Resource partitioning among South African delphinids

In order to better understand trophic relationships among four species of coastal delphinids, we compared isotopic composition of skin to attempt to assess potential inter- and intra-specific resource partitioning. Skin samples were collected from Indo-Pacific bottlenose dolphins (Tursiops aduncus) (n = 132), long-beaked common dolphins (Delphinus capensis) (n = 78), humpback dolphins (Sousa chinensis) (n = 27), and striped dolphins (Stenella coeruleoalba) (n = 3) along the coastline of South Africa. While the latter species tends to be found offshore, the other species have overlapping distributions and feed on similar prey, carbon (δ13C) and nitrogen (δ15N) isotope analyses revealed resource partitioning with differences in diet and habitat use. Striped dolphin δ13C values (− 16.97 ± 0.25‰, SD) were consistent with evidence that they typically forage offshore, while δ13C values of humpback dolphins reflected their use of inshore habitats (− 15.16 ± 0.65‰). Common and bottlenose dolphins had δ13C (− 15.48 ± 0.66‰ and − 15.76 ± 0.71‰ respectively) values that fell between these two extremes. Mean values for δ15N ranged from 11.92 ± 0.20‰ for striped dolphins to 15.19 ± 0.73‰ for humpback dolphins, suggesting either that these species were feeding at different trophic levels or that they were feeding in different trophic systems. Common and bottlenose dolphins had δ15N values of 13.49 ± 0.50‰ and 14.40 ± 0.74‰ respectively. Male bottlenose dolphins were significantly more enriched in δ15N compared to females suggesting dietary differences. No sex related differences were found in other species. Isotopic niche width determinations using corrected standard elliptical area (SEAc) were calculated. Humpback and bottlenose dolphins had the largest SEAc reflecting a broader trophic niche, while striped dolphins had the smallest SEAc reflecting a more specialized niche. Overall, these sympatric species appear to reduce potential competitive pressure through a combination of differing prey selection and habitat utilization.