Stock assessment of Pacific saury (Cololabis saira) in the Western North Pacific Ocean through 2019

This paper describes the stock assessment of the Pacific saury (Cololabis saira) in the Western North Pacific Ocean (WNPO) based on the guideline of the 2019 SSC PS05. The assessment consisted of applying the Bayesian state-space surplus production model for estimating the biomass from 1980 to 2019 with available catches from 1980 to 2018. Abundance indices available for WNPO saury consisted of standardized catch-per-unit-effort (CPUE) of stick-held dip net fisheries from Japan (1980 - 2018), Chinese Taipei (2001 - 2018), Russia (1994 - 2018), Korea (2001 - 2018) and China (2013 - 2018), and biomass survey from Japan (2003 - 2019). 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 2005 and 2008, after then dramatically decreased overtime and below BMSY in 2015 - 2019. It should be noted that the models estimate an increase in biomass in 2018 (median B2018/BMSY = 0.87, 80 percentile range 0.61 - 1.30) and following a slightly decrease in 2019 (median B2019/BMSY = 0.70, 80 percentile range 0.48 - 1.05). A steady increase in fishing mortality is estimated to have occurred from 2004 to 2018 and the recent average fishing mortality is estimated to be above FMSY (median F2016-2018/FMSY = 1.26, 80 percentile range 0.65 - 2.35). The ensemble MCMC results from the two base cases indicated that the 2018 stock status are likely within the red quadrant (Prob[B2018<BMSY and F2018>FMSY] = 61.35%) and the stock is likely overfished and is likely experiencing overfishing relative to MSY-based reference points.