2025 update on Pacific saury stock assessment in the North Pacific Ocean using Bayesian state-space production models

The base case stock assessment for North Pacific saury has been updated. The assessment employed the state-space surplus production model, which has been used as an interim stock assessment model since SSC-PS01. This model accounts for process errors in the population dynamics and observation errors in the abundance indices. Parameter estimation was conducted within a Bayesian framework using the Markov Chain Monte Carlo method. The stock status and future projections were presented following the template agreed upon at the 5th SSC-PS meeting, with some modifications to accommodate the updated data period.

As for the combined base case stock assessment result, the 2025 median depletion level was 22.1% (80% CI: 12.7–34.4%) of the carrying capacity. Furthermore, the B-ratio (=B/Bmsy) and F-ratio (=F/Fmsy) in 2024 were 0.377 (80% CI: 0.252-0.499) and 1.085 (80% CI: 0.771–1.622), respectively. For the three-year average values, the B-ratio over 2023–2025 and the F-ratio over 2022–2024 were 0.394 (80% CI: 0.264–0.544) and 1.015 (80% CI: 0.752–1.460), respectively. In addition, the probability of the stock being in the green Kobe quadrant in 2024 was estimated to be approximately 0%, while the probabilities of being in the yellow and red Kobe quadrants were assessed as 53% and 47%, respectively. Note that there is a large difference in the biomass series between the two base cases, whereas relative quantities, such as the B- and F-ratios and depletion level, differ little.