Connectivities with shellfish farms and channel rivers are associated with mortality risk in oysters

International audienceOyster diseases have major consequences on fisheries and aquaculture. In France,young Pacific oysters Crassostrea gigasare severely hit by the ostreid herpesvirus, whereas adultssuffer mortalities presumably caused by pathogenic bacteria. Here we investigated the origin andspread of mortalities that affect both young and adult oysters, and we identified and comparedtheir risk factors. Mortality was monitored in 2 age classes of oysters deployed in early spring at39 sites spread over a 37 km2surface area inside and outside of shellfish farms. Environmentaldata obtained from numerical modelling were used to investigate risk factors. Mortality of youngoysters associated with ostreid herpesvirus occurred in the oyster farming area. Hydrodynamicconnectivity with oyster farms was associated with higher mortality risk, whereas chlorophyll aconcentration was associated with a lower risk. Adult oysters experienced 2 mortality events thatwere associated with different risk factors. The first event, which occurred after deployment andwas probably caused by endogenous pathogens, was mainly associated with connectivity to chan-nel rivers and salinity. The second mortality event observed at the end of the summer was mainlyassociated with connectivity to oyster farms, suggesting pathogen transmission. The risk factorsinvolved in young and adult oyster mortalities were partly different, reflecting distinct origins.Connectivity with oyster farms is a mortality risk factor for both young and adult oysters; thus, dis-ease management strategies that focus on oyster farming areas will impact overall disease risk