Management of juvenile oyster disease (JOD) in Maine

Abstract

Juvenile Oyster Disease (JOD) is a syndrome that affects juvenile oysters, Crassostrea virginica during the first growing season. Signs of JOD include reduced meat weight, uneven valve growth, and characteristic conchiolin deposits on inner valve surfaces. Since 1988, JOD has been responsible for cumulative mortalities of up to 96% in the Damariscotta River, Maine. Efforts to minimize the impact of this disease on commercial oyster production have involved both short-term and long-term approaches. Initial research revealed that mortality caused by JOD was inversely related to oyster size. Oysters with a mean shell height of 12.1 mm had a cumulative mortality of 56.2% while larger oysters (25.9 mm mean shell height) had a cumulative mortality of 13.6%. A subsequent study determined that mortality caused by JOD was seasonal in nature; cohorts placed in the river before June or after mid-August had cumulative mortalities <20% while those deployed between June and August had cumulative mortalities of 64-96%. Thus short term management strategies involve early spawning and deployment to achieve maximal size prior to the onset of disease. Longer term management has been accomplished through genetic selection. Selected oysters (Flowers F sub(3)) had a cumulative mortality of 11.2% compared to 95.7% for unselected oysters. Ultimately, further management strategies will depend on the identification of an etiological agent. Recent experiments showed that oysters exposed to antibacterial agents had a lower cumulative mortality (55%) than control groups (81%). Further, bacteriological analysis revealed that a novel alpha-proteobacterium is numerically dominant in oysters exhibiting signs of JOD and not detected in healthy oysters. Challenge experiments with this suspect pathogen are ongoing.