Growth dynamics of bottlenose dolphins Tursiops truncatus at the southern extreme of their range

Populations living at extreme range edges have physiological adaptations related to size and growth patterns, allowing them to survive in dynamic habitats. The Critically Endangered bottlenose dolphins Tursiops truncatus inhabiting the fiords of southwestern Aotearoa/New Zealand are living at the southern extreme of the species’ range. Previous efforts to quantify the size of these dolphins using vessel-mounted photogrammetry systems yielded small sample sizes. Here, we used an uncrewed aerial system to collect video over 1.5 yr of 143 individuals from 2 small sub-populations of Fiordland bottlenose dolphins. Individuals were identified based on the curation of marks on the dorsal body, allowing integration with life history information available through longitudinal study. Photogrammetric measurements of total length and blowhole to dorsal fin length were taken from 1218 images using the software ‘whalength’. Growth dynamics were estimated within a hierarchical multivariate Bayesian model with individual random effects using measured lengths to fit von Bertalanffy growth curves. Across the population, the median posterior predictive distribution for total length was 2.94 m (90% credible interval: 2.68-3.18 m) with estimates of known reproductive females ranging from 2.73 to 3.30 m. Our results indicate that slower growth rates tend to be correlated with larger adult dolphins, which has local implications for survival probability and reproductive success. These results provide an improved understanding of Fiordland bottlenose dolphin morphology, which adds perspective to the global structure of this species. Furthermore, we demonstrate the utility of incorporating photogrammetric methods into monitoring programmes of dolphins to assess population health and resilience.