Abundance of Indo-Pacific bottlenose dolphins (Tursiops aduncus) along the south coast of South Africa

Abstract

Coastally distributed dolphin species are vulnerable to a variety of anthropogenic pressures, yet a lack of abundance data often prevents data-driven conservation management strategies from being implemented. We investigated the abundance of Indo-Pacific bottlenose dolphins (Tursiops aduncus) along the south coast of South Africa, from the Goukamma Marine Protected Area (MPA) to the Tsitsikamma MPA, between 2014 and 2016. During this period, 662.3h of boat-based photo-identification survey effort was carried out during 189 surveys. The sighting histories of 817 identified individuals were used to estimate abundance using capture-recapture modelling. Using open population (POPAN) models, we estimated that 2,155 individuals (95% CI: 1,873-2,479) occurred in the study area, although many individuals appeared to be transients. We recorded smaller group sizes and an apparent decline in abundance in a subset of the study area (Plettenberg Bay) compared to estimates obtained in 2002-2003 at this location. We recorded declines of more than 70% in both abundance and group size for a subset of the study area (Plettenberg Bay), in relation to estimates obtained in 2002-2003 at this location. We discuss plausible hypotheses for causes of the declines, including anthropogenic pressure, ecosystem change, and methodological inconsistencies. Our study highlights the importance of assessing trends in abundance at other locations to inform data-driven conservation management strategies of T. aduncus in South Africa.

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).…”