Length-based assessment of Nemipterus bathybius in northern South China Sea

Yellowbelly threadfin bream (Nemipterus bathybius), an economically important demersal fish species in the northern South China Sea, has been overexploited in recent years. To provide technical support for the scientific management and sustainable utilization of N. bathybius stocks, based on the biological data of 3 059 individuals of N. bathybius collected during bottom trawl surveys in the northern South China Sea from 2014 to 2019, we assessed the stock status by using two assessment models Length-based Bayesian biomass (LBB) and length-based spawning potential ratio (LBSPR) under data-poor conditions. Results show that the asymptotic fork length (L_inf), relative natural mortality (M/K), and 50% sexually mature body length (L₅₀) of N. bathybius in the northern South China Sea from 2014 to 2019 were 23.7 cm, 2.33 and 11.76 cm, respectively. The LBB assessment results show that the relative biomass level (B/B_MSY) and ratio of length of 50% of the individuals captured by the gear to optimal length-at-first-capture (L_c/L_{c_opt}) were 0.89 and 0.85, respectively, indicating that N. bathybius was experiencing moderate overfishing and growth overfishing. The LBSPR results show a spawning potential ratio (SPR) of 0.19, indicating that N. bathybius was experiencing overfishing. Uncertainty analysis of the priori parameters reveals that LBB and LBSPR results were extremely sensitive to the setting of L_inf and sensitive to the setting of M/K. Therefore, the above two parameters should be set with caution when using LBB and LBSPR.