First large-scale abundance estimates of fin whales (Balaenoptera physalus) in the North Pacific: Implications for management

The fin whale ( Balaenoptera physalus ) was exploited by global commercial whaling well beyond sustainable population levels in most regions since the early 1870s. In the post-whaling era, fin whale populations in several regions may have stabilized or begun recovering from historical depletion. However, large-scale abundance information has remained limited owing to the high operating costs of surveys and the challenges of international cooperation. We report the first large-scale abundance estimates for the entire North Pacific during the boreal summer (July to September) using vessel-based line transect sighting data collected through long-term domestic and international research programs over two periods: 2008–2014 and 2017–2022. During the second period, the survey area included the Bering Sea. Assuming a detection probability on the trackline g (0) of 1, the abundance estimates were 6633 (CV = 0.453) and 31,835 (CV = 0.264) individuals for the western and eastern North Pacific, respectively, in the first period; and 4405 (CV = 0.241), 37,297 (CV = 0.181), and 9885 (CV = 0.201) for the western and eastern North Pacific and Bering Sea, respectively, in the second period. Considering g(0) corrections, the corrected abundance estimates for the entire North Pacific were 44,523 (CV = 0.234) and 45,344 (CV = 0.167) in the first and second periods, respectively. For the Bering Sea, the estimate was 10,234 (CV = 0.202) in the second period. These new estimates are available for the conservation and management purposes of this species in the North Pacific. • Information regarding the fin whale population in the North Pacific was lacking. • Large-scale domestic and international sighting surveys were conducted previously. • Distance sampling was used for abundance estimation of fin whales. • Perception bias was estimated with the mark-recapture distance sampling method. • Estimates are available for managing the North Pacific fin whale population.