Abstract
Sperm whales (Physeter macrocephalus) have a major impact on the marine ecosystem as top predators, nutrient vectors, and indicators of ecosystem health. Male sperm whales have been studied off the coast of Kaikōura, New Zealand for the past three decades, where the submarine canyon and surrounding area are a known foraging ground. Outside of Kaikōura, there have been no comprehensive studies on male sperm whales elsewhere in New Zealand. My research is the first to expand our understanding of the sperm whales visiting New Zealand in another location. The topographic and oceanographic features off Otago form a habitat that has the potential to be an important foraging ground for male sperm whales. Therefore, the overall aim of this thesis was to obtain data on the sperm whales around Otago and compare it to those from Kaikōura to investigate whether Otago was being used in a similar way and by a similar demographic of individuals. This thesis presents findings from 14 systematic surveys conducted over Otago’s submarine canyon complex and 32 surveys over the Kaikōura Canyon and surrounding area. At both locations, the boat-based visual and acoustic surveys were carried out over three field seasons (two winters and one summer) between June 2022 and July 2023. The surveys collected data that characterised the occurrence, distribution, size structure, and behaviour of sperm whales around Otago and Kaikōura during the study period. Comparisons of seasonal distribution and behaviours were made within locations if the sample size was large enough. Size was acoustically estimated from the multi-pulse structure of their echolocation clicks using acoustic recordings obtained during surveys. Surface and diving behaviour metrics (i.e., surface interval, blow interval, time to first echolocation click and last echolocation click to surface) were used to investigate their foraging behaviour. Although the density of sperm whales visiting Otago was lower than in Kaikōura, a similar demographic of individuals were utilising both habitats, mainly sub-adult males. The similarities in size structure, spatial dispersion, lack of social interaction, and foraging behaviours observed in both locations, along with the encounter rates over winter and instances of site fidelity, suggest that Otago is another important foraging ground for sub-adult male sperm whales in New Zealand. However, some of their foraging behaviours around Otago were significantly different (i.e., surface interval and time to first click), suggesting that they may be employing different feeding strategies compared to the whales visiting Kaikōura. This was also the first study to demonstrate habitat connectivity in New Zealand through the photo identification of the same individual at both foraging grounds. Future studies on sperm whales around the Otago Canyons are essential to continuing to develop our knowledge of the species and discern whether conservation management is required in the area.