Comparative field study of Crassostrea gigas (Thunberg, 1793) and Crassostrea virginica (Gmelin, 1791) in relation to salinity in Virginia

To evaluate and compare the performance of triploid juvenile C. gigas (mean shell height = 19.2 mm) and triploid juvenile Crassostrea virginica (mean shell height = 31.7 mm), 600 oysters of each species were deployed for 1 year in floating mesh cages at three replicate sites within low, medium, and high salinity regimes (respectively, <15%, 15-25%, > 25%) in the Chesapeake Bay and the Atlantic Coast of Virginia. The comparative performance of the two oyster species varied with salinity. At low salinity sites, cumulative mortality of C. virginica (10%) was significantly (P < .05) lower than that of C. gigas (63%), and over-all mean growth rate of C. virginica (2.9 mm mo(-1)) was significantly (P < .05) higher than that of C. gigas (1.6 mm mo(-1)). At medium salinity sites, survival and growth rate of C. virginica and C. gigas were nor significantly (P > .05) different. Both species experienced moderately high cumulative mortality at the medium salinity sites-35% for C. virginica and 53% for C. gigas-but considerable variation among sires was observed. Ar high salinity sites, mean cumulative mortality was similarly low (<11%) for both species; whereas, over-all mean growth rate of C. gigas (7.1 mm mo(-1)) was significantly (P < .05) higher than that of C. virginica (3.6 mm mo(-1)). At all sites, C. gigas was less susceptible than C. virginica to Perkinsus marinus infections. Infections by Haplosporidium nelsoni were present in C. virginica and absent in C. gigas. Infestations by mud-worm Polydora spp. were more prevalent and severe for C. gigas than for C. virginica at low and medium salinity sites in October 1997, but similar for both species at other times and locations. Condition index was significantly (P < .05) higher for C. virginica than for C. gigas at low salinity in May 1998, but similar for both species for other times and locations. Crassostrea virginica outperformed C. gigas in low salinity sires in the Chesapeake Bay, C. gigas outperformed C. virginica at high salinity sites in the Atlantic Coast, and performance was similar for both species at medium salinity sites in the Chesapeake Bay.