Summer spatial distribution of cetaceans in the Strait of Gibraltar in relation to the oceanographic context
Résumé
The Strait of Gibraltar, the only passage between the Mediterranean Sea and the Atlantic Ocean, and characterised by a surface inflow of Atlantic waters and a deep outflow of Mediterranean waters, is inhabited by a large number of cetacean species. The present study focuses on the occurrence and the spatial distribution of cetacean species within the strait in relation to oceanographic features. Shipboard visual surveys were conducted during the summers of 2001 to 2004, covering 4926 km. A total of 616 sightings of 7 cetacean species were made. The spatial distributions of 6 species (short-beaked common dolphins Delphinus delphis, striped dolphins Stenella coeruleoalba, bottlenose dolphins Tursiops truncatus, long-finned pilot whales Globicephala melas, sperm whales Physeter macrocephalus and killer whales Orcinus orca) were examined with respect to depth and slope. The analyses indicate that these species could be ordered into 3 groups. A first group, with a northward tendency, is composed of common and striped dolphins. Due to its at-sea location and feeding habits, this group is likely to feed on mesopelagic fishes or squids associated with the surface Atlantic waters. The second group, constituted by bottlenose dolphins, long-finned pilot whales and sperm whales, is mainly found over the deep waters of the central part of the strait. While the foraging ecology of bottlenose dolphins is still unclear, both sperm whales and pilot whales are most likely to feed on squids occurring in deep Mediterranean waters. The third group, formed by killer whales Orcinus orca, was associated with blue fin tuna Thunnus thynnus fisheries in the southwestern part of the strait.