Catches by mud crab collectors were used to assess the distribution and movement of Scylla spp in replanted mangroves in Buswang, Philippines. Comparison of catches in traps set in the mangroves and a stakenet set in a mangrove creek suggested S. olivacea to be the most resident and most numerically dominant of the three mud crab species found at the replanted mangroves. No juveniles less than 3.25cm external carapace width (ECW) were caught inside the mangrove even though the gear mesh size suggested this was possible indicating recruitment into the mangroves occurs at a larger size. Stakenet catches suggest mature females are more mobile than mature
males, and more likely to leave the mangroves at ebb tide. Catches contained both gravid and berried females suggesting that females do not spawn and release far offshore.
Trapping grids were used to compare the densities in typical natural, replanted and degraded habitats. Reforestation was demonstrated to be effective in restoring ecological function, as suggested by mud crab densities. The use of the trapping grid as a method for estimating the relative abundance of mud crabs was assessed using depletion fishing, single mark-recapture and multiple-mark single-recapture experiments and burrow excavation to estimate absolute abundance. Density estimations from these methods broadly agreed with CPUE suggest that trapping grids are a good method of comparing mud crab density.
A socio-economic study showed mangrove replanting to be a highly successful
method of raising local incomes, ensuring food security and improving livelihoods at this site. The mangrove contributed 578-2568 kg of mangrove-associated fisheries products ha-1 yr-1 worth US$463-2215 ha-1 yr-1 to coastal and mangrove fisheries.
Additional income of US$41 ha-1 from tourism and US$61 ha-1 from timber sales was also raised annually. The fisheries productivity was similar to that of brackish water aquaculture ponds suggesting that it may not pay to convert mangroves to ponds. Benefits of the protective function of mangroves against storm and cyclone seawater inundation and erosion were not included in the analysis but the survey of mangrove benefits suggested that most of the fishers recognised coastal protection (95%) and the nursery function (91 %) as being important mangrove benefits. Almost all of the fishermen (98%) wanted to protect the mangrove from aquaculture pond conversion.
Habitat partitioning by different life stages of Scylla paramamosain in the Mekong delta in Vietnam was examined using 5 different fishing methods. A single cohort was followed from recruitment to maturity. Crabs as small as 3mm ICW settled along the mangrove fringe attaching to the pneumatophores between December and February. Larger juvenile crabs were still found on the mud surface until 3cm internal carapace width (ICW) when they either began to dig burrows or live subtidally migrating into the mangroves daily with the tide. Just prior to maturity most live subtidally, and although offshore mating and spawning is important it is hypothesised that habitat partitioning maybe a factor as only 14% of females caught offshore had developed ovaries. This study highlights the importance of the mangrove fringe in maintenance
of S. paramamosain stocks and the habitat partitioning by the different life stages.
The close association of both S. olivacea and S. paramamosain with mangroves
emphasises the importance of mangroves for many commercially important species and the need for their conservation. The socio-economic survey highlights the value of the services replanted mangroves provide to the local community, which can be at least as great as those from natural mangroves and seafood production can be more than equivalent to that of brackish water aquaculture ponds.