Assessment and Management Implications for Chub Mackerel (Scomber japonicus) in the North Pacific: Integrating Length-Based Bayesian and Catch-MSY Models

Chub mackerel (Scomber japonicus) is a key pelagic commercial fish species in the North Pacific Ocean and is one of the priority species managed by the North Pacific Fisheries Commission (NPFC). This study assessed the stock status and maximum sustainable yield (MSY) of S. japonicus using the Length-based Bayesian Biomass (LBB) estimator and the Catch-MSY model. The assessment was based on length-frequency data collected from the light purse seine fishery on the high seas of the North Pacific (6061 individuals sampled during April–December, 2019–2024) combined with historical catch data spanning 1995 to 2024. Management implications were further analyzed using the Kobe plot and the Kobe II Strategy Matrix (K2SM). The results showed that: (1) The LBB model revealed significant interannual variability in stock status of S. japonicus. Although the stock was within sustainable limits (B/BMSY ≥ 1.0) in 2019, 2021, and 2022, it suffered from overfishing or was heavily overfished in 2020, 2023, and 2024. Notably, integrated data analysis indicated that during 2023–2024, the B/BMSY ratio dropped to 0.51, reflecting a marked deterioration in the stock condition. (2) The Catch-MSY model estimated the maximum sustainable yield (MSY) at 278,000 t, with a 95% confidence interval (CI) ranging from 162,000 t to 475,000 t. (3) Based on catch data, the Kobe plot indicated that in the final assessment year, the probability of the stock being in a sustainable state was only 23.2% at the 95% confidence level. The probabilities of being recruitment-overfished, fishing-overfished, and severely overfished were 37.6%, 14.9%, and 24.3%, respectively. With relative biomass B/BMSY < 1 and relative fishing mortality F/FMSY < 1, the stock status in the final assessment year was diagnosed as recruitment overfishing. (4) Integrating the outputs of the two models, the Kobe plot, and the probabilistic projections of future stock status from the Kobe II Strategy Matrix (K2SM), it is recommended that the total allowable catch (TAC) be set within 60–80% of the baseline TAC during the medium-term management phase (5–10 years), corresponding to a range of 211,300 t to 281,700 t. In the long term, if the stock exhibits positive recovery trends, the TAC could be gradually increased to the MSY level to achieve the maximum sustainable utilization of this fishery resource.