Updates of stock assessment of Pacific saury (Cololabis saira) in the Western North Pacific Ocean through 2023

This paper describes the updates of stock assessment of the Pacific saury (Cololabis saira) in the Western North Pacific Ocean (WNPO) based on the guideline of the 2024 SSC PS14. The assessment consisted of applying the Bayesian state-space surplus production model for estimating the biomass from 1980 to 2024 with available catches from 1980 to 2023. Abundance indices available for WNPO Pacific saury consisted of updated standardized catch-per-unit-effort (CPUE) indices of stick-held dip net fisheries from Japan (1980 – 2023), early and late Chinese Taipei (2001 – 2010; 2011 – 2023), Russia (1994 – 2021), Korea (2001 – 2023), and China (2013 – 2023), joint CPUE (1994 – 2023), and biomass survey from Japan (2003 – 2024). Two base case models were considered for the assessment outputs, differing in their use of individual fleet CPUE indices versus the joint CPUE index. The results of two base case models indicated that the estimated biomass had a similar trend over years. The ensemble time-series of biomass is estimated to have an increasing pattern since 2000 with peak in mid-2000s, after then dramatically decreased overtime and below B_MSY in 2009 – 2022. It should be noted that the models estimate the lowest biomass level in 2020 and 2021(median B₂₀₂₀/B_MSY = 0.217, 80 percentile range 0.164 – 0.297; median B₂₀₂₁/B_MSY = 0.219, 80 percentile range 0.162 – 0.303) and following a slight increase in 2022 and 2023 (median B₂₀₂₂/B_MSY = 0.27, 80 percentile range 0.20 – 0.37; B₂₀₂₃/B_MSY = 0.339, 80 percentile range 0.242 – 0.470). In the recent three years (2022 – 2024), the biomass was estimated below the B_MSY (median B_2022-2024/B_MSY = 0.352, 80 percentile range 0.251– 0.482). The fishing mortality was below F_MSY before 2007, and then the fishing mortality increased above F_MSY andreached a high level in 2014 and 2018, respectively. A decreased trend in fishing mortality was found from 2021 to 2023, and the recent average fishing mortality is estimated to be close to F_MSY (median F_2021-2023/F_MSY = 0.988, 80 percentile range 0.738 – 1.333). It should be noted that the models estimate a slightly decreasing in fishing mortality in 2023 (median F₂₀₂₃/F_MSY = 0.903, 80 percentile range 0.643 – 1.301). The ensemble MCMC results from the two base cases indicated that the 2023 stock status is likely within the yellow quadrant (Prob [B₂₀₂₃<B_MSY and F₂₀₂₃<F_MSY] = 64%).