Beyond encounter risk: Fleet-level heterogeneity is associated with at-vessel mortality of false killer whales in the Western and Central Pacific tuna purse seine fishery

Reducing mortality of non-target species caused by fisheries bycatch is a longstanding objective of global fisheries governance. Yet, the operational processes shaping mortality after capture remain poorly understood. Based on publicly available Annual Report Part 1 data submitted to the Western and Central Pacific Fisheries Commission, linked with environmental covariates, this study applied a regularized beta-binomial model to examine factors associated with at-vessel mortality of false killer whales ( Pseudorca crassidens ) incidentally captured in industrial tuna purse seine fisheries during 2016–2020 and 2023–2024. A total of 239 false killer whale interaction events reported by six reporting entities-China, Taiwan Province of China, the Federated States of Micronesia, Kiribati, Papua New Guinea, and Nauru-were included in the analysis. These events involved 863 individuals, of which 91 were recorded as dead. The results showed that fleet was the main statistical factor explaining variation in at-vessel mortality, with a relative contribution of 53.49%, followed by year (31.93%). In contrast, environmental variables made a much smaller overall contribution (10.69%), while the two-dimensional spatial structure contributed the least (3.88%). The leave-one-fleet-out sensitivity analysis indicated that the fleet effect was sensitive to the exclusion of some fleets. This suggests that differences among fleets may reflect the combined influence of operational practices, handling procedures, reporting protocols, and other unobserved factors, rather than being attributable to any single operational variable. For the management of small-cetacean bycatch in industrial tuna purse seine fisheries, greater attention should be given not only to compliance with existing safe-release measures, but also to fishing operations during the period between accidental encirclement and retrieval alongside the vessel. Earlier detection, reduced crowding and compression within the net, and improved release procedures may help lower potential mortality risk and provide a process-based basis for marine animal conservation in industrial fisheries.