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This experiment was designed to evaluate the effects tidal zonatlon and bot· tom placement of the eastern oyster, Crassostrea v/rglnica, have on the prevalence and Intensity of the oyster parasite, Perk/nsus marlnus1 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 lntertldally and subtldally 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 Perkin· sus mar/nus, using the thloglycollate method. Oyster mortality and shell length data were also evaluated. Neither prevalence (p = 0.3505) nor Intensity levels (p = 0.2993) of Perklnsus marinus In oysters were slgnlflcantly different among the treatments. Although there were no sign ill· cant differences In prevalence or Intensity of the pathogen among treatments, the inter· tidal off·bottom treatment had the lowest values most frequently. Perklnsus 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. Subtldal bottom oysters experienced higher mortalities (p = 0.0022), but the prevalence and Intensity of Perklnsus mar/nus In oysters were not significantly different between treatments. It appears therefore, that the oyster parasite, Perklnsus mar/nus Is not the discern· ing factor In the higher mortalities witnessed In oysters placed subtldally on the bottom In the southeastern U.S. coastal waters. The eastern oyster, Crassostrea virgin/ca, occurs subtidaiiy along much of the eastern seaboard, but primarily in the intertidal zone In the southeastern U.S. (Dean, 1892; McKenzie and Badger, 1969; Harris, 1980; Burrell et al., 1984). Adult oysters transplanted from intertidal beds of Wassaw Sound, Georgia and placed in subtldai areas in coastal Georgia have suffered high mortalities (Oemler, 1889). The subtldal oyster mortalities have, in the past, been attributed to predation, parasitism by the boring sponge, Cliona ce/ata, and high siltation In the subtldal zone (Harris, 1980; Burrell et al., 1984; Oflara and Stevens, 1987). Recent 1Corresponding Author 79 research, however, indicates that these factors may not be the major causes of adult oyster mortalities in the s.ubtidal zone in the southeastern U.S. Adams et al. (1994) placed oysters in predator exclu· sion bags on and above various substrates in both intertidal and subtidal areas of coastal Georgia. Subtidal onbottom oysters suffered significantly higher mortalities than the other treatments. It was speculated that the oyster pathogen, Perkinsus marinus, 'Dermo', may be a contributing factor In the lower survival rates for oysters cultured subtldally on the bottom. Perkinsus mar/nus affects many 80 O'Belrn, F. X., C. C. Dean, and R. L Walker populations of Crassostrea virgin/ca along the eastern and Gulf coasts of the United States (Andrews, 1988). The parasite was detected In oyster populations in Georgia In 1966, and occurred in epizootlc proportions between 1985-1987, when massive oyster mortalities occurred along the entire coastline (Lewis et al., 1992). This study was initiated to determine the extent of the pathogen Perkinsus mar/nus In a Georgia tidal creek. It also examined the effects that bottom placement and tidal zonation had on the prevalence and intensity of the pathogen within oysters. Another perceived benefit of this study is that It will add valuable Insight into the establishment of a grow· out protocol for oysters In a coastal Georgia marlcultural operation.

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