Distribution and relative abundance of sperm whales in the Mediterranean Sea

The distribution of sperm whales in the Mediterranean Sea was investigated over 4 consecutive years. Field surveys took place in 1997, 1998, 1999 and 2000 between June and August from a 12 m survey boat equipped with towed hydrophones. The boat cruised at a mean speed of 6 knots on zig-zag lines. The total transect length was 12 709 km, and 3903 acoustic stations were assigned along the transects to monitor the underwater acoustic environment for sperm whale clicks. The Mediterranean Sea was divided into 6 regions for data analysis: the Ligurian Sea, the Gulf of Lions, the southwestern basin, the Alboran Sea, the Tyrrhenian Sea and the Ionian Sea. Relative frequencies and relative abundances of sperm whales were calculated from both visual and acoustic data. Acoustically, the Gulf of Lions yielded the highest relative abundance, with an average of 2.15 x 10(-2) whales heard km(-1) effort. High abundance was also seen in the southwestern basin and the Ionian Sea (1.90 x 10(-2) and 1.21 x 10(-2) whales heard km(-1) respectively). Visual results indicated high relative abundance in the southwestern basin, with 4.88 x 10(-2) sperm whales sighted km(-1) effort. Intermediate values were obtained in the Ligurian Sea, and there were few sightings in the Alboran and Tyrrhenian Seas. Most of the sightings south of the 41degrees parallel consisted of sperm whale groups, of 5 to 7 individuals. Analysis of sperm whale distribution with respect to bathymetry did not establish a significant preference for either continental-slope waters or the open sea. High biological productivity in the northwestern basin might explain high sperm whale relative abundance, noticeably in the Gulf of Lions.