Occurrence, feeding ecology, and population structure of two dolphin species, Tursiops aduncus and Delphinus delphis, off the Wild Coast of South Africa

Dolphins are apex predators and their movements, foraging activities and population dynamics play an important role in shaping their environment. Understanding their occurrence, movement patterns, and trophic ecology is essential to their conservation, especially as they are considered sentinel species. The Wild Coast of South Africa’s Eastern Cape, is characterized by the annual sardine (Sardinops sagax) run, which serves as an important foraging event for apex predators, including Indo-Pacific bottlenose (Tursiops aduncus) and long-beaked common dolphins (Delphinus delphis). Despite the ecological significance of this event, no dedicated surveys for these species have previously been conducted in this area. The main objectives of this research were to: investigate long-term occurrence of dolphins during the sardine run to determine if there have been any changes over the past 19 years, as has been predicted from common dolphin dietary investigations and anecdotal evidence; to determine short-term occurrence and movement patterns of selected inshore delphinids within the area, which is characterized by three marine protected areas (MPAs); and to use stable isotope analyses to determine trophic relationships and population structure of bottlenose and common dolphins within the region. Long-term data consisted of 131 opportunistic aerial surveys conducted between May and July over the period 1996 to 2014. Results from these surveys indicate that common dolphins, typically associated with sardines, decreased significantly in average group size over the study period (p=0.0343). Bottlenose dolphins, demonstrated no such trend (p=0.916). For both species, greater majority (> 70% of total counts) of sightings were made inside the MPAs. Short-term boat-based surveys were conducted three times a year between June 2014 and December 2016, contributing to a total of 47 days of surveys divided into three locations: Amathole, Hluleka, and Pondoland, each containing a MPA. Density and group size data were analyzed for both species and photographic identification analysis was performed for photographs of bottlenose dolphin dorsal fins. Results indicate that animal and sighting density did not differ temporally (bottlenose dolphin: sighting density – p=0.398, individual density –p=0.781; common dolphin: sighting density –p=0.472, individual density – p=0.204). Environmental factors (sea surface temperature, depth, substrate, and distance from shore) appeared to have limited effect on individual and sighting density and group size for both species (p>0.05). Photographic identification of bottlenose dolphins resulted in 2149 individuals, with a 11.8% resighting rate, with the highest resighting rate within the Pondoland MPA (16.1%). The resighting count did not differ temporally between monthly survey based on generalized linear models (p=0.866), but did differ between study areas (p