Addressing Oyster Mortalities in Florida’s Off-Bottom Oyster Aquaculture Industry

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. This pilot study addresses recent unexplained oyster mortalities experienced by growers in late spring and early summer, with reported 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.

Discussion concerning these mortality events began at a Florida oyster growers’ meeting in 2020. Although consensus was not reached about probable causes, there was agreement that a systematic approach was needed to better understand factors. This was followed by a Gulf of Mexico regional workshop where experts from several universities discussed what is known and not known. Industry input was sought to develop priorities for future work.

With University of Florida funding, a monitoring and assessment plan was initiated to examine relationships between production and health of cultured oysters and key environmental factors. The objectives were three-fold: monitor production at commercial farms; examine water quality parameters and phytoplankton abundance; and assess prevalence and severity of shell parasitism and Dermo disease. 

Methods: 

Four farms in two aquaculture use zones were identified in be involved in the study – two in Alligator Harbor in Franklin County and two at Oyster Bay in Wakulla County. Oysters were spawned at the Auburn University Shellfish Lab in May 2020. Two triploid lines were produced using males from the traditional LSU tetraploid stocks and the newly developed Florida tetraploids. These were half-siblings with shared diploid females. This provided an opportunity to evaluate the performance of two genetic lines. Seed were nursed by UF at Cedar Key to a size (R6) that is typically purchased by growers for planting. Seed were distributed on September 29, 2020, with about 1,000 of each line given to growers. Average seed size was 21 mm for Florida and 19 mm for Louisiana, both averaged 1.4 grams in weight. Seed were deployed into Vexar bags supported by floats. Growers maintained oysters during the culture period.

Sampling was initiated in late January 2021, 4 months post-deployment. Bimonthly sampling was undertaken until most oysters reached market size. There were three sample periods for Alligator Harbor with harvest on May 25^th, while Oyster Bay had four sample periods with harvest on July 13. Water temperature and salinity were continuously monitored using a YSI sonde at Alligator Harbor and Onset loggers at Oyster Bay. At each sample period, oysters were measured for shell height and whole wet weight and live and dead oysters counted per replicate bag. Phytoplankton kits were provided to a grower at each lease area to collect and preserve water samples weekly during March through July, months when mortalities have been observed. Two sets of samples were analyzed monthly at the UF Algal Ecology Lab. In addition, oysters collected from each sample period were analyzed for health at the UF Aquatic Pathobiology Lab.

Results: 

Water temperatures were similar for both lease areas, declining throughout sample period 1, increasing in the following sample periods. Seasonal differences followed expected patterns (Figure 1). The growout period at Oyster Bay extended into June and July with monthly average values reaching 83°F. At Alligator Harbor, salinity was high with little variation as sample period averages ranged from 28.3 to 30.7 (Figure 2). Lower salinities were observed at Oyster Bay as the site is influenced by adjacent rivers and seasonal variations due to run-off and prevailing winds. Sample period averages ranged from 17.8 over the summer months to 23.1 in the winter. 

Figure 1. Continuous water temperature (°F) data from October 1, 2020, to May 25, 2021, at the Alligator Harbor (AH) lease location and from October 1, 2020, to July 13, 2021, at the Oyster Bay (OB) lease location. Means for each sample period are displayed.