Updates of stock assessment of Pacific saury (Cololabis saira) in the Western North Pacific Ocean through 2024

This paper presents the latest stock assessment update for Pacific saury (Cololabis saira) in the Western North Pacific Ocean (WNPO), following the modeling framework recommended by SSC PS15 of the North Pacific Fisheries Commission (NPFC). A Bayesian state-space surplus production model was applied using fishery catches from 1980 to 2024 and standardized catch-per-unit-effort (CPUE) indices provided by NPFC Members. The abundance indices included updated CPUE series from Japan, Chinese Taipei (two-period approach: 2001 - 2010 and 2011-2024), Korea, Russia and China, a joint CPUE index from 1994 to 2024, and an updated Japanese biomass survey index from 2003 to 2025. Two base case models were considered, which differed in the treatment of individual fleet CPUEs versus using the joint CPUE index. Both base case models showed consistent biomass trends after 2011, but differed notably from 1990 to 2010. Base case 2 displayed smoother variability and yielded higher biomass estimates than base case 1 during this period. The ensemble biomass estimates indicate an increasing pattern from 2000, peaking in the mid-2000s, followed by a continuous decline below B_MSY since 2009. The lowest stock condition was estimated in 2020 and 2021 (median B₂₀₂₀/B_MSY = 0.233; 80% CI: 0.176 - 0.331; median B₂₀₂₁/B_MSY = 0.238; 80% CI: 0.176 - 0.338), with a slight increase observed in recent years. Biomass in 2025 is below B_MSY (median B₂₀₂₅/B_MSY = 0.510; 80% CI: 0.340 - 0.749), showing slightly recovery from the 2020 - 2021 lows but not yet reach to B_MSY. Over the most recent three years (2023 - 2025), biomass was consistently estimated below B_MSY (median B_2023–2025/B_MSY = 0.429; 80% CI: 0.309 - 0.596). The fishing mortality (F) was below F_MSY prior to 2007, increased above F_MSY thereafter, and reached high levels in 2014 and 2018. A declining pattern in F was estimated from 2021 to 2024, with recent fishing mortality close to the F_MSY (median F_2022–2024/F_MSY = 0.927; 80% CI: 0.684 - 1.278). Estimated fishing mortality in 2024 increased slightly (median F₂₀₂₄/F_MSY = 0.972; 80% range: 0.684 – 1.408) compared with 2023 (median F₂₀₂₃/F_MSY = 0.893; 80% range: 0.646 – 1.243). The ensemble MCMC stock status results suggest that the stock is likely in the yellow quadrant of the Kobe plot, below B_MSY while fishing mortality is near or below F_MSY (Prob[B₂₀₂₄ < B_MSY and F₂₀₂₄ < F_MSY] = 54%).