Stock assessment of Pacific saury (Cololabis saira) in the Western North Pacific Ocean through 2018

This paper describes the stock assessment of the Pacific saury (Cololabis saira) in the Western North Pacific Ocean (WNPO) conducted in 2019. The assessment consisted of applying the Bayesian state-space surplus production model with available catches of Pacific saury from Japan, Chinese Taipei, Korea, China, Russia and Vanuatu from 1980 to 2017. Abundance indices available for WNPO saury consisted of standardized catch-per-unit-effort (CPUE) of stick-held dip
net fisheries from Japan (1980-2017), Chinese Taipei (2001-2017), Russia (1994-2017), Korea (2001-2016), and China (2013-2017) and biomass survey from Japan (2003-2018). Further developments to the stock assessment have been undertaken to address the recommendations of the 2018 3rd TWG-PASS meeting in response to the uncertainty of the Japanese survey’s catchability and the potential change of catchability of the Japanese stick-held dip net fishery in the 1980s. Six base case models were considered for the assessment outputs. The results of stock assessment indicated that the absolute estimated biomass trends before 2000 were sensitive to the early Japanese index (1980-1993), while the estimated biomass levels were sensitive to the catchability from the Japanese survey. Time-series of biomass showed an increasing pattern since 2000 with the peaks in 2005 and 2008, after then dramatically decreased until 2017. It should be noted that the estimated biomass has slightly increased in 2018 for all base case models examined. The ensemble stock condition from the results of the structural uncertainty grid indicated that the overall median depletion and the ratio of biomass to BMSY in 2017 were estimated at 0.47 (80 percentile range 0.28-0.66) and 0.70 (80 percentile range 0.43- 1.01). Recent fishing mortality is estimated to be below FMSY (median F2015-2017/FMSY =0.78, 80 percentile range 0.39-1.88).