VIBRIO SPP. LEVELS IN WILD AND AQUACULTURE-REARED EASTERN OYSTERS (CRASSOSTREA VIRGINICA) AND WILD RIBBED MUSSELS (GEUKENSIA DEMISSA) AT COMMERCIAL SHELLFISH AQUACULTURE SITES IN SOUTH CAROLINA, USA.  

Wild populations of oysters in many U.S. east coast estuaries are typically not harvested in the summer months.  From a sustainable resource management perspective, seasonal closures allow populations to recover from harvesting activities; from a marketing perspective, oyster palatability is often reduced during the summer months due to the presence of gametes pre-spawning or low meat weights post-spawning.  Such closures also serve to reduce the likelihood of human illnesses caused by the consumption of shellfish containing high concentrations of Vibrio spp. bacteria driven by elevated water temperatures.  For triploid shellfish aquaculture, however, only the last of these rationales remains relevant.

To improve our understanding of the risks to human health of consuming shellfish harvested from aquaculture operations during summer months, researchers from the SCDNR, SCDHEC and FDA collaborated on a quantitative evaluation of Vibrio spp. levels in shellfish grown in South Carolina waters.  In June-September 2015, monthly samples of wild and floating cage-reared Eastern oysters (Crassostrea virginica), and wild ribbed mussels (Geukensia demissa) were collected from two commercial shellfish sites.  Additionally, the effects on Vibrio spp. levels of two husbandry practices were investigated, specifically: 1) air exposure by flipping the OysterGro cage and subsequent re-immersion (one time investigation in September-October); and 2) two-week storage of ribbed mussel samples through constant immersion in a wet storage facility (conducted monthly).  During each sampling event, two replicate samples of each shellfish type were collected and shipped overnight to FDA for testing of Vibrio spp. levels.

Shellfish samples were analyzed using a three-tube most probable number (MPN) method with enrichment in alkaline peptone water.  MPN estimates were derived from standard tables.  The presence of V. parahaemolyticus and V. vulnificus were confirmed from enrichments using real-time PCR.  Preliminary results (Figure 1, July 2015 data) indicate that wild oysters harbored slightly higher levels of V. parahaemolyticus compared to the aquaculture oysters or wild mussels.  The wet storage depuration of wild mussels appeared to reduce V. parahaemolyticus concentrations by an order of magnitude.  Data from this study will help to inform future decisions related to shellfish permit conditions that serve to both support the industry and protect consumers.