Long-Term Assessment of Spatio-Temporal Association Patterns of Dusky Dolphins (Lagenorhynchus obscurus) off Kaikoura, New Zealand

The challenges of monitoring fully aquatic and long-living mammals have limited research on cetaceans that spans several decades to only a few populations. We report the first long-term assessment of association and residency patterns of dusky dolphins (Lagenorhynchus obscurus) off Kaikoura, New Zealand. Dorsal fin images, spanning 30 years of data collection, were used for photo-identification and social structure analyses. A total of 4,022 uniquely marked individuals were identified. Most (80%) were sighted only one time. The population of dusky dolphins off Kaikoura appears to be open, with the possibility of immigration and emigration. Dusky dolphins showed preferences for assembly into either large or small groups, which appear to drive association patterns. They did not demonstrate significant preferred or avoided associations with each other, and had overall weak association patterns. Individuals reside in the Kaikoura area for approximately eight years, and also have temporally stable associations lasting approximately eight years. Dusky dolphins fit the model of a fission-fusion society, and their association patterns may reflect unique local ecological conditions and foraging opportunities.