Addressing Cultured Oyster Mortalities: Project Results

The new off-bottom oyster aquaculture industry in Florida faces many challenges, such as limited seed availability and year-round need for biofouling control. Risks, such as storms and hurricanes, and economic feasibility are still being assessed. Further, recent unexplained oyster mortalities experienced by growers in late spring and early summer have resulted in losses of 50 to 90% of adult oysters reaching market size. This is not just a Florida issue but a problem facing growers in the Gulf of Mexico and Southeastern US.

In response to industry concerns, a monitoring and assessment plan was initiated in 2020 to examine relationships between production and health of cultured oysters and key environmental factors. The objectives were three-fold: monitor oyster production at commercial farms; examine water quality parameters and phytoplankton biomass; and assess external and internal parasitism.

“Sentinel” farms were established at two lease locations, Alligator Harbor and Oyster Bay. Triploid seed, produced using males from the traditional LA tetraploid stocks and the newly developed Florida tetraploids, were provided to two growers at each lease location. Oysters were sampled at bimonthly intervals through harvest in May 2021 at Alligator Harbor (AH) and July at Oyster Bay (OB) to determine growth and mortality, as well as prevalence and severity of Polydora (mudworm) and Dermo disease. From April through June, encompassing the period when mortalities have occurred, water samples were analyzed for phytoplankton abundance and composition.

To summarize results, shell growth rates were significantly affected by sample period (SP), location, and their interaction. At AH, rate in SP1 was 1.6–2.7 times higher than other periods and at OB. During SP2&3, rates were higher at OB compared to AH. Salinity influenced growth with no effect from temperature, health indices or genetic stock. In terms of weight gain, AH had higher growth rates compared to OB; this effect was greater in SP3 versus SP1&2. Rates increased significantly at OB in SP3&4; effect was 3-5 times greater than differences between locations. Higher temperatures and salinities affected weight gain. Genetic stock also influenced weight gain with FL triploids at OB having significantly higher rates than LA triploids during SP3&4.

Mortality rates increased considerably in SP3&4; SP was the strongest effect observed. OB had lower overall mortality than AH with a significant interaction between location and sample period. Higher salinities and temperatures influenced mortality. Dermo was significantly higher in SP4 than any other SP but did not demonstrate any patterns related to mortality; there were no effects from Polydora on mortality. Florida triploids demonstrated lower mortality than Louisiana triploids. Mortality differences between genetic stocks indicate potential to develop a line with higher resistance to environmental stressors and mortality events.

View the entire project report here. This study was funded by the UF/IFAS Support for Emerging Enterprise Development Integration Teams (SEEDIT) Program, 2020-1.

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