Off-bottom aquaculture of oysters Crassostrea virginica in Florida represents a new industry sector with more than 170 growers operating on 965 acres of leases located primarily in the state’s Panhandle . In 2019, 4.7 million oysters were produced on these leases. Recently, unexplained mortality events during late spring and early summer, amounting to 50-80% loss of triploid oysters reaching market size, have been reported by growers. A variety of factors may account for these mortalities; h owever, diversity in location, gear, seed, and operational scale, make it challenging to extricate potential causes. In response to industry concerns, “sentinel” farms were established at two lease locations and a monitoring program initiated to examine basic but important relationships between production and health of cultured oysters , and environmental factors.
Juvenile triploid oysters (19-21 mm shell height, SH; n=2000), half produced from traditional Louisiana (LA) tetraploid stocks and half from new Florida (FL) tetraploids were deployed in October 2020 at farm sites in the Alligator Harbor (AH) Aquaculture Use Zone (AUZ) and Oyster Bay (OB) AUZ, where oyster mortalities have occurred. The AH lease site is characterized by high salinity with monthly means ranging from 27.8-32.2 psu in this study, while salinities at OB ranged from 16.2-26.1 psu. Growers provided gear (floating bags) and maintenance during the culture period. Oysters were sampled at bimonthly intervals through harvest in May at the AH site and July at the OB site to determine growth and mortality and assess prevalence and severity of shell parasitism and Dermo disease. From April through June encompassing the period when mortalities have occurred, water samples were analyzed for phytoplankton abundance and composition.
At AH, oysters reached market-size (75.6+11.1 mm SH ) within six months (March) but were maintained another two months (May). Oysters at OB averaged 53.7+6.2 mm SH after six months and did not reach market size (76.2+8.9 mm SH) until 3.5 months later (July). Cumulative mortali- ties at harvest were 30.2-32.0 % (AH ), 25.4% ( OB, FL stock), and 40.4% ( OB, LA stock) with significant differences between stocks at the OB site (Table 1) . Water quality, phytoplankton, and parasitism will be presented that most influenced oyster production both positively and negatively.