Dietary plasticity of two coastal dolphin species in the Benguela upwelling ecosystem

Defining the trophic relationships of marine predators and their dietary preferences is essential in understanding their role and importance in ecosystems. Here we used stable isotope analysis of skin samples (δ 15 N values reflecting trophic level and δ 13 C values reflecting foraging habitat) to investigate resource partitioning and spatial differences of the feeding ecology of dusky dolphins Lagenorhynchus obscurus and Heaviside’s dolphins Cephalorhynchus heavisidii from 2 coastal study sites separated by 400 km along the coast of central (Walvis Bay) and southern (Lüderitz) Namibia in the Benguela upwelling ecosystem. Overall, isotopic niches of both predators were significantly different, indicating partitioning of resources and foraging habitats. Despite their smaller body size, Heaviside’s dolphins fed at a significantly higher trophic level than dusky dolphins. Stable isotope mixing models revealed that both species fed on high trophic level prey (i.e. large Merluccius spp., large Sufflogobius bibarbatus , and Trachurus t. capensis ) at Walvis Bay. The diet of both dolphin species included smaller pelagic fish and squid at Lüderitz. Spatial differences highlight that Heaviside’s and dusky dolphins may exhibit dietary plasticity driven by prey availability, and that they likely form distinct population segments. Important prey for both dolphin species, specifically Merluccius spp. and T. t. capensis , are the main target of trawl fisheries in the Benguela upwelling ecosystem, highlighting potential resource overlap between dolphins and fisheries.