Multi-Model Stock Assessment of Japanese Flying Squid (Todarodes pacificus) Under a Special Fishing License System in the Yellow Sea: Healthy Biomass but Declining Size Structure

The Japanese flying squid (Todarodes pacificus) is a commercially important cephalopod in the Northwest Pacific. Since 2021, a pilot special fishing license system has been implemented in Weihai, Shandong Province, China, yet no systematic stock assessment has been conducted to inform quota setting. We applied a hierarchical multi-model framework comprising Length-Based Bayesian Biomass estimation (LBB), Catch-Maximum Sustainable Yield (CMSY), and the Bayesian Schaefer Model (BSM) to length-frequency, catch, and CPUE data collected from the Weihai special fishing program during 2021–2025. All three models consistently classified the stock as healthy throughout the study period. LBB showed that B/Bmsy increased from 1.08 (2021) to 1.20 (2025). The BSM estimated MSY at 6.7 × 103 t, with B2025/Bmsy = 1.15 and F2025/Fmsy = 0.95. However, length at first capture (Lc) declined from 22.5 cm to 18.7 cm (−16.9%), and mean mantle length (ML) decreased from 24.0 cm to 20.8 cm, signaling ongoing size truncation despite healthy biomass levels. While the stock remains biologically healthy under current catch limits, the persistent decline in Lc warns of emerging growth overfishing risk. We recommend transitioning from a single-focus total catch control approach to an integrated management system that combines catch constraints, minimum landing size regulations, Lc-based monitoring, and strengthened data collection infrastructure. This study demonstrates the value of multi-model cross-validation for assessing data-limited, short-lived cephalopod stocks. Despite the short time series, which limits the separation of fishing effects from natural variability, the consistent model outputs provide initial support for the current management framework.