Seasonal changes in carbohydrate metabolism and its relationship with summer mortality of Pacific oyster Crassostrea gigas (Thunberg) in Marennes–Oléron bay (France)

This paper investigates the biological responses of Crassostrea gigas under traditional culture conditions on a mudflat in Marennes–Oléron bay. Summer mortality has been regularly observed in recent years in oysters reared using “on bottom” culture conditions. The present study attempts to provide a better understanding of the mortality phenomenon through biological parameters. Classical ‘field parameters’ such as mortality and growth rates, and quality index (dry meat weight / dry shell weight × 1000) were monitored. Additional parameters, as biochemical composition of oyster meat and glucose incorporation capacity, were included as potential new bioindicators. The work highlighted a critical timing (May–June) preceding the summer mortality and characterised by an arrest in lipid synthesis and a decrease in carbohydrate content. During this period, growth (especially gonad maturation) either slowed down significantly or even stopped. The first mortality event occurred during a growth renewal period at the end of June. Short-term analysis (15 days) provided information to identify such responses which may indicate a physiological stress and demonstrating the need for further investigation. The seasonal food availability (estimated from chlorophyll a levels) did not facilitate the mortality understanding which occurred after water temperature went above 18–19 °C. Nevertheless, this study shows carbohydrate anabolism contributed in the physiological stress leading to mortality events.