Mortalités massives de l’Huître creuse: causes et perspectives

The objectives of this study are to increase our understanding of the mass mortality phenomena in Pacific oysters Crassostrea gigas in the Thau lagoon and to examine the effect of cultivation practises. Mortality events in 2009 and 2010 always coincide with simple or dual infections involving primarily the Herpes virus OsHV-I and secondarily the bacteria Vibrio splendidus, when seawater temperature is between 17°C and 24°C. In the Thau lagoon, mortality starts early May and last until fall with an interruption period when seawater temperature is higher than 24°C in July and August. Mortality of oysters varies as a function of zootechnical life-history, hydrodynamics and cultivation practises. First, mortality depends on whether or not oysters have been exposed to a mortality event in the past. Mortality-free oysters transferred into an infected area die massively irrespective of their size and age, whereas oysters which survive to a mass mortality event in the past are resistant thereafter. Therefore, it seems interesting to expose the oysters to the mortality as soon as possible to select resistant individuals at low cost. Second, mortality and Herpes virus infection spread within the lagoon by means of currents. It is likely that the Herpes virus move over long distances as a function of hydrodynamics and remained infectious outside of its host. Third, mortality varies a function of cultivation practises, which influence the infection by V. splendidus and the physiological conditions of oysters. Given that oyster mortality reflects the effect of infectious disease and varies with zootechnical life-history and hydrodynamics, one needs to know precisely oyster’s life history and characteristics of the farming area. It is pressing to display a traceability setup and a certification test for oysters. Young oysters suffered heavy losses irrespective of their origins and ploidy so that it is not possible to act on these factors to reduce the mortality in the Thau lagoon. Rearing of oysters free of mortality is possible in deep open-sea.