Stock Assessment and Management Recommendations for the Sustainable Fishery of Tropical Shad (Terubok), Tenualosa toli, in Sarawak Waters, Malaysia

Abstract

The population of Tenualosa toli is primarily distributed in the coastal and estuarine waters of Sarawak, Malaysia, where landings have declined steadily from 1031 tons in 2003 to 586 tons in 2013. To assess stock status and support sustainable management, samples were collected between May 2016 and October 2017. Length-based stock assessment methods, including the TropFishR package and the Length-Based Bayesian Biomass (LBB) model, were used to estimate growth, mortality, exploitation, and biomass indicators. Growth analysis yielded an asymptotic length (L∞) of 57.81 cm, a growth coefficient (K) of 0.34 yr−1, a growth performance index (φ′) of 3.09, and a theoretical age at zero length (t0) of −0.40. Mortality estimates indicated total mortality (Z) of 1.82 yr−1, with natural mortality (M) of 0.49 yr−1 and fishing mortality (F) of 1.33 yr−1. The exploitation rate (E) was estimated at 0.73, substantially higher than the optimal level (E = 0.50), indicating excessive fishing pressure. The length at first capture (Lc50 = 19.16 cm) was considerably lower than the length at first maturity (Lm50 = 31.91 cm), suggesting that most individuals are harvested before reaching maturity. The optimal length (Lopt) was estimated at 35.0 cm (Lopt/L∞ = 0.67), while the observed mean length was much smaller (Lmean/Lopt = 0.73), indicating growth overfishing. Biomass indicators were critically low, with B/B0 = 0.017 and B/BMSY = 0.046, confirming that the stock is severely depleted and far below sustainable levels, suggesting recruitment overfishing. Overall, the results indicate that the fishery is experiencing both growth and recruitment overfishing due to intense harvesting of immature individuals and high fishing pressure. Therefore, management measures such as increasing mesh size, reducing fishing mortality to sustainable levels (F = M), and protecting spawning grounds and peak spawning seasons are urgently required. If these measures are effectively implemented, the T. toli stock could recover and reach sustainable production levels in the waters of Sarawak, Malaysia.