Impact of feeding on oyster depuration efficacy under conditions of high salinity and low temperature

Feeding regimes of laboratory-cultured algae reportedly maintain meat quality as well as reduce mortality of oysters during prolonged depuration. However, feeding regimes are costly and their impact on depuration efficacy is not understood. The impact of a feeding regime of laboratory-cultured Chaetoceros gracilis (added at a manufacturer-recommended dose of 1.4 × 109 cells/oyster/day) on depuration efficacy for batches of Pacific oysters collected from four sites in Japan in 2016 and 2017 was evaluated. Depuration was done in filtered seawater (FSW) under conditions of low temperature and high salinity (17 °C and 28 to 31 ppt respectively). Indicators of depuration efficacy (Vibrio parahaemolyticus and V. vulnificus counts, condition index as well as mortality) for oyster batches that were fed were compared with those for batches that were not fed over a depuration period of 14 days. Generally, feeding did not significantly affect removal of V. parahaemolyticus and V. vulnificus, condition index or mortality of oysters (P > .05). However, for one batch of oysters in an exceptionally poor physiological state (condition index <32), condition index improved significantly after 14 days of depuration with feeding. From the results it was concluded that under conditions of low temperature and high salinity, feeding does not have a significant impact on depuration efficacy unless initial condition index of the batch was very low. This is the first study to show that maintaining high salinity and low temperature during oyster depuration eliminates the need for feeding regimes.