Multi-disciplinary surveys reveal a small subpopulation of Hector's dolphins is frequently exposed to the risk of bycatch

Hector's dolphin (Cephalorhynchus hectori) is an endangered species endemic to the coastal waters of Aotearoa New Zealand. To address the unsustainable bycatch of Hector's dolphins, restrictions on commercial gillnetting within 4 nautical miles of the coast were implemented in 2008, covering most of the South Island's east and south coasts. A small subpopulation of Hector's dolphins, estimated at 42 (95% CI: 19-92) in 2012, is located off Dunedin, on the south-east coast of the South Island. It is known from other locations that Hector's dolphins are distributed well beyond 4 nautical miles from the coast and hence are vulnerable to ongoing gillnetting. In this study, a combination of boat-based surveys, passive acoustic monitoring (PAM) and analysis of fishing effort were used to generate an up-to-date abundance estimate for Hector's dolphins off Dunedin and assess their vulnerability to commercial fishing. In the summer of 2021, abundance was estimated at 41 individuals (95% CI = 31-54), providing no evidence for population change since 2012. Analysis of visual sighting data indicated ongoing overlap between Hector's dolphins and commercial fishing effort. Using PAM, Hector's dolphins were detected on 94% of days at a location approximately 0.3 nautical miles from the coast, suggesting that shallow waters provide critical habitat for these dolphins year-round. At an offshore mooring, approximately 4.5 nautical miles from the coast and therefore outside the area in which gillnetting is prohibited, detections were made on 35% of days, indicating potential for direct overlap with commercial fisheries. The inherent vulnerability of small populations to extinction highlights the need to take a precautionary approach to the management of Dunedin's Hector's dolphins if population recovery is to be facilitated.