Population ecology of the mud crab Scylla paramamosain (Estampador) in an estuarine mangrove system; a mark-recapture study

Mud crabs of the genus Scylla are commercially important mangrove residents that are extensively fished throughout their range in the Indo-West Pacific. Despite this high level of exploitation very little is known about the population dynamics of any of the Scylla species. The present study concentrated on an exploited population of Scylla paramamosain in a natural estuarine mangrove on Can Coc Island in the mouth of the Hau River, Mekong Delta, Vietnam. A total of 6,114 juvenile crabs captured on the seaward mangrove fringe were internally tagged by injection of coded microwire tags and released over a period of 29 days. Recaptures were monitored over the following 144 days. There was little migratory movement within the estuary; of 285 recaptured crabs, 93% were recovered within the island mangrove study area and only two individuals were recovered from mangroves lining the opposite mainland riverbanks. A von Bertalanffy growth function was fitted to growth increment data from recaptured crabs, with Lₘₐₓ = 150 mm to derive a growth constant, k = 2.39 year⁻¹. The theoretical age at which carapace width is zero (t₀) was derived from known size at recruitment at instar 1, giving a value of −0.0095 years. Previous studies of the same population have shown that female S. paramamosain reach maturity at a mean size of 102 mm carapace width. The present study indicates that they attain this size at around 160 days from first settlement in the mangrove fringe. Abundance of juvenile crabs in the study area was estimated by the Petersen method as 1,101,500 (95% CI 4,17,300–1,785,800, representing 1,102 crabs ha⁻¹ of mangrove. Estimation of mortality from tag returns and from the age-catch curves during a period of constant recruitment were comparable (Z = 1.11 and 1.04 month⁻¹, respectively). Fishing only accounted for 14% of total mortality, suggesting that at the time exploitation was not at a critical level despite the apparent high level of fishing activity.