Crepidula fornicata L. (mollusque, gastéropode) en baie de Marennes-Oléron : cartographie des fonds par sonar à balayage latéral et estimation du stock

Extensive and detailed subtidal ground mapping of the Marennes-Oleron Bay and Fouras-Aix areas was performed with a side-scan sonar. Side-scan sonographs gave a comprehensive view of sedimentological bottom environments and also allowed us to outline locations of grounds colonised by the gastropod mollusc Crepidula fornicata L. with a precise estimate of their surface. A sedimentological map of the centre of the Marennes-Oleron Bay is given, describing spatial organisation of the following four types of sediment: pure mud, fine sand, coarse sand and rocks. Crepidula beds were also recognised on sonographs and were estimated to cover 181 hectares in the Fouras-Aix area and 615 hectares within the Marennes-Oleron Bay. Standard sampling methods were then combined with the side-scan sonar mapping results, allowing an estimate to be made of the stocks of Crepidula fornicata (live specimens and dead shells). A stratified sampling procedure was performed in April-May 1995, including 80 stations sampled with a Smith McIntyre grab (two grabs per station). The live stock of Crepidula was estimated to be 2 494 +/- 3 344 tons at Fouras-Aix and 2 644 +/- 1 137 tons within the bay of Marennes-Oleron. This latter estimate is similar to the previous one performed in this bay in spring 1984, i.e. 1 800 +/- 900 tons, due to the overlap of 95 % confidence intervals. These comparable estimates within the Marennes-Oleron Bay give no evidence to suggest that dredging operations (1 000-1 500 tons . yr(-1)) performed in the bay for 15 years are not efficient. However, annual landing operations of slipper limpet cannot prevent further spread of the species, as pointed out by the large development of a new Crepidula ground in the north-western centre of the bay (north Lamouroux sand bank). Both natural processes (current action and bed load transport) and bottom-trawl activities can facilitate spread of the species in the bay, as revealed by side-scan sonar image analysis.