From May to September 2004 a survey of cetacean distribution and relative abundance was conducted on board a ‘ship of opportunity’—a vessel used as a survey platform but not chartered for this purpose—off the west coast of Ireland. In total, 508 hours of survey effort were completed, with 304.2 hours in Beaufort sea-state 3 or less. Two hundred and thirty one sightings of eight species—common dolphin Delphinus delphis (L.), Atlantic white-sided dolphin Lagenorhynchus acutus (Gray), bottlenose dolphin Tursiops truncatus (Montagu), harbour porpoise Phocoena phocoena (L.), Risso’s dolphin Grampus griseus (Cuvier), pilot whale Globicephala melas (Traill), minke whale Balaenoptera acutorostrata (Lacepede) and fin whale Balaenoptera physalus (L.)—were recorded, totalling 2,933 individuals. The greatest diversity and relative abundance were recorded on the Rockall Bank, where Atlantic white-sided dolphin was the most abundant species. The common dolphin was the most abundant species recorded on the continental shelf to the south-west of Ireland, while the relative abundance of cetaceans off the north coast was very low. The data suggest that cetacean species composition and relative abundance can vary greatly between adjacent marine habitat types and over relatively small geographical distances.
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