Moving beyond controversy: is the chub mackerel (Scomber japonicus) stock recovering?

Chub mackerel (Scomber japonicus) is a commercially significant and highly migratory species in the Northwest Pacific. Despite various management efforts, previous stock assessments have yielded inconsistent results, leading to ongoing debates regarding its stock status. To provide a robust assessment, this study employed two complementary models: CMSY++, which utilizes an Artificial Neural Network (ANN) to objectively estimate prior biomass ranges, and a Bayesian State-Space (BSS) model. Crucially, the BSS model incorporated a correction for technical creep to address biases in Catch Per Unit Effort (CPUE) data—a factor frequently overlooked in previous research. The results from both models were consistent. In 2024, the relative biomass (B/BMSY) was estimated at 0.63 (CMSY++) and 0.59 (BSS), indicating that the stock remains overfished and below the Biomass at Maximum Sustainable Yield (BMSY). Kobe plot analysis further revealed that sustained high fishing mortality since the early 2000s has impeded recovery. This study demonstrates CMSY++ as an effective tool for reconciling discrepancies in data-limited assessments and underscores the necessity of correcting for technical creep to ensure accurate management decisions. Enhanced scientific surveys and the systematic collection of high-quality fisheries data are essential for the sustainable management of this transboundary stock.