Stock Assessment of the Scylla serrata (Giant mud crab) in Achchankulam Lagoon, Sri Lanka

The Scylla serrata (Giant mud crab) is a highly valued species in Sri Lanka’s coastal fisheries due to its nutritional value and economic importance. In recent years, concerns have arisen regarding overexploitation and the sustainability of its stocks, particularly in confined ecosystems such as lagoons. This study assessed the stock status of Scylla serrata in the Achchankulam Lagoon, located in the Mannar District of Sri Lanka. Fortnightly field surveys were conducted at major landing sites from September 2017 to August 2019. On each sampling day, approximately 5 kg of crabs were randomly collected, and carapace width measurements were taken from a total of 520 individuals. Monthly length-frequency data were analyzed using the TropFishR package in R. The results showed that von Bertalanffy growth parameters estimated an asymptotic carapace width (L∞) of 21.13 cm and a growth coefficient (K) of 1.89 yr⁻¹. The anchor age at zero length (t_anchor = 0.279 years) suggested that peak recruitment occurs around mid-April. The growth performance index (φ') was calculated as 2.925, indicating favorable growth conditions in the lagoon. The estimated natural mortality (M), fishing mortality (F), and total mortality (Z) were 2.392 yr⁻¹, 4.979 yr⁻¹, and 7.371 yr⁻¹, respectively. The current exploitation rate (E = 0.67) exceeded the precautionary limit of 0.5, suggesting that the stock is experiencing high fishing pressure. However, yield-per-recruit (YPR) analysis demonstrated that the current fishing effort (4.979 yr⁻¹) is approaching the maximum sustainable yield (Fmax), estimated at 5.000 yr⁻¹. These findings underscore the need for precautionary management to ensure the long-term sustainability of the Scylla serrata fishery in Achchankulam Lagoon.