CREPIDULA FORNICATA L. (MOLLUSCA, GASTROPODA) IN THE MARENNES-OLERON BAY - SIDE-SCAN SONAR MAPPING OF SUBTIDAL BEDS AND STOCK ASSESSMENT

Extensive and detailed subtidal ground mapping of the Marennes-Oleron Bay and Fouras-Aix areas was performed with a side-scan sonar. Side-sc an sonographs gave a comprehensive view of sedimentological bottom env ironments and also allowed us to outline locations of grounds colonise d by the gastropod mollusc Crepidula fornicata L. with a precise estim ate of their surface. A sedimentological map of the centre of the Mare nnes-Oleron Bay is given, describing spatial organisation of the follo wing four types of sediment: pure mud, fine sand, coarse sand and rock s. Crepidula beds were also recognised on sonographs and were estimate d 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 she lls). A stratified sampling procedure was performed in April-May 1995, including 80 stations sampled with a Smith McIntyre grab (two grabs p er 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 M arennes-Oleron. This latter estimate is similar to the previous one pe rformed in this bay in spring 1984, i.e. 1 800 +/- 900 tons, due to th e overlap of 95 % confidence intervals. These comparable estimates wit hin 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 yea rs are not efficient. However, annual landing operations of slipper li mpet cannot prevent further spread of the species, as pointed out by t he large development of a new Crepidula ground in the north-western ce ntre of the bay (north Lamouroux sand bank). Both natural processes (c urrent 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. (C) Elsevier, Paris.