Habitat use and preferences of cetaceans along the continental slope and the adjacent pelagic waters in the western Ligurian Sea

The physical habitat of cetaceans occurring along the continental slope in the western Ligurian Sea was investigated. Data were collected from two different sighting platforms, one of the two being a whale-watching boat. Surveys, conducted from May to October and from 1996 to 2000, covered an area of approximately 3000 km2 with a mean effort of about 10,000 km year−1. A total of 814 sightings was reported, including all the species occurring in the area: Stenella coeruleoalba, Balaenoptera physalus, Physeter macrocephalus, Globicephala melas, Grampus griseus, Ziphius cavirostris, Tursiops truncatus, Delphinus delphis. A Geographic Information System was used to integrate sighting data to a set of environmental characteristics, which included bottom gradient, area between different isobaths, and length and linearity of the isobaths within a cell unit. Habitat use was analysed by means of a multi-dimensional scaling, MDS, analysis. Significant differences were found in the habitat preference of most of the species regularly occurring in the area. Bottlenose dolphin, Rissoʹs dolphin, sperm whale and Cuvierʹs beaked whale were found strongly associated to well-defined depth and slope gradient characteristics of the shelf-edge and the upper and lower slope. The hypothesis of habitat segregation was considered for Rissoʹs dolphin, sperm whale and Cuvierʹs beaked whale. Canonical discriminant functions using depth and slope as predictors outlined clear and not overlapping habitat preferences for Rissoʹs dolphin and Cuvierʹs beaked whale, whereas a partial overlapping of the habitat of the other two species was observed for sperm whale. Such a partitioning of the upper and lower slope area may be the result of the common feeding habits and suggests a possible competition of these three species. A temporal segregation in the use of the slope area was also observed for sperm whales and Rissoʹs dolphins. Fin whales, and the occasionally encountered common dolphin and long-finned pilot whales showed a generic preference for the pelagic area. In contrast, striped dolphins were generically found in any type of physical habitat, in agreement with their known opportunistic feeding habits. Interannual and seasonal movements were observed in fin whales and in all the other regularly occurring species, indicating that changes in the environmental and climatological conditions have a remarkable influence on habitat use. Finally, a relationship between group size and habitat use was observed for the species known to forage in groups. This relationship suggests that these species respond to environmental variability either by moving within the study area or by behavioural changes that may reflect changes in the preference for prey items.