Effect of gear type on Vibrio spp. levels in farm-raised oysters (Crassostrea virginica) after routine handling and resubmersion.

During routine handling, cultured oysters are removed from the water and exposed to elevated temperatures, causing growth of Vibrio vulnificus and V. parahaemolyticus within. Farmers can resubmerse oysters in the water, allowing elevated Vibrio spp. levels to return to ambient levels within the oysters. Previous resubmersion research is limited to one aquaculture gear type during studies performed from June-September. This study aims to expand existing knowledge on the recovery times needed for elevated Vibrio levels in handled oysters from two common gear types (adjustable longline system [ALS] and OysterGro system [OG]) during early and mid-summer periods. Oysters held in both gear types were subjected to being tumbled and refrigerated (TR) or  desiccated, then resubmersed into water in May and July of 2018 and 2019. Vibrio spp. levels were measured before (pre) and after (post) the treatments, and 3, 7, and 14 days after resubmersion, and compared to levels in submersed oysters. All samples were tested for V. vulnificus, total V. parahaemolyticus, and pathogenic V. parahaemolyticus (tdh+/trh+). Water temperatures in May were significantly lower (~5ºC; p ≤ 0.009) than in July, corresponding to lower V. vulnificus levels (-0.67 log MPN/g) and higher tdh+/trh+ levels (+0.56-0.63 log MPN/g) in control oysters. The average  Vibrio spp. levels in control oysters from each trial did not differ between the gear types (p ≥ 0.08). Elevated V. vulnificus levels recovered to ambient levels after seven days in May and three days in July, regardless of gear or handling. For V. parahaemolyticus, the desiccated oysters required 14 days to recover in May, and 7 days in July, while the TR oysters required 14 days or more in both months. This study had limited replication in each month, but the data suggest that the resubmersion times differ between the gear types, treatment types and months. Future studies with more replication are needed to determine if these trends continue.