Effects of culture methods in the Pacific Northwest on the levels of Vibrio spp. in farm-raised oysters (Crassostrea gigas)

Abstract

Oyster aquaculture utilizes both on-bottom and off-bottom culture techniques to harvest raw oysters for the half-shell market. In the Pacific Northwest, the farming practices for growing the Pacific oyster, Crassostrea gigas, in the intertidal zone expose them to elevated air temperatures for up to several hours each day. These periods of tidal desiccation can improve oyster quality by reducing biofouling but increasing air temperatures in summer months may result in growth of the naturally occurring human pathogens Vibrio parahaemolyticus and V. vulnificus. Following tidal desiccation, oysters are resubmersed by the incoming tide and can begin filter feeding again, allowing for elevated Vibrio spp. levels to return to levels seen prior to air exposure. This study compared the effects of two types of intertidal culture methods used in Samish Bay, Washington on the levels of Vibrio spp. in farmed oysters following tidal desiccation and after resubmersion. The two culture methods, beach culture (bottom culture) and flip bag (off-bottom), were compared to determine whether culture method affects the recovery of Vibrio spp. levels. Following maximum exposure to air temperatures, Vibrio spp. levels generally increased within oysters from both culture methods, although high variation among trials was observed. Subsequent resubmersion from the incoming tide resulted in a decrease in Vibrio spp. levels, although variability made general conclusions on the recovery of Vibrio spp. difficult. Additionally, while Vibrio spp. levels often were not affected by culture method, when they were, levels in flip bag raised oysters were higher than those in beach culture oysters in all cases but one. This study is intended to help oyster growers and public health officials in the region ensure that their culture methods and regulations are not increasing consumer risk from these bacteria.