Isotopic niche overlap between sympatric Australian snubfin and humpback dolphins

Ecological niche theory predicts the coexistence of closely related species is promoted by resource partitioning and leads to the use of different ecological niches. Australian snubfin (Orcaella heinsohni) and humpback (Sousa sahulensis) dolphins live in sympatry throughout most of their range in northern Australia. We compared stable isotope ratios of carbon (δ13C) and nitrogen (δ15N) in their skin to investigate resource partitioning between these ecologically similar species. Skin samples were collected from live Australian snubfin (n = 31) and humpback dolphins (n = 23) along the east coast of Queensland in 2014-2015. Both species had similar δ13C and δ15N values and high (>50%) isotopic niche space overlap, suggesting that they feed at similar trophic levels, have substantial dietary overlap, and both rely on similar basal food resources. Despite similarities, snubfin dolphins were more likely to have a larger δ15N than humpback dolphins, indicating they may forage on a wider di...