Les risques conchylicoles en Baie de Quiberon. Première partie : le risque de mortalité virale du naissain d’huître creuse Crassostra gigas. Rapport final du projet Risco 2010-2013

This study (“Risco”), implicating both industry, socio-economic experts and biologists, was funded by the Regional Council of Brittany, for 3 years (2010-2013), to investigate upon the origin of oysters (Crassostrea gigas) mortalities in the bay of Quiberon (South Brittany, France). Specific mortalities of adult oysters were recorded in this bay in 2006, whereas mortalities of one year old spat were observed since in 2008 in this bay as at a national scale. The experimental protocol in 2010, including a monthly survey of oyster batches at 15 experimental stations, allowed to assess the risk for one-year oysters, due to viral disease. A clear spatial distribution of this risk was evidenced, with a western area spared from contamination and mortality, and a central and north-east sector strongly affected. The virus analysis clearly demonstrate the responsibility of the OsHV-1 virus in the mortalities. The oysters on the bottom with a lower growth appear less sensitive to the viral disease than the surelevated ones. In order to interpret this spatial distribution, an epidemiological model has been tested : it is based on emission of virus from the stock of spat in surrounding farming areas, dispersion by the hydrodynamic conditions, and inactivation of the virus by solar radiation. The resulting simulations suggest that the contamination would be endogenous to the bay (from contaminated stocks of spat seeded nearby). A decrease of densities for rearing spat, as well as the seeding of non contaminated spat may be recommended.