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 NPFC SSC PS07. The assessment consisted of applying the Bayesian state-space surplus production model for estimating the biomass from 1980 to 2021 with available catches from 1980 to 2020. Abundance indices available for WNPO Pacific saury consisted of standardized catch-per-unit-effort (CPUE) of stick-held dip net fisheries from Japan (1980 – 2020), Chinese Taipei (2001 – 2020), Russia (1994 – 2020), Korea (2001 – 2020), and China (2013 – 2020), and biomass survey from Japan (2003 – 2021). Two base case models were considered for the assessment outputs. The results of two base case models indicated that the estimated biomass trends before 2000 were sensitive to the early Japanese CPUE index (1980 – 1993). The ensemble time-series of biomass is estimated to have an increasing pattern since 2000 with the peaks in 2003, 2005 and 2008, after then dramatically decreased overtime and below B_MSY in 2015 – 2021. It should be noted that the models estimate the lowest biomass level in 2020 (median B₂₀₂₀/B_MSY = 0.43, 80 percentile range 0.27 – 0.66) and following a slightly increase in 2021 (median B₂₀₂₁/B_MSY = 0.55, 80 percentile range 0.35 – 0.85). An increasing trend in the fishing mortality is estimated from 2004 to 2018 and the recent average fishing mortality is estimated to be below F_MSY (median F_2018-2020/F_MSY = 0.94, 80 percentile range 0.48 – 1.90). It should be noted that the models estimate a slightly decreasing in the fishing mortality in 2020 (median F₂₀₂₀/F_MSY = 0.75, 80 percentile range 0.40 – 1.42). The ensemble MCMC results from the two base cases indicated that the 2020 stock status is likely within the yellow quadrant (Prob [B₂₀₂₀<B_MSY and F₂₀₂₀<F_MSY] = 71.56%).