Influence of husbandry practices on OsHV-1 associated mortality of Pacific oysters Crassostrea gigas

In 2010 Ostreid herpesvirus-1 (OsHV-1) was detected in Australia and had a disastrous impact on Pacific oyster Crassostrea gigas aquaculture and coastal communities. The acronym POMS (Pacific Oyster Mortality Syndrome) was created in Australia to refer to mass mortalities due to OsHV-1. While management of this disease mainly involves active surveillance, rigorous biosecurity protocols and mollusc breeding programs targeting production of resistant animals, the effects of aquaculture practices on mortality outbreaks are still poorly understood. The present study aimed to determine the effect of growing heights on OsHV-1 associated mortality in C. gigas in Woolooware Bay (Australia) during the summer 2011/2012. Growing height influences the immersion time of inter-tidally cultivated oysters during each tide cycle, and could be an important risk factor for OsHV-1 exposure and mortality. Pacific oysters of different ages (2 and 12 month) were placed in intertidal rearing structures at three different sites in the bay. Mortality and growth rates, infection prevalence and seawater parameters (temperature and salinity) were monitored over 7 months. The outbreak started in November 2011 and mortalities were observed until late April 2012. The pattern of disease expression was time and site dependent as the mortalities started immediately after infection with OsHV-1 at two of the three sites, while the infection preceded the onset of mortality by two months at the third site. No clear difference in salinity or temperature of water was observed among sites suggesting that other environmental features influence the onset of the disease. Extreme mortalities were observed in the younger class of oysters, while the modification of growing height led to a significant increase in survival of adult oysters. Infection prevalence and intensity decreased in surviving oysters suggesting that some individuals may be able to clear the virus. Differences in mortality among sites and growing heights are discussed in relation to OsHV-1 infection intensity and prevalence in oysters, and the environmental data recorded during the outbreak.