Abstract
The closed population of bottlenose dolphins inhabiting Doubtful Sound, New Zealand, has access to several potential food sources within the boundaries of its home range. Some of these sources are in the form of fishes strongly associated with benthic habitats inside this deep fjord and likely represent local production. The other main food source available is pelagic fishes, which represent a resource that has the potential to provide the system with subsidies from the outer coast or Tasman Sea. In this study, stable isotope analysis of carbon and nitrogen was used to investigate the importance of local food sources versus subsidies in the diet of this population of bottlenose dolphins.
Exfoliated skin tissue could be retrieved from live dolphins with minimal consequences to the animals and was used for the stable isotope analyses. This tissue was an appropriate tissue to use for stable isotope analysis as estimates of δ15N and δ13C values for this tissue could be related reliably to those of muscle tissue, which is a better proven indicator of diet.
Fishes were collected from Doubtful Sound to define isotopic patterns of the potential food sources to the dolphins. δ13C and δ15N values of fishes mainly reflected the habitats these fishes utilised. Fishes associated with the deep basins or estuarine habitat, for example had low δ15N and δ13C values compared to other fishes. Mobile pelagic species and a few local demersal species formed a distinct group and so did upper trophic level fishes and fishes with strong association to reef habitats. Tissue from fishes in this last group displayed the highest values for δ15N and δ13C.
Exfoliated skin tissue retrieved from live bottlenose dolphins was analysed for stable carbon and nitrogen isotopes and compared to isotopic signature of the different groups of fishes. The findings of these comparisons were consistent with the idea that the diet of this population was likely to have consisted mainly of fishes with a strong association to the rocky reef and other closely related benthic habitats. It was found unlikely that the population had consumed a large amount of pelagic prey species during the late summer/autumn and winter of 2002. These findings indicated that the population was supported mainly by local production during the course of this study.