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

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.