Abundance, spatial distribution and threats to Indo-Pacific bottlenose dolphins (Tursiops aduncus) in an Important Marine Mammal Area in Tanzania

Abstract

Abundance estimates of cetaceans in the western Indian Ocean are rare, but important, as many cetacean populations are under threat, especially those in coastal habitats. This study aimed to generate first estimates of abundance for Indo-Pacific bottlenose dolphins (Tursiops aduncus), assessed as Near Threatened on the IUCN Red List, in an area identified by the Marine Mammal Protected Area Task Force as an ‘Important Marine Mammal Area’. Two study sites were surveyed along the east and west coastlines of the Pemba Channel, Tanzania. Between 2014 and 2016, four boat-based visual surveys conducted a total of 2467 km of survey effort sighting a total of 16 groups of T. aduncus in west Pemba. Abundance was estimated using mark-recapture models of photo-identified individuals as 89 individuals (CV 7.7 %, 95 % CI 76-103) in the 1084 km2 study area. In the Tanga study area in 2016 two boat-based visual surveys covered 1254 km of effort during which 15 groups of T. aduncus were sighted, resulting in a photo-ID based mark-recapture abundance estimate of 177 individuals (CV 8.6 %, 95 % CI 150-210) in the 1562 km2 study site. Group encounter rate for this species in Tanga was at least double that recorded in the Pemba study site. A total of 23 % of identified dolphins bore the scars of interactions with fishing gear.