Our research group has shown that constant immersion aids survival of Pacific oysters after exposure to OsHV-1 in artificial seawater. It follows our previous epidemiological observations that the survival of adult oysters is enhanced in intertidal culture if growing height is increased. We rationalise these apparently contradictory observations.
Evans O, Kan JZF, Pathirana E, Whittington RJ, Dhand N, Hick P (2019). Effect of emersion on the mortality of Pacific oysters (Crassostrea gigas). Aquaculture 505: 157-166.
Summary: Microvariant genotypes of Ostreid herpesvirus-1 (OsHV-1) have been responsible for mass mortalities in farmed Pacific oyster (Crassostrea gigas) populations in Europe, New Zealand and Australia since its first detection in France in 2008. Previous studies conducted in the Georges River estuary, New South Wales, Australia demonstrated a significant protective effect of increased emersion time for adult oysters, when oysters were grown 300mm above the standard intertidal growing height used by local farmers. The aim of this study was to investigate the protective effect of tidal emersion on infection by OsHV-1 in a controlled laboratory environment. Adult C. gigas (24 months old 70–90 mm) and C. gigas spat (5 months old, 20–40 mm) were infected with OsHV-1 by intramuscular injection and housed with either twice daily emersion or constant immersion. Oysters were monitored daily for gaping of the valves, as an indicator of clinical disease. Contrary to observations in prior field studies, adult oysters subjected to emersion in this study had significantly higher mortalities (67.2%) compared to the constantly immersed adults (11.3%). No significant difference was observed between treatments for the spat. Real-time quantitative PCR confirmed that all mortality was associated with a high concentration of OsHV-1 DNA. Constant immersion appeared to have a protective effect on infected adult oysters, as many gaping oysters recovered. These results suggest that the beneficial effect of high growing height on adult oysters in the field is due to avoidance of infection with OsHV-1, rather than an effect on oyster physiology. Furthermore, it suggests that if infected oysters can be immersed in such a way that predators and secondary infections are avoided, then many may survive the infection.
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