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.
Fig S1. Survey tracks along the study area.
Table S1. Search effort per section of the study area, year and season.
Table S2. Model selection and abundance estimates for T. aduncus obtained from POPAN open population Jolly-Seber models.