Abstract
The Critically Endangered Fiordland bottlenose dolphin (Tursiops truncatus) lives within the fiords of southern Aotearoa/New Zealand. This habitat is at the southern range edge for this species and seasonal fluctuations in water temperatures include cold winters approaching freezing temperatures. The fiords are included within the Te Moana o Atawhenua-Fiordland Marine Area (FMA) which provides broad, ecosystem-level protections. This includes the prohibition of commercial fishing within the fiords, as well as targeted measures to specifically protect the dolphins,
including voluntary agreements with tourism operators to avoid transit in specific areas and to leave dolphin encounters to chance. The Fiordland bottlenose dolphin population is comprised of four small sub-populations, two of which have been extensively studied since 1990 and 2006, the Doubtful and Dusky sub-populations, respectively. These pods are known to inhabit discrete fiord complexes year-round and have only ever been sighted interacting with individuals from their own pod.
Rooted in longitudinal life history data, Fiordland bottlenose dolphin habitat use, as well as somatic growth and population dynamics are investigated here. Photographs and life history data collected in the Patea/Doubtful and Tamatea/Dusky Sound complexes as well as other areas and fiords were organised into a relational database. This provided more flexibility in accessing demographic details, identifying individuals from overhead images, accessing relevant capture histories, and compiling details related to female reproductive state.
To explore bottlenose dolphin use of other fiords in the FMA, passive acoustic monitoring was conducted between February 2022 and November 2023, and opportunistic sightings were aggregated. Different acoustic devices, monitoring/recording schedules and detection approaches were used in this study. Occupancy was estimated by modeling detection data within a Bayesian framework where detection probabilities were estimated for the applied approach for collecting and processing acoustic data. These results demonstrate that bottlenose dolphins regularly use fiords that are not currently considered part of their habitat range, and may often use spaces outside the FMA.
An uncrewed aerial system was used to collect overhead images over a 1.5-y period to measure the length of individuals using photogrammetry. A hierarchical multivariate Bayesian approach was used to assess growth dynamics by fitting von Bertalanffy growth curves with individual random effects to individual length data. The population-level estimate of maximum total length was 2.94 m (90% CI = 2.68–3.18), which is similar to bottlenose dolphins in the Northern hemisphere living in habitats with similar water temperatures. Growth rate and maximum length were correlated indicating that slower growing dolphins tend to reach longer lengths. These results suggest that growth dynamics are not indicative of sex or pod membership, and the individual variability in size and growth, particularly among females, is highlighted.
Finally, state-space Cormack-Jolly-Seber hidden Markov models were applied within a hierarchical Bayesian framework to assess trends in Fiordland bottlenose dolphin population dynamics over a 19- and 16-y period for the Doubtful and Dusky pods, respectively. These models were used to estimate trends in survival, capture, abundance,
and fecundity over these study periods. Results indicate that adult females are most often in a state reflective of maternal investment. Overall, periods of low abundance seem to be buoyed by the high survival probability and rate of reproduction.