Studies conducted in seawaters around New Zealand have shown the numbers of human pathogenic Vibrio spp. are usually low, but high numbers sometimes occur during warmer summer/autumn months (January - April). In this study, Pacific oysters (Crassostrea gigas) were grown at Kaipara Harbour and Mahurangi Harbour in New Zealand at different heights from the seafloor in different ways: fixed positons intertidally and subtidally, and as floating long lines over the 2013 and 2014 summer periods. Two geographically distinct commercial harvest areas: Coromandel Harbour (North Island) and Croisilles Harbour (South Island) in New Zealand were also compared in 2015 where oysters are grown under different methods. Detection and enumeration of Vibrio spp. was performed according to the Bacteriological Analytical Manual using the Most Probable Number approach and real-time polymerase chain reaction technique. The only significant growing method effect was observed in Mahurangi Harbour, where intertidal oysters at 1.5 m from the seafloor had higher numbers of trh + Vibrio parahaemolyticus than other intertidal samples from Kaipara Harbour and Coromandel Harbour. All other samples showed a relationship with surface seawater temperature, but not with distance from seafloor or farming method. Overall, there is no clear evidence that different oyster farming methods (floating, subtidal or intertidal at different depths) affect Vibrio spp. population sizes, which were dominated by seasonal changes and environmental parameters.
Keywords: Pacific oysters; Sea depth; Tide exposure; Vibrio parahaemolyticus; Vibrio vulnificus.
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