Foraging Ecology of Cephalorhynchus Dolphins in Aotearoa New Zealand

Abstract

Understanding the foraging ecology of large marine predators gives insights into trophic relationships and habitat use. This typically requires knowledge of diet and environmental factors influencing spatiotemporal distribution of prey. Hector’s and Māui dolphins (Cephalorhynchus hectori hectori, and C. h. maui) are coastal species endemic to Aotearoa New Zealand. The critically endangered Māui dolphin is distributed along the west coast of the North Island (WCNI). Hector’s dolphins are discontinuously distributed around the coast of the South Island; an area where these dolphins are vulnerable and poorly understood is the north coast of the South Island (NCSI). This region has complex bathymetry and geochemistry, and oceanographic variability gives rise to very different local environments at fine-scales. Here I focus on Māui and NCSI Hector’s dolphins; these populations were identified by the Department of Conservation as high research priorities. The aim of this thesis was to investigate broad-scale temporal trends and fine-scale spatial patterns in foraging ecology of Māui and NCSI Hector’s dolphins, respectively. To achieve this I used stable isotope analysis; a method where biochemical markers in tissue can provide dietary information. Analyses used dolphin tissue from a long-term archive (~30 years) curated at the University of Auckland. For Māui dolphins, I found significant interannual variation over time, where the largest variation coincided with an intense El Niño event and was likely driven by a climate-mediated shift in prey distribution. Subsequently I used trawl survey data and remotely sensed environmental data in a species distribution model, to explore the distribution of key prey species along the WCNI, and identified environmental relationships affecting prey species richness. For NCSI Hector’s dolphins, I found significantly different isotope values between west and east NCSI, likely driven by different prey distributions and underlying oceanographic variability of each region. This work addressed key knowledge gaps in the foraging ecology of two vulnerable populations of Cephalorhynchus dolphins, and highlighted areas of data paucity in coastal New Zealand. Future research should focus on collection of long-term isotopic and environmental data to better characterise the dynamic coastal environments, which can benefit future studies of marine predators and other coastal species.