Estimating the abundance of the Hector’s dolphins (Cephalorhynchus hectori hectori) that use Porpoise Bay, New Zealand

ABSTRACT

Hector’s dolphin, an endangered species endemic to the coastal waters of Aotearoa New Zealand, has a highly fragmented population structure. Low abundance of some local subpopulations, coupled with a stepping-stone mode of gene flow, suggests that these are at high risk of extinction. During the austral summer, dolphins from the small, genetically distinct subpopulation found along the South Island’s south-east coast frequently use Porpoise Bay, an important foraging and calving area. The estimate of abundance of dolphins using the bay has not been updated since the introduction of protection measures in 2008 intended to mitigate bycatch in fisheries. Photo-ID surveys, including random photography to estimate mark rate, were conducted during summer 2020. Using Chapman’s version of the Lincoln-Petersen estimator, it was estimated that 63 Hector’s dolphins (95% CI = 46–79), including three new calves, used Porpoise Bay during the study period. While regression analyses showed no evidence of change since previous abundance estimates in 1996–97, 2002–03, and 2008, the very low estimated abundance validates fears for the long-term persistence of this subpopulation. In this context, potential threats need to be managed within a precautionary framework, with a clear goal of facilitating recovery.

KEYWORDS:

• Abundance
• small population
• capture–recapture
• multi-decadal
• comparison

Acknowledgements

The authors would like to thank staff from the Department of Conservation, especially Ros Cole, for their support and involvement in the project. We would also like to thank the volunteers who gave up their time to help with fieldwork, administrative staff from the University of Otago, and the Porpoise Bay locals whose local knowledge was invaluable to this project. The authors appreciate the support of ngā rūnaka ki Murihiku. M.H. was supported by a University of Otago Masters Research Scholarship.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This research was supported by funding from the Department of Conservation, New Zealand Whale and Dolphin Trust, and the Department of Marine Science, University of Otago.