Abstract
Commercial off-bottom aquaculture of the eastern oyster Crassostrea virginica is challenged by repeated summer mortality events that appear to affect triploid oysters disproportionally. Many farmers believe common farming practices, especially when performed during hot summer months, may increase triploid mortality. The goal of this study was to investigate how diploid and triploid oysters react to common stressors imposed by farmers: tumbling during size grading and desiccation to prevent biofouling. Triploid and diploid oysters were deployed in floating cages at 3 farm sites along the US Gulf of Mexico coast. In May and July, oysters in 7 cages were subjected to 1 of 6 stress treatments: 1 of 3 levels of desiccation (18, 24, or 48 h) mixed with 1 of 2 levels of tumbling (tumbled or not tumbled), along with a never handled, submersed control. The mortality and growth rate of oysters were assessed in June, August, and September. Growth rates of both ploidies were affected by compounding stressors; that is, treatments with both tumbling and the longest desiccation period exhibited slower growth. Triploid oysters exhibited a greater increase in mortality in response to farm stress than diploid oysters, and mortality increased in all oysters subjected to extreme stress treatments when compared to the control. Based on this study, farmers should limit the desiccation time of oysters (particularly triploids) in the summer months to avoid any mortality or reduced growth resulting from compounding stressor effects.
Highlights
Eastern oysters Crassostrea virginica are grown commercially in aquaculture operations throughout the east coast of North America and the Gulf of Mexico
We found that triploid oysters experienced higher mortality rates than diploid oysters
The act of putting oysters into a mechanical grader, tumbling, could have caused this reduction in growth rate due to increased shell breakage (Ring 2012). This shell breakage may have outweighed any physiological effects the stress treatments had on growth rate
Summary
Eastern oysters Crassostrea virginica are grown commercially in aquaculture operations throughout the east coast of North America and the Gulf of Mexico. The estimated value of aquaculture production for the US eastern oyster in 2015 was $173 million (NOAA Fisheries 2016). The industry is growing quickly, especially in the northern Gulf of Mexico along the coasts of Alabama, Mississippi, Florida, and Louisiana (Casas et al 2017). Summer mortality is a concern for both diploid and triploid oysters, both of which are commonly used in aquaculture. Triploid oysters have reduced gametogenesis and rarely spawn. They expend less energy on reproduction and may grow faster and reach market size (75 mm) sooner than diploid oysters. One study indicated that whole wet weight of Publisher: Inter-Research · www.int-res.com
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