Niche partitioning among odontocetes in a marine biogeographic transition zone of the western South Atlantic Ocean

Toothed cetaceans represent a diverse and important component in poorly studied pelagic food webs. We used the variation in δ13C and δ15N values in tooth dentin and bone collagen sampled from 185 specimens of 23 species (families Delphinidae, Physeteridae, Kogiidae, Phocoenidae, and Ziphiidae) stranded in the western South Atlantic Ocean (WSAO) to estimate the isotopic niche width and assess the interspecific isotopic niche overlap. Two gradients of increasing δ13C values among species were evidenced: ocean-coast and high-to-low latitudes. Variation in δ13C and δ15N values indicated the existence of at least four trophic and habitat-related guilds: neritic high trophic-level predators (Pseudorca crassidens, Tursiops truncatus gephyreus, T. truncatus truncatus and Phocoena spinnipinis); oceanic intermediate trophic-level predators (Orcinus orca, Steno bredanensis, Kogia breviceps, Lagenodelphis hosei, Physeter macrocephalus, Delphinus delphis and Stenella frontalis), oceanic low trophic-level predators (Grampus griseus, P. crassidens—mass stranded group, Mesoplodon grayi, M. densirostris, Ziphius cavirostris, Globicephala melas, S. coeruleoalba, and K. sima), all from the subtropics, and a temperate/polar grouping of neritic/oceanic low trophic-level predators (Berardius arnuxii, M. layardii, M. hectori and P. dioptrica). Furthermore, our results also suggest the occurrence of putative ecotypes of O. orca and P. crassidens. The isotopic niche width was highly variable among species, with high niche overlap among several oceanic taxa. The results of the present study suggest a wide variation in trophic position, habitat and, therefore, the ecological role of different odontocete species in the WSAO ecosystems.