed the typically observed pattern of maximum values in summer and fall and minimum levels in winter. Subtidal bottom oysters experienced higher mortalities (p = 0.0022), but the prevalence and Intensity of Perkinsus marinus in oysters were not significantly different between treatments. It appears therefore, that the oyster parasite, Perkinsus marinus is not the discerning factor in the higher mortalities witnessed In oysters placed subtidally on the bottom in the southeastern U.S. coastal waters.