SEASONAL-VARIATION OF THE BATHYMETRIC DIS TRIBUTION OF CETACEANS IN THE LIGURO-PROVENCAL BASIN (WESTERN MEDITERRANEAN)

The Liguro-provencal basin is one of the most attractive areas for cet aceans in the Mediterranean Sea. During the summer season, high levels of abundance have been estimated in the area. But the cetaceans' stat us during the other seasons remains comparatively unknown, we pooled t he data of seven years of sampling to get a first description of the s easonal variation of the bathymetric affinities. The sampling was perf ormed on a 30 feet sailboat, cruising at 5 knots on diesel propulsion and concerns the central portion of the Liguro-provencal basin.The tot al effective effort in the area amounts to 11,000 km. 850 observations of six species were made during the four seasons and three species we re also observed throughout the different seasons: the striped dolphin , Risso's dolphin and the fin whale. Four bathymetric strata were defi ned between the coastline, the 500 m depth line, the 1,000 m depth lin e and the 2,000 m depth line. We called bathymetric affinity a distrib ution indicator corrected for sampling heterogeneities. In summer, the striped dolphin and the fin whale are distributed in all strata, with increasing preferences for deeper areas, whereas Risso's dolphin clea rly favours the continental slope, particularly the upper part of it. The pilot whale and the spermwhale are markedly more frequent in the d eep slope stratum and in the open sea stratum. For the three species p resent all year round, we observe an increasing affinity for the deepe r strata in autumn and winter, while the continental slope seems to be favoured in spring. From an ecological point of view, our results on the seasonal variation of abundance make sense for the fin whale, whos e population can feed on euphausiid swarms as soon as February-March. The striped dolphin feeds on a high diversity of prey items, hence its wide range in the area. Risso's dolphin relies mainly on various fami lies of cephalopods, for which the continental slope is a very favoura ble stratum. More information on the biology of the preys is needed be fore a more precise diagnostic of the delphinids' habits can be made i n this very interesting area of the Mediterranean sea.