Investigating risk factors for mortality and reovirus infection in aquaculture production of soft-shell blue crabs (Callinectes sapidus)

Crustacean aquaculture is prone to mortality from the combined effects of disease agents and the stresses associated with crowded, closed conditions. The culture practice of producing soft-shell blue crabs is no exception, suffering from mortality of about 25%. The virus, Callinectes sapidus reovirus 1 (CsRV1), has been reported at high viral loads in crabs dying in soft-shell shedding facilities. We investigated the relationship between crab mortality and CsRV1 prevalence and load in soft-shell crab production and whether death and virus infection correlated with identifiable aquaculture practices, environmental stresses, crab characteristics, or geographic regions. The patterns of CsRV1 prevalence, infection intensity, and mortality in blue crab aquaculture were studied in the Chesapeake Bay and Gulf of Mexico, USA. Using a genome-targeted assay, we compared virus loads in live and dead aquaculture crabs by individual sex and injury state from recirculating and flow-through systems of variable salinity, temperature, and crabs per aquaculture tank. Mortality was two-fold higher in flow-through aquaculture systems (33%) than in recirculating aquaculture systems (16%). Flow-through aquaculture systems had higher daily water temperature variability than recirculating aquaculture, and hypoxic events were observed only in flow-through systems during this study. High CsRV1 intensity was found in 62% of all pre-molt mortalities in production compared with 7% of successfully molted soft-shell crabs. The CsRV1 virus load in dead crabs was elevated in higher salinity conditions. In a mixed-effect model analysis, the random effects of location and time were more significant than salinity in predicting CsRV1 load in all crabs and dead crabs. Our results support previous research showing that recirculating aquaculture has lower mortality in soft-shell production, and confirms the association of high viral loads of CsRV1 with crab mortality in these production systems. Moreover, the findings indicate that although CsRV1 is ubiquitous in these systems, management of culture conditions such as salinity and temperature may limit virus-associated mortality.