Updates of stock assessment for Pacific saury in the North Pacific Ocean up to 2020

This working paper presents the results of update of stock assessment for the North Pacific Ocean Pacific Saury stock using the Bayesian state-space production model. The assessment was conducted based on the model specification (2 base cases and 2 sensitivity cases) updated in the 7th Meeting of the Small Scientific Committee on Pacific saury. The model parameters were estimated based on Bayesian framework with a Markov chain Monte Carlo method. The assessment results were diagnosed with the Gelman and Rubin’s statistic, standardized residual plots, the shapes of posterior distributions for key parameters, and retrospective analysis. The main assessment results were concluded as follows: The estimated median B2020 from the two base case scenarios was 390,700 (80%CI 186,700-580,500) and 404,050 (80%CI 133,100-805,600) metric tons, respectively. The median B2020/BMSY and F2020/FMSY over the two base case scenarios were 0.33 (80%CI 0.19-0.54) and 0.99 (80%CI 0.55-1.64), respectively. Over two base case scenarios, large interannual variability was shown in biomass trajectory during the most recent years. A decreasing biomass trend was found in 2019 and 2020, followed by an increase in 2021. The probability of the population being in the red Kobe quadrant in 2020 was estimated to be greater than 48%.