Abstract
The small population of Hector’s dolphins that ranges along the Catlins coast forms a crucial genetic link between the dolphins in Otago and in Southland’s Te Waewae Bay. During the austral summer, dolphins from this population frequently use Porpoise Bay, which appears to be an important foraging and calving area. A perceived increase in the number of visitors to the bay raised concerns over the potential effects of interactions with swimmers, and resulted in the need for a comprehensive study to guide management of the dolphins. This thesis details the results of a study conducted during 2019/20 which aimed to update the estimate of abundance, assess the potential effects of recreational swimmers, and quantify the use of Porpoise Bay by Hector’s dolphins. Using capture-recapture modelling of photo-identification data it was estimated that 53 Hector’s dolphins (95% CI = 41-71), including three new calves, used Porpoise Bay during summer 2020. Linear regression analyses showed no evidence of a change in abundance from previous estimates in 1996/97, 2002/03, and 2008, despite the establishment of the Catlins Coast Marine Mammal Sanctuary and the introduction of restrictions on potentially harmful fishing methods in 2008. Land-based theodolite tracking was used to assess the potential effects of recreational swimmers on the behaviour of Hector’s dolphins and evaluate the effectiveness of management guidelines. Dolphins showed significant behavioural responses to swimmer presence, including a reduction in time spent ‘long-diving’ and some evidence of avoidance of swimmers when calves were present. During swimmer-dolphin interactions observed in this study, management guidelines were frequently contravened, particularly section 20(b) of the Marine Mammals Protection Regulations (i.e. persons may not swim with dolphin calves or a pod of dolphins that includes calves). Kernel density estimation and passive acoustic monitoring were used to quantify use of Porpoise Bay by Hector’s dolphins. All analyses indicated that the dolphins’ preferred area was in the south-west corner of the bay, suggesting long-term stability in the core summer habitat of this population. Dolphin presence was greatest during the summer sampling months (January to March), although dolphins were detected in the bay during non-summer months for the first time. In addition, new evidence of variation in the dolphins’ use of the bay over diel and tidal cycles was found. Given that small populations inherently have a high risk of extinction, a precautionary approach to management of the Catlins coast Hector’s dolphins’ key threats is justified. This thesis provides management recommendations designed to minimise disturbance and promote recovery of this important, vulnerable local population of Hector’s dolphins.