Why does the introduced gastropod Crepidula fornicata fail to invade Arcachon Bay (France)?

Arcachon Bay was the last important shell culture area of the French Atlant ic coast without a stock assessment of the accidentally introduced Crepidul a fornicata (Gastropoda). Following a stratified sampling strategy: 205 sta tions were sampled with a dredge. The total biomass of C. fornicata was est imated at 155 tn (confidence limits: 72 tn) scattered on 2.4 km(2), i.e. ab out 5% of the infralittoral area. This biomass is very low compared to othe r exploited shellfish sites. Amongst mean environmental parameters (such as bathymetry temperature, salinity sediment grain-size, anthropic activity), the major keyfactors differentiating Arcachon Bay from other French sites were searched to explain the differences in C. fornicata colonization patte rn. It seems that the low biomass of C. fornicata, 30 y after its occurrenc e was first recorded in the ball, could br correlated to: ii) the scarcity of subtidal muddy sediments which are suitable for C. fornicata settlement; (2) the presence of large Zostera spp. beds on both intertidal and subtida l areas; and (3) the absence of bottom trawl fishing. Growth, production and productivity (P/B) ratios were compared between five stations characterized by different hydrological and sediment parameters. Growth and productivity (0.3 y(-1)) were not affected by mean biomass sugge sting a lack of population regulation by intraspecific competition.