Pan-tropical spotted dolphins (Stenella attenuata) and other cetaceans around St Helena in the tropical south-eastern Atlantic

Abstract

The occurrence, distribution and structure of cetacean communities in the tropical South Atlantic beyond the shelf edge are poorly known with little dedicated research occurring within this region. At 15°58′S 005°43′W, the island of St Helena is one of the few areas of land within this region and the only one that lies in the tropical south-eastern Atlantic. As a result, St Helena offers a unique opportunity to study cetaceans within this area using small boats and land-based observations. This paper describes the results of a preliminary, short-term survey of the cetacean community around St Helena in the austral winter of 2003. Pan-tropical spotted dolphins (Stenella attenuata) were the most numerous species recorded, followed by bottlenose dolphins (Tursiops truncatus) and rough-toothed dolphins (Steno bredanensis), a species not previously reported from St Helena. This last species was only recorded occurring in mixed groups with bottlenose dolphins. Pan-tropical spotted and bottlenose dolphins differed in their spatial distribution around St Helena. While pan-tropical spotted dolphins were primarily recorded resting in large groups in the lee of the island during daylight hours, bottlenose dolphins and rough-toothed dolphins were recorded closer to shore and on both the windward and lee sides. Humpback whales (Megaptera novaeangliae) were also recorded once during the survey, but interviews with local fishermen suggest that this species regularly occurs in the waters around St Helena in small numbers during the austral winter. The results of this preliminary survey suggest that the cetacean community around St Helena during this survey was relatively simple, consisting of up to three species that are present year-round and one seasonally occurring species in the nearshore waters, with a small number of additional species occurring occasionally in deeper offshore areas.