Population ecology of the mud crab Scylla olivacea: a study in the Ranong mangrove ecosystem, Thailand, with emphasis on juvenile recruitment and mortality

The abundance, recruitment and mortality of the mud crab S. olivacea were studied in mangrove forests associated with the Kra Buri River in Ranong, southern Thailand. Data were obtained for mud crabs from two catch series. From March 1998 to March 2000, mud crabs were collected at monthly intervals from four to six fishermen who trapped the crabs at three different locations in the Kra Buri River estuary (commercial catch series). In addition, mud crabs were trapped experimentally in a 2000 ha mangrove area from March 1999 to May 2000 (experimental catch series). The external carapace width (ECW) and sex of each crab caught in this area were recorded and the population estimated from the average number of fishermen operating there. The size distribution in the mud crab catch was mono-modal with a size range of 5.0-13.5 cm ECW. Fewer female crabs were caught than male crabs and berried females were observed only in offshore catches (ECW: 8.2-11.6 cm). The size distributions from the two catch series were analysed using catch-at-length analysis with natural mortality rates set to 0.05, 0.1, 0.2. Mortality rates were lower for females (0.49 and 0.53 per month) than for males (0.90 and 1.00 per month). Estimated abundance increased with the size of the selected natural mortality rate, but irrespective of the rate set, the estimate of mud crab abundance remained within a narrow range with no significant upward or downward trend. In the experimental fishing area, average abundance was estimated to be 21-28 crabs/ha (min: 14 ha−1, max: 38 ha−1) including both males and females. Recruitment into the fishery was continuous throughout the year without any strong seasonal fluctuation, but the trends for male and female mud crabs were slightly different. Females also recruited into the fishery at a smaller average size compared to males.