Cultured oysters, Crassostrea virginica, genetically selected for fast growth in the Damariscotta River, Maine, are resistant to mortality caused by Juvenile Oyster Disease (JOD)

Growth and mortality of hatchery-produced juvenile oysters, Crassostrea virginica, selected for fast growth were compared with unselected, wild oysters at sites in Maine (Damariscotta River) and Massachusetts (North Bay) where Juvenile Oyster Disease (JOD) is enzootic. Over the course of the study, JOD occurred primarily in Maine, even though prevailing temperature and salinity at both sites were conducive for JOD development. From July to November 1996, mean shell height of selected oysters was greater than that of wild oysters at both sites. At the end of the study, mean shell height of both selected and wild oysters was greater in Massachusetts than in Maine. Mean cumulative mortality of both groups was greater in Maine than Massachusetts. Only in Maine was mean cumulative mortality of wild oysters greater than that of selected oysters. Differences in growth and mortality of oysters between sites were due primarily to the differential timing and severity of JOD occurrence. In addition, the difference in survival between selected and wild oysters in Maine was not related to differences in size between groups at the time of initial exposure to JOD. Thus, we conclude that occurrences of JOD are site specific (not dependent on source of seed), and that under challenge from JOD, selected oysters not only grow faster than wild (unselected) oysters, but exhibit a genetically based tolerance of this disease.