Nutrition and reproduction are key parameters in the interaction process leading to Crassostrea gigas oyster summer mortality in France

Complex interactions between oyster, environment and pathogens were observed during the MOREST (2001-2005) project on summer mortality events of Crassostrea gigas oysters in France. The talk will show how nutrition level increasing reproductive intensity generates a risk for pathogen infection under synergistic effect of temperature and stress. Moreover, a genetic component is associated to specific reproductive performances and spawning behaviour. To better understand origin of such resistance or susceptibility to summer mortality, other characteristics as stress susceptibility, energy management and hemocyte activities of oysters from the two phenotypes were compared. This first comparison suggests interactions between stress, reproduction and defence mechanisms. Molecular tools are now necessary to better identify the concerned genes.However, none of the different factors involved in the environment/host/pathogen interaction process, can separately account for summer mortality, and all of these conditions seem necessary to reproduce the event. In the present case study of C. gigas oyster summer mortality, these necessary interactions provide different possibilities to prevent or reduce the problem by breaking one of them. Short term and long term options of reducing risk will be presented and discussed. Short term options include culture practices (temperature, trophic conditions, sediment distance, density, rearing steps, reduction of reproduction and stress). Options can depend also on biological material, with possibility of using triploids. Long term options by genetic selection of resistant oysters or by management of nutrients and organic matter from river watersheds are also possibilities. A risk analysis is necessary for each growing area to adapt the best strategy to the local environmental and culture conditions. The assessment of the economical aspects will be necessary before validation of these choices. This case study presents synergistic activities of scientists and resource managers in enhancing or restoring oyster populations in France.