Theme Session (CC): Population Biology Summer Mortality of the Pacifie cupped oyster Crassostrea gigas in the Bay of Marennes-Oléron (France)

A comprehensive research program has been developed since 1995 to focus on C. gigas abnormal mortality rates occurring regularly during the summer period in the Bay of Marennes Oleron (France). Several subprojects were carried out including in-vitro ecophysiological studies to asses l) the relationship between reproductive status, scope for growth and mortality, 2) effects of stress temperature on physiological functions. Meanwhile, field studies concerned monitoring surveys of calibrated oyster populations at various emersion time and geographic sites. A database was developed and incorporated into a geographic information system (GIS) to overlay several layers of information and establish relationship among environmental parameters and mortality rates. Concomitantly, seawater quality monitoring surveys were developed to assess carrying capacity and overall environmental conditions affecting oysters. Temperature probes were deployed at the near vicinity of oysters as weIl as a multiparameter probe to record, temperature, salinity, turbidity, pH and 02 concentration on a continuous basis. Moreover, a valvometer recording continuously the oyster valve cl0 sure activity was deployed near the previous probe to establish relationship with environmental conditions. C. gigas summer mortality rates will be discussed and a general hypothesis will be proposed to explain mortality occurrence.