Impact of extended fishing moratorium on life-history parameters of different-sized fishes in the Bohai Sea via coupled aLBI-LBB methods

The summer fishing moratorium (SFM) aims to protect spawners and larvae for fishery resource conservation. To evaluate the conservation effectiveness of an extended SFM in the Bohai Sea, this study compared the population status and biological parameters of fish species with different body shapes before and after the policy adjustment. Bottom trawl survey data from late August were analyzed across two periods: before (2014-2016) and after (2017-2019) the SFM adjustment. Nine representative species across six body shapes were selected: compressed (Konosirus punctatus, Thryssa kammalensis, Engraulis japonicus, Sardinella zunasi), ribbon-like (Enedrias fangi), depressed (Callionymus beniteguri), eel-like (Chaeturichthys stigmatias), asymmetrical (Cynoglossus lighti), and fusiform (Scomber japonicus). Kernel density estimation was used to compare length-frequency distributions. Allometric growth parameters b were analyzed using length-weight power functions. Biological priors, including asymptotic length (L_inf), length at first maturity (L_mat), and optimum length (L_opt), were estimated using the aLBI (a length-based indicators) method. The robustness of L_inf estimates across different bin sizes (0.50–3.00 cm) was measured by relative error (RE). Finally, L_inf and L_mat were used as priors for the LBB (length-based bayesian biomass) model to estimate and compare resource indicators, such as relative biomass (B/B₀) and the ratio of natural mortality to growth (M/K). Results showed significant differences in responses to the extended SFM among fish species. Body lengths showed an increasing trend for T. kammalensis, S. zunasi, and K. punctatus, while a decreasing trend was observed for E. fangi, C. beniteguri, C. lighti and S. japonicus. Changes in body length for C. stigmatias were not significant. The extension of the summer fishing moratoria resulted in species-specific responses in the allometric growth parameter b. Specifically, for E. fangi, T. kammalensis, and E. japonicus the parameter b was significantly greater than 3 (P<0.001) across both periods, indicating positive allometric growth. RE increased with larger bin sizes; for K. punctatus and C. beniteguri, RE increased and L_inf was overestimated when bin sizes exceeded 1.25 cm. B/B₀ showed an improving trend for K. punctatus, E. fangi, C. stigmatias, E. japonicus, and S. zunasi, but declined for C. beniteguri, C. lighti, and S. japonicus. M/K values remained relatively stable, fluctuating around 1.5 for most species. In conclusion, the effectiveness of the extended SFM varies by species, and assessment models are highly sensitive to bin size settings. Practical applications should optimize bin sizes based on biological traits and sample sizes. Although the extended SFM helps improve the population structure of pelagic fishes, the miniaturization of demersal fishes and the insufficient spawning biomass in certain populations warrant further attention.