Updated stock assessment of Pacific saury (Cololabis saira) in the Western North Pacific Ocean through 2021

This paper describes the updated 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 2022 with available catches from 1980 to 2021. Abundance indices available for WNPO Pacific saury consisted of standardized catch-per-unit-effort (CPUE) of stick-held dip net fisheries from Japan (1980 – 2021), Chinese Taipei (2001 – 2021), Russia (1994 – 2021), Korea (2001 – 2021), and China (2013 – 2021), joint CPUE index (1994 – 2021) and biomass survey from Japan (2003 – 2022). Two base case models were considered for the assessment outputs. The results of the 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 B_MSY in 2009 – 2022. It should be noted that the models estimate the lowest biomass level in 2020 (median B₂₀₂₀/B_MSY = 0.29, 80 percentile range 0.18 – 0.51) and following a slight increase in 2021 and 2022 (median B₂₀₂₁/B_MSY = 0.33, 80 percentile range = 0.19 – 0.56; median B₂₀₂₂/B_MSY = 0.45, 80 percentile range = 0.23 – 0.77). In the recent three years (2020 – 2022), the biomass was estimated below the B_MSY (median B_2020-2022/B_MSY = 0.36, 80 percentile range = 0.22 – 0.60). A steady increase in fishing mortality is estimated to have occurred from 2004 to 2018, but a decreasing trend in fishing mortality was found from 2019 to 2021. The recent average fishing mortality is estimated to be above F_MSY (median F_2019-2021/F_MSY = 1.21, 80 percentile range = 0.71 – 2.16) while the fishing mortality in 2021 is less than F_MSY (median F₂₀₂₁/F_MSY = 0.75, 80 percentile range = 0.43 – 1.45). The ensemble MCMC results from the two base cases indicated that the 2021 stock status is likely within the yellow quadrant (Prob [B₂₀₂₁<B_MSY and F₂₀₂₁<F_MSY] = 72.82%)