Mussel culture relies on the mussel self-attachment capacity through byssus production. By doing so, no cages or containment devices are needed. It has been previously suggested that thread production requires a non-negligible part of the energy expenditure in blue mussels Mytilus edulis. Therefore our work investigates the relationships between byssal thread production, mussels energetic reserves and phospholipids profiles in the foot. The relationship between thread production and heterozygosity was also examined. The study was realized in a small lagoon of the Îles-de-la-Madeleine, southern Gulf of St. Lawrence, Canada, on four sampling dates between June and September encompassing the pre- to post-spawning periods. Our results show a significant relationship between the thread numbers and attachment strength. However, no correlations were found between energy reserves, phospholipids composition of the foot or heterozygosity level and byssal production. Our results suggest that mussels in suspension culture in this lagoon were not energy-limited so that the energy reserves did not influence the byssal thread production.

We sampled 27 month-old mussel populations grown on collector ropes in Cascapédia Bay,Quebec, to test whether density-dependent growth was present concomitantly toself-thinning, a process which was previously shown to occur in this system and thought tobe driven by spatial constraints. Biomass-density curves of raw samples were curvilinear,suggesting density-dependent growth. However, at least two cohorts were present.Fractionating the samples on the basis of age yielded a linear relationship for the main,2 year-old cohort. This implies density-independent growth and rules out food regulationin these populations. Therefore, our results are consistent with inferences drawnpreviously from the values of the self-thinning exponent, that is, space-regulatedself-thinning. Our results suggest that curvilinearity of the raw biomass-density curvesresulted from a bias caused by including the 1 year-old cohort and spat of the year in theanalysis. This conclusion is supported by a model showing that samples with mixed cohortscan yield linear, concave or convex biomass-density curves without density-dependentgrowth. The shape of the curves depends on the scaling relationships between cohortabundances. It appears that the shape of biomass-density curves may be a usefulcomplementary criterion – in addition to the value of self-thinning exponents – for theidentification of food or space as factors regulating cultured populations.