Abundance of Indian Ocean humpback dolphins (Sousa plumbea) along the Garden Route of South Africa

Human development is placing growing pressure on coastal ecosystems worldwide. Coastal dolphins are consequently exposed to a range of anthropogenic threats, yet for many species, limited knowledge of population size and dynamics hinders effective conservation planning. Four species of humpback dolphin (Sousa spp.) inhabit shallow coastal waters globally, and all are threatened by human activities. Along the South African coast, the endangered Indian Ocean humpback dolphin (Sousa plumbea) occurs in a small, declining population whose abundance is thought to be fewer than 500 individuals. Here, we estimate the abundance and group structure of this species along the Garden Route on South Africa’s south coast. We conducted photo-identification surveys from March 2014 to June 2015 and fitted a Bayesian open-population (Jolly-Seber) model to encounter histories of 65 identified individuals. Dolphins were encountered most consistently in the west of the study area, particularly inside the Goukamma marine protected area, and never in large groups. We estimated that approximately 89 individuals occurred in the study area over the survey period, representing around one-fifth of the latest abundance estimate for the species in South Africa. We also analyzed intermittently collected photo-identification data from Plettenberg Bay, a subset of the study area, to evaluate temporal changes in local abundance. This suggested a decline in abundance over comparable 12-month periods, from a posterior mean of 112 individuals in 2002/03 to approximately 60–70 individuals in 2012/13 and 2014/15, consistent with declines reported elsewhere across the species’ range. This study underscores the importance of the Garden Route region for conservation of this species and provides a valuable baseline against which future population change can be assessed.