Preliminary updates of stock assessment of Pacific saury (Cololabis saira) in the Western North Pacific Ocean through 2022

This paper describes the preliminary updates of stock assessment of the Pacific saury (Cololabis saira) in the Western North Pacific Ocean (WNPO) based on the guideline of the 2021 SSC PS09. The assessment consisted of applying the Bayesian state-space surplus production model for estimating the biomass from 1980 to 2023 with available catches from 1980 to 2022. Abundance indices available for WNPO Pacific saury consisted of updated standardized catch-per-unit-effort (CPUE) of stick-held dip net fisheries from Japan (1980 – 2022), Chinese Taipei (2001 – 2022), Russia (1994 – 2021), Korea (2001 – 2022), and China (2013 – 2022), joint CPUE index (1994 – 2022) and biomass survey from Japan (2003 – 2023). Two base case models were considered for the assessment outputs. 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 two peaks in 2003 and 2005, after then dramatically decreased overtime and below BMSY in 2009 – 2022. It should be noted that the models estimate the lowest biomass level in 2020 (median B2020/BMSY = 0.25, 80 percentile range 0.18 – 0.36) and following a slight increase in 2021, 2022 and 2023 (median B2021/BMSY = 0.25, 80 percentile range 0.18 – 0.37; median B2022/BMSY = 0.32, 80 percentile range 0.22 – 0.47; median B2023/BMSY = 0.42, 80 percentile range 0.26 - 0.66). In the recent three years (2021 – 2023), the biomass was estimated below the BMSY (median B2021-2023/BMSY = 0.34, 80 percentile range 0.23 – 0.49). A steady increase in fishing mortality is estimated to have occurred from 2004 to 2018, but a decrease trend in fishing mortality was found from 2020 to 2022, and the recent average fishing mortality is estimated to be above FMSY (median F2020-2022/FMSY = 1.08, 80 percentile range = 0.78 – 1.51). The ensemble MCMC results from the two base cases indicated that the 2022 stock status is likely within the yellow quadrant (Prob [B2022<BMSY and F2022<FMSY] = 75.41%).