Spatial variation in isotope values of Hector’s dolphins from the north coast of the South Island, New Zealand

Abstract

The north coast of the South Island, Aotearoa New Zealand, is a region with complex bathymetry and biogeochemistry, where oceanographic variability gives rise to very different local environments at fine scales. This variation also influences the region’s isoscape, providing the ability to assess the fine-scale foraging behaviour of top-level marine predators through isotope analyses. Hector’s dolphin Cephalorhynchus hectori hectori, an endemic coastal dolphin, is resident of the north coast of the South Island, but there is limited information describing its foraging ecology and population dynamics. We analysed carbon and nitrogen stable isotope ratios of skin samples (n = 111) from Golden Bay in the west (n = 14), and Queen Charlotte Sound (n = 41) and Cloudy Bay (n = 56) in the east, to investigate spatial variation in isotope values and niche space, found significant differences between the 2 regions. This is likely driven by a combination of differing prey distributions, underlying oceanographic variability, and varying isotopic baselines that may act as an ecological boundary preventing movement between the 2 regions. The isotopic niche space between the west and east differed, but within the east, Queen Charlotte Sound was a subset of Cloudy Bay. This suggests a common prey source and possible movement of individuals between Queen Charlotte Sound and Cloudy Bay. This research highlights the value of stable isotope analysis to investigate regional-scale variation of top-level marine predators and can provide insight into environmental factors that influence resource use.