SIZE-DEPENDENT MORTALITY IN HATCHERY-REARED POPULATIONS OF OYSTERS, CRASSOSTREA-VIRGINICA, GMELIN-1791, AFFECTED BY JUVENILE OYSTER DISEASE

Recurring mortalities of hatchery reared juvenile oysters in the Damariscotta River, Maine, caused by juvenile oyster disease (JOD), prompted a monitoring program to track growth and survival of juvenile eastern oysters, Crassostrea virginica, during the summer of 1993. Three cohorts of oysters ranging in initial mean shell height from 12.1 to 25.9 mm were deployed on July 16 in growout trays adjacent to a commercial shellfish nursery operation. Mortalities commenced August 18 and continued beyond September 21, 1993 in all three cohorts. Dying oysters displayed cupping of the left valve, unequal shell growth with the left valve outgrowing the right, tissue emaciation, mantle retraction and conchiolin deposition on the inner shell surfaces. Cumulative mortality was highly correlated to mean cohort size (shell height). The smallest size class (Cohort 3) suffered the highest cumulative mortality (56.2%); the largest size class (Cohort 1) experienced relatively low cumulative mortality (13.6%). The results of this study suggest that the impacts of JOD can be minimized by attaining mean shell height of >25 mm prior to mid-July (requires early spawning) or by deploying seed after mid-August (requires overwintering).