Updates of stock assessment for Pacific saury in the North Pacific Ocean up to 2025

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) agreed in SSC PS15 meeting. Results showed that the estimated median B₂₀₂₄ and B₂₀₂₅ over the two base case scenarios were 567,100 (80%CI 354,000-1,066,000) and 678,600 (80%CI 444,900-1,174,000) metric tons, respectively. The median B₂₀₂₅/B_MSY and F₂₀₂₄/F_MSY over the two base case scenarios were 0.44 (80%CI 0.26-0.64) and 1.09 (80%CI 0.77-1.85), respectively. For the most recent three years average values, B_2023-2025 was estimated below the B_MSY (B_2023-2025/B_MSY=0.38, 80%CI 0.24-0.51), sand F_2022-2024 was estimated to be above F_MSY (F_2022-2024/F_MSY=1.02, 80%CI 0.77-1.63). 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 2025. Since 1999, the harvest rate has shown an upward trend, but it has rapidly declined since 2018. The probability of the 2024 stock status being in the red quadrant of Kobe plot (Prob[B₂₀₂₄<B_MSY and F₂₀₂₄>F_MSY]) was estimated to be 61%. The assessment results were considered unrealistic because they still failed to reflect the improving trend in the stock status of Pacific saury due to retrospective pattern and scaling issues. Conversely, the assessment suggested that the stock status was worsening, with B/B_MSY remaining fixed and an increased probability of falling into the red zone of the Kobe plot over the years. This outcome contradicts multiple indicators and observations, including increasing trend of abundance indices since 2020 and the larger body sizes observed in the 2025 fishing season.