If These Walls Could Talk: Investigating Bottlenose Dolphin Occupancy and Range in Fiord Ecosystems Using Passive Acoustic Monitoring and Visual Surveys

Abstract

Effective management of protected species requires a comprehensive understanding of their ecology. Within the Fiordland (Te Moana o Atawhenua) Marine Area (FMA) in south-western Aotearoa/New Zealand, two of the four recognised bottlenose dolphin (Tursiops truncatus) subpopulations are considered resident to discrete fiord systems. Opportunistic sightings outside the Patea/Doubtful and Tamatea/Dusky Sound complexes, however, indicate these subpopulations occupy a larger space than is currently recognised. To investigate bottlenose dolphin use of remote spaces in the FMA, passive acoustic monitoring equipment was deployed in six of the southern fiords. Acoustic data were collected between February 2022 and November 2023 using two types of devices: F-PODs to detect click trains of odontocetes and SoundTraps to collect recordings to explore a variety of call types. We used automated classifiers/detectors and manual browsing to detect various vocalisation types. To better understand dolphin occupancy at each acoustic monitoring site, we constructed a hierarchical Bayesian model to estimate detection probabilities of the different data collection and signal detection approaches. Photo-identification was used to identify individual dolphins in remote areas, and differences in detection between visual and acoustic methods were explored. This study demonstrates that dolphins, including two resident subpopulations of Fiordland bottlenose dolphins, regularly use fiords that are not currently considered part of their habitat range and may often use spaces outside the FMA. This increased understanding of the spatial ecology of Fiordland bottlenose dolphins underscores the need for a re-evaluation of their habitat and potential threats throughout this region.