Population status assessment of Coilia mystus in Pearl River Estuary based on LB-SPR model

Coilia mystus is a commercially important fish species in the Pearl River Estuary, yet its exploitation status has long lacked systematic quantification. To accurately evaluate its exploitation status and reproductive potential under data-limited conditions, we analyzed the bottom-trawl survey samples collected from 2020 to 2024 (n=2 890, total length 24−224 mm) to estimate the length-mass relationship, growth, mortality, and maturity parameters. Besides, we assessed the stock status by using the length-based spawning potential ratio (LB-SPR) model. The length-mass relationship was expressed as W=4 × 10⁻⁶L^3.005 (r² = 0.9191). The estimated asymptotic length (L_∞) was 236.25 mm, and the growth coefficient (K) was 0.54 a⁻¹. The total mortality rate (Z) was 1.77 a⁻¹, with natural mortality (M) and fishing mortality (F) estimated at 0.67 a⁻¹ and 1.10 a⁻¹, respectively, yielding an exploitation rate (E) of 0.62. The LB-SPR model estimated a spawning potential ratio (SPR) of 0.19 and an F/M ratio of 1.77. The gear selectivity parameters (L_c50=100.49 mm and L_c95=141.72 mm) were both lower than the maturity parameters (L_m50=132.77 mm and L_m95=219.08 mm), indicating that a substantial proportion of individuals were subjected to selective fishing before reaching sexual maturity. The length-frequency distribution showed a high proportion of small individuals and a relatively low proportion of large mature individuals. According to the sensitivity analysis, SPR was most sensitive to changes in L_∞, followed by M/K (Ratio of natural mortality to growth parameter), but was relatively insensitive to L_m50. In conclusion, the results indicate that the C. mystus population in the Pearl River Estuary is currently under high fishing pressure, and its reproductive potential is close to the limit reference point. Accordingly, management measures such as increasing the minimum capture size, optimizing gear selectivity, and reducing juvenile catch are recommended to promote stock recovery and sustainable exploitation.