The initial assessment of an important pelagic fish, Mackerel Scad, in the South China Sea using data-poor length-based methods

Objective

Scads Decapterus spp. occur widely throughout tropical, subtropical, and temperate oceans and are both economically and ecologically important species. They are among the main commercial fish species caught by the light falling-net fishery in the South China Sea, which is dominated by Mackerel Scad Decapterus macarellus and Shortfin Scad D. macrosoma. Stock assessment is an approach used to guide effective management and maintain fishery sustainability.

Methods

Two data-poor methods, namely length-based Bayesian biomass estimation and length-based spawning potential ratio, were applied to assess the status of Mackerel Scad, the most dominant Decapterus species in the South China Sea. Electronic length–frequency analysis was used to estimate von Bertalanffy growth parameters (asymptotic length L^inf and growth coefficient K) to serve as priors.

Result

The results show the estimated L^inf and K of Mackerel Scad in the South China Sea as 36.0 cm and 0.37/year, respectively. Estimated natural mortality was 0.74/year, and mean fork length at 50% sexual maturity was 24.29 cm. Length-based Bayesian biomass estimation analysis showed that the estimated relative stock size had decreased from 1.3 in 2012–2014 to 0.7 in 2019–2021, indicating an overfished status. Length-based spawning potential ratio analysis showed that the estimated spawning potential ratio for this fishery decreased from 13% to 12%, which is significantly below the limit reference point of 20%.

Conclusion

Our results obtained through length-based methods indicate the lower SPR of Mackerel Scad stock was due to catching an excess of juveniles. Therefore, we highlight the need to increase the minimum mesh size in the fishing nets and strengthen international cooperation for conserving this shared stock in the South China Sea.