Body size, population density and factors regulating suspension-cultured blue mussel (
We sampled 27 month-old mussel populations grown on collector ropes in Cascapédia Bay, Quebec, to test whether density-dependent growth was present concomitantly to self-thinning, a process which was previously shown to occur in this system and thought to be 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 regulation in these populations. Therefore, our results are consistent with inferences drawn previously from the values of the self-thinning exponent, that is, space-regulated self-thinning. Our results suggest that curvilinearity of the raw biomass-density curves resulted from a bias caused by including the 1 year-old cohort and spat of the year in the analysis. This conclusion is supported by a model showing that samples with mixed cohorts can yield linear, concave or convex biomass-density curves without density-dependent growth. The shape of the curves depends on the scaling relationships between cohort abundances. It appears that the shape of biomass-density curves may be a useful complementary criterion – in addition to the value of self-thinning exponents – for the identification of food or space as factors regulating cultured populations.