Croissance et productivité d'une population du Sphaerium corneum (L.) (Bivalvia: Pisidiidae)

Abstract

The growth pattern of Sphaerium corneum from a small stream population (Mérantaise, Île-de-France) has been established from the analysis of annual modal evolution. Preliminary data obtained by other authors suggested that the von Bertalanffy growth model was the most suitable, but that the growth rate varied with the season. The largest and smallest heights of animals observed in the field allowed us to determine the parameters of the model: H (asymptotical height) = 11.5 mm and h0 (initial height) = 2 mm. The daily growth rate (k) varied from 2.5 × 10−4 in November to 4.3 × 10−3 in May and its seasonal variation is described with a model that allows its incorporation into the von Bertalanffy growth function. Mortality was studied considering the evolution of cohort size; the annual mortality rate was about 80%, which suggests a 3-year life-span. Productivity was estimated by analysis of a theoretical cohort evolution and by field data analysis. Productivity (P) determined from monthly samplings varied between 9 and 10.3 g m−2 according to the method; the productivity to biomass per square meter ratio (P/B) was between 3.7 and 4.4. The same ratio obtained from the theoretical cohort was 4.5. Comparison of these values with those reported by other workers reveals notable variations that might be related to ecological differences in the ecosystem.