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.  In west Pemba, 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. Abundance was estimated using mark-recapture models of photo-identified individuals as 83 animals (CV 7.8%, 95% CI 72-97) in the 1084km² 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 196 (CV 8.9%, 95% CI 165-233) individuals in the 1562 km² study site. Group encounter rate for this species in Tanga was double that recorded in the Pemba study site. A total of 23% of identified dolphins bore the scars of interactions with fishing gear.