Estimation de biomasse des bivalves intertidaux (moule, coque) exploités en Baie de Saint - Brieuc (Manche Ouest)

The aim of this report is to provide total biomass estimates of the blue mussel (Mytilus edulis, M. galloprovincialis) livestock harvested on mussel beds, and of the cockle Cerastoderma edule in the Bay of Saint-Brieuc. The sampling scheme for reared mussels combines stratification and three-stage sampling. The biomass of mussels, estimated twice in a year, remains close to 3 500 metric tons of wet weight (coefficient of variation : 10 to 14 %). A nonrandom, systematic sampling was performed to estimate the total biomass of cockles, the data being processed according to the geostatistical methods the biomass of the stock exhibits seasonal variability : 4 800 tons (CV : 15 %) at the end of summer, vs. 3 700 tons (CV: 17 %) at the beginning of spring. The consistency of these last two results is checked with a differential model of the evolution of cocklets population size-structure during winter.