We examined the relationship of summer mortality to reproductive events for suspensioncultured mussels h/lytilus edulis L, in the Magdalen Islands. Parameters associated with gametogenesis and spawning were followed along w t h the tuning and intensity of mortality from mid-June to midSeptember 1991. For mussels maintained In a shallow lagoon (the usual mussel culture s~t e ) , summer mortality began in late July as a major spawning event was e n d~n g 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.

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We considered Cushing's match/mismatch theory in a heterotrophic environment and hypothesized that settlement and recruitment success in blue mussel are higher when the food supply is rich in polyunsaturated and essential fatty acids (PUFA/EFA). To test this hypothesis, we monitored larval development as well as fatty acid composition in trophic resources during two successive reproductive seasons. The decoupling we found between the presence of competent larvae in the water column and settlement rates strongly suggests that metamorphosis is delayed until conditions are suitable. In both years, the major mussel settlement peak was synchronized with a phytoplanktonic pulse rich in EFA, consisting of a large autotrophic bloom in 2007 and a short but substantial peak of picoeukaryotes in 2008. These results suggest a "trophic settlement trigger" that indirectly affects recruitment by strongly improving the settlement rate. Despite similar larval settlement rates during both years, the lower 2007 recruitment likely resulted from a mismatch with a high lipid-quality trophic resource. The seasonal trophic conditions differed greatly between the two years, with fatty acids profiles reflecting heterotrophic plankton production in 2007 and mostly autotrophic production in 2008. In agreement with Cushing's theory, our results highlight a match/mismatch, related to the food lipid quality rather than food quantity. For the first time, we show that the recruitment in marine bivalves may be dependent on phytoplanktonic pulses characterized by high levels of PUFA.

Mussels secrete byssal threads regularly to the substratum, and the strength of these threads can fluctuate with time. The present study examined weekly variations in the attachment strength of 2 yr old cultured mussels Mytilus edulis on submerged longlines in a semi-enclosed lagoon from late May to mid-October. Some possible factors influencing attachment strength were investigated: environmental factors (temperature, food availability, wind velocity and hydrodynamic conditions such as current velocity, turbulence and wave height) and reproductive condition were measured concurrently. Attachment strength was measured directly on cultured mussels using a dynamometer. Attachment strength varied 2-fold from summer to fall, a difference related not only to the number of byssal threads but also to their individual strengths. The hierarchical influence of each factor on attachment strength is discussed. Our results suggest that spawning seemed to be correlated with an important decrease (-32%) in attachment strength, and that water temperature (negative relationship) and turbulence (positive relationship) were the most important factors explaining the variation in mussel attachment strength. In contrast to previous studies carried out in intertidal zones, no trade-offs were observed between reproduction and attachment strength.

Biofilm ageing is commonly assumed to improve mussel settlement on artificial substrata, but the structure and taxonomic composition of biofilms remains unclear. In the present study, multi-species biofilms were characterized at different ages (1, 2, and 3 weeks) and their influence on settlement of the blue mussel, Mytilus edulis, was tested in the field. As biofilms can constitute a consistent food resource for larvae, the lipid quality, defined as the proportion of related essential fatty acids, may be a selection criterion for settlement. Overall mussel settlement increased on biofilms older than 1 week, and the enhanced settlement corresponded to the abundance and composition of the biofilm community, rather than to essential fatty acid levels. However, during a pulse of phytoplankton, the positive influence of biofilm was not detected, suggesting that pelagic cues overwhelmed those associated with biofilms. The influence of biofilms on mussel settlement could be more crucial when planktonic resources are limited.