Challenges of Monitoring Endangered Bryde's Whales During a Period of Rapid Environmental Change

Estimating population size is challenging for species that occur at low densities and are difficult to detect during sampling. Challenges are further intensified by climate and anthropogenic impacts causing phenological mismatches for populations of conservation concern. In Aotearoa New Zealand (NZ), Bryde's whales ( Balaenoptera brydei ) are listed as Nationally Critical —numbering less than 250 mature individuals—in part due to previous vulnerabilities to vessel‐strike mortality. This population exemplifies the challenges associated with estimating abundance of a rare and elusive species. It is a small population distributed widely throughout northeastern NZ, where prior estimates have focused only on whales sighted in a core habitat—the Hauraki Gulf (‘the Gulf’). Here, we estimated Bryde's whale abundance in the Gulf over 10 years (2014–2023) using opportunistic photo‐identification data. We also expanded the dataset to include whales sighted 200 km north of the Gulf. To account for complex residency patterns, we developed a custom‐formulated POPAN model incorporating transience to estimate the total number of whales. Based on photo‐identification data corrected for mark rate, annual estimates ranged from 80 whales in 2014 (95% CI: 60–102) to 67 whales in 2023 (95% CI: 45–97). The superpopulation estimate (2014–2023) was 161 whales, with 30% estimated as transients. Including individuals from the wider area revealed connectivity, suggesting a single population. Despite no vessel‐strike mortality since 2014, abundance did not increase as anticipated. Over the same period, multiple marine heatwaves occurred in the Gulf with a concomitant shift in Bryde's distribution. We suggest our estimate reflects climate‐induced shifts beyond the inner Gulf or reduced carrying capacity in a degraded habitat. Our study highlights the necessity of accounting for drivers of change when estimating abundance of endangered species.