Reproduction and summer mortality of blue mussels Mytilus edulis in the Magdalen Islands, southern Gulf of St. Lawrence

We examined the relationship of summer mortality to reproductive events for suspension-cultured mussels Mytilus edulis L. in the Magdalen Islands. Par ameters associated with gametogenesis and spawning were followed along with the timing and intensity of mortality from mid-June to mid-September 1991. For mussels maintained in a shallow lagoon (the usual mussel culture site) , summer mortality began in late July as a major spawning event was ending and as summer temperatures peaked (>20 degrees C), Mussels from this group showed low energetic contents after spawning and during the period of highe st mortality, glycogen content decreased rapidly and shell growth ceased. T hus, mussels weakened by a major spawning were at the same time submitted t o stressful temperatures. Summer mortalities were observed until the end of the experimental period (mid-September) even though surviving mussels seem ed to have improved their condition in late August (when glycogen content, tissue mass and shell length had increased). By mid-September approximately 65 % of the mussels had been lost. In contrast, no summer mortality was ob served for mussels from the same stock that were maintained at 16 m depth i n the open sea. These mussels were exposed to lower temperatures and spawne d less extensively than those in the lagoon. They had no major spawning in late July and were never completely empty of gametes. Our results suggest t hat suspension-cultured mussels in Magdalen Islands lagoons pay a high repr oductive cost in terms of survival when a major spawning is completed durin g a period of stressful environmental conditions, such as high water temper atures.