ABSTRACT: Isolated populations can be strongly influenced by patterns in the local production of food, and by subsidies from outside sources. We used stable-isotope analysis to investigate the relative importance of autochthonous food resources versus pelagic subsidies in the diet of the isolated population of bottlenose dolphins Tursiops sp. inhabiting Doubtful Sound, New Zealand. Samples of the primary carbon sources (macroalgae, phytoplankton and chemoautotrophs) as well as potential food sources (fish and squid) were collected from Doubtful Sound and analysed for δ¹³C and δ¹⁵N. Isotopic signatures of fishes fell along a gradient from very depleted values for deep benthic species, particularly hagfish Eptatretuscirrhatus (δ¹³C –23.6, δ¹⁵N 6.3), to intermediate values for pelagic species (δ¹³C –18.1, δ¹⁵N 11.3) and more enriched values for reef-associated species (δ¹³C –16.1, δ¹⁵N 14.4). Exfoliated skin tissue was collected from live dolphins (n = 11) and used to estimate the isotopic signature of dolphin diet (δ¹³C –15.4, δ¹⁵N 14.2). The position of this estimate at the most enriched end of the range of isotopic signatures indicated that there was likely minimal mixing of resources from different habitats and that a majority of the dolphin population’s diet came from rocky reef and demersal habitats. Estimates of δ¹³C and δ¹⁵N for dolphin diet were compared with isotopic signatures of the different primary carbon sources using a multiple-source mixing model. Both results suggest that the diet of this population was primarily made up of autochthonous carbon production with a large contribution from benthic macroalgae, rather than pelagic subsidies from outside of the Sound.