GROWTH AND MORTALITY OF EASTERN OYSTERS, CRASSOSTREA-VIRGINICA (GMELIN, 1791), AND PACIFIC OYSTERS, CRASSOSTREA-GIGAS (THUNBERG, 1793) UNDER CHALLENGE FROM THE PARASITE, PERKINSUS-MARINUS

Stocks of oysters Crassostrea virginica and C. gigas were produced in the Virginia Institute of Marine Science (VIMS) hatchery in April 1991 and grown in two quarantined flumes receiving water from the York River, VA. From July 1991 through November 1992, growth and mortality of both species were compared. Also, beginning in June 1992 one flume (containing one group of each species) was ''dosed'' (oysters infected with P. marinus added) and the other flume remained ''undosed'' (no infected oysters added). Disease prevalence and intensity, mortality, and shell height were compared both within (between undosed and dosed groups) and between species. Mean shell height of C. gigas was significantly greater (P less-than-or-equal-to 0.05) than that of C. virginica in all but four monthly samples. In November 1992 (at age 19 months), mean shell height of C. gigas was 55 mm and that of C. virginica was 41 mm. Cumulative mortality of C. gigas was 70% compared to 59% for C. virginica. Mortality of C. gigas occurred primarily in April-May and September 1992 in conjunction with salinity below 20 ppt; mortality of C. virginica was greatest in September-November 1992 in conjunction with infection by P. marinus. Prevalence and intensity of P. marinus infections were greater in C. virginica than in C. gigas. The dosed C. virginica group had 100% prevalence with heavy infections by August 1992; maximum prevalence in the dosed C. gigas group was 80% with only 1 heavy infection throughout the study. There were no differences in mean shell height between dosed and undosed groups of either species. Mortality in the dosed C. virginica group was significantly greater (P less-than-or-equal-to 0.05) than in the undosed group in September and October 1992. Thus even though C. gigas is more tolerant of P. marinus and grows faster than C. virginica, a high non-disease mortality may be indicative of a lack of adaptability of C. gigas to the environmental factors prevailing in lower Chesapeake Bay.