Updates of stock assessment for Pacific saury in the North Pacific Ocean up to 2024

This working paper presents the update results 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 13th 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 B₂₀₂₃ and B₂₀₂₄ over the two base case scenarios were 378,300 (80%CI 240,200-799,100) and 509,200 (80%CI 343,900-923,600) metric tons, respectively. The median B₂₀₂₄/B_MSY and F₂₀₂₃/F_MSY over the two base case scenarios were 0.43 (80%CI 0.25-0.62) and 0.92 (80%CI 0.66-1.51), respectively. For the most recent three years average values, B_2022-2024 was estimated below the B_MSY (B_2022-2024/B_MSY=0.34, 80%CI 0.23-0.46), and F_2021-2023 was estimated to be above F_MSY (F_2021-2023/F_MSY=1.01, 80%CI 0.77-1.52). Over the years, there have been decadal and interannual fluctuations in the biomass trend. Approximately after 2005, the biomass displayed a downward trajectory, albeit with interannual variation. The lowest biomass was recorded in 2020, but a recovery trend has been noted in from 2020 to 2024. Since 1999, the harvest rate has shown an upward trend, but it has rapidly declined since 2018. The probability of the 2023 stock status being in the yellow quadrant of Kobe plot (Prob[B₂₀₂₃<B_MSY and F₂₀₂₃<F_MSY]) was estimated to be 62%.