Further observations on the influence of husbandry practices on OsHV-1 μVar mortality in Pacific oysters Crassostrea gigas: Age, cultivation structures and growing height

Abstract

Abstract Management of mass mortality events associated with Ostreid herpesvirus-1 microvariant (OsHV-1 μVar) is vital for aquaculture of Crassostrea gigas . As a consequence, the understanding of transmission mechanisms and risk factors enabling husbandry solutions to be developed constitutes an international research priority. In this context, a longitudinal intervention study was set up in Woolooware Bay, Australia, during summer in 2012–2013. The primary objective was to improve the external validity of a study completed in 2011–2012 that demonstrated mortalities in adult oysters but not spat could be halved by reducing their immersion time in an inter-tidal tray cultivation system (by elevating growing height by 300 mm). It is unknown whether an even higher growing height (+ 600 mm) could further improve survival or benefit the survival of spat. Consequently Pacific oysters of different ages (9.5 and 17.5 month) were placed in inter-tidal and sub-tidal rearing structures (trays and baskets) at three different sites in the bay. Mortality, growth, infection prevalence and viral load were monitored for 8 months (October–May). As in the previous summer, the disease was not continuously active over time in Woolooware Bay; mortalities were observed mainly in November 2012 and March 2013. The mortality was highly correlated with the prevalence of OsHV-1, which was in turn highly correlated with viral load, confirming the strong relationship between the virus and oyster mortality. The protective effect of high growing height on adult oysters was confirmed, with a final cumulative mortality below 40%, associated with significantly lower infection prevalence. Even though infection prevalence was significantly lower in baskets than in trays considering all ages and all sites, the final cumulative mortalities for spat were high (> 60%) regardless of the height and the type of cultivation structure; other mitigation strategies need to be developed for young oysters. The disease showed temporal and spatial variability suggesting that substantial variations in exposure to the virus occurred within and between years.