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. Parameters 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°C). Mussels from this group showed low energetic contents after spawning and during the period of highest mortality, glycogen content decreased rapidly and shell growth ceased. Thus, mussels weakened by a major spawning were at the same time submitted to stressful temperatures. Summer mortalities were observed until the end of the experimental period (mid-September) even though surviving mussels seemed 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 observed for mussels from the same stock that were maintained at 16 m depth in the open sea. These mussels were exposed to lower temperatures and spawned 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 that suspension-cultured mussels in Magdalen Islands lagoons pay a high reproductive cost in terms of survival when a major spawning is completed during a period of stressful environmental conditions, such as high water temperatures.