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. 2020 Oct 12;15(10):e0227085.
doi: 10.1371/journal.pone.0227085. eCollection 2020.

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

O Alejandra Vargas-Fonseca  1   2 , Stephen P Kirkman  3 , W Chris Oosthuizen  1   2   4 , Thibaut Bouveroux  1   2 , Vic Cockcroft  1 , Danielle S Conry  1   2 , Pierre A Pistorius  1   2
Affiliations

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

O Alejandra Vargas-Fonseca et al. PLoS One. .

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.

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Conflict of interest statement

The authors have declared that no competing interests exist.

Figures

Fig 1
Fig 1. Map of South Africa and the study area, which extended from the western boundary of the Goukamma MPA to the eastern boundary of the Tsitsikamma MPA.
Boat surveys were conducted approximately 100 m from the coast (dashed black line). Locations and bioregions mentioned in the text are shown on the South African map: (1) Kosi Bay; (2) Durban; (3) Ifafa (all within the Natal Bioregion). (4) Algoa Bay; (5) study area; (6) False Bay (all within the Agulhas Bioregion).
Fig 2
Fig 2. The number of new T. aduncus individuals identified from photographs per month (bar plot), and the cumulative discovery curve (per survey) for new individuals (black line).
In total, 189 surveys were conducted from March 2014 to February 2016.
See this image and copyright information in PMC

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