Spring awakening temperature and survival of sediment-covered eastern oysters Crassostrea virginica

• Oysters in mesh bags are susceptible to accidental burial, particularly in winter.
• In this study, oysters awakened in spring when temperature increased to ~ 2.61 °C.
• Oyster buried under 20 mm of silt survived and resumed normal behaviour.
• However, oyster buried under 40 or 60 mm of silt invariably died within 12 days.
• Stocks should be re-suspended quickly following the melting of winter ice.

In the Gulf of St. Lawrence estuaries, mesh bags containing cultured oysters (Crassostrea virginica) are lowered onto the bottom in autumn prior to the formation of a thick ice cover. The oysters remain quiescent and unattended in near-freezing waters for four to five months, during which time they are susceptible to accidental burial by sedimenting particles. The objectives of this study were (1) to gain insight into the mechanism cueing the spring awakening of oysters, (2) to determine the approximate burial depth that oysters can withstand, and (3) to estimate the time it takes for mortality to occur under conditions of excessive siltation. Results indicate that water temperature is the primary factor controlling the timing of awakening, with the majority of oysters suddenly opening their valves when temperatures increased to 2.61 ± 0.66 °C. Supplementing the diet to mimic spring bloom conditions had no modulating influence on awakening behaviour. Oysters buried under 20 mm of sediment initially exhibited erratic valve movements and sometimes remained closed for days. Within 2 weeks, however, they did succeed in expulsing the overlying silt around their valve margins and in resuming normal valve movements, including circadian rhythmicity. By comparison, burial under 40 or 60 mm of silt invariably led to death within 11.7 ± 1.3 days. It is concluded that oysters should be re-suspended as soon as the ice cover breaks apart and moves offshore in the spring.