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.

Spatio-temporal trends in cetacean strandings and response in the south-western Indian Ocean: 2000-2020

“…Previous research efforts which have led to abundance estimates of cetacean populations at a local level have focused on Indo-Pacific bottlenose dolphins (Tursiops aduncus), spinner dolphins (Stenella longirostris), Indian Ocean humpback dolphins (Sousa plumbea), IWC | S. Plön et al: Spatio-temporal common bottlenose dolphins (Tursiops truncatus), southern right whales (Eubalaena australis) and humpback whales (Megaptera novaeangliae). The focus on these species is likely due to their accessibility and the relative ease with which individuals can be distinguished using photo-identification (Reisinger and Karczmarski, 2010;Findlay et al, 2011;Pusineri et al, 2014;Webster et al, 2014;Braulik et al, 2017;Dulau et al, 2017;Estrade and Dulau, 2020;Vargas-Fonseca et al, 2020;Kasuga et al, 2022;Neveceralova et al, 2022).…”

Genetic Diversity and Maternal Lineage of Indo-Pacific Bottlenose Dolphin (Tursiops aduncus) in the Andaman Sea of Thailand

“…However, T. truncatus are considered rare for Thai waters and are unlikely to be found in these areas [3,4]. T. aduncus is a coastal species that has been impacted by natural events such as morbillivirus [5,6], parasites [7], and human activities such as marine pollution [8,9], dolphin-watching tourism [10,11], and the most dominant threat, bycatch fisheries [1,[12][13][14][15]. Until recently, T. aduncus were listed as data deficient by the International Union for Conservation of Nature's Red List of Threatened Species (IUCN Red List); however, in 2019, their status was changed to near-threatened [1].…”