Assessment of the effectiveness of mangrove rehabilitation using exploited and non-exploited indicator species

Abstract

Mangrove forests have been cleared at an alarming rate over the last century to allow space for settlements, agriculture and aquaculture and are still used today for fuel and construction. However, in the last few decades the value of the range of services and products that mangroves supply are being increasingly appreciated by policy makers. Mangrove replanting is frequently used as a method of restoring ecological function and associated goods and services but this may not be justified as once diverse forests are often replanted with mono-genus stands. In the present study the abundance of the commercially important mud crab Scylla olivacea, a top benthic predator, was used as an indicator of the ecological function of mangrove habitats. Abundance was estimated using catch per unit effort (CPUE) data obtained from an experimental standardized trapping grid. The same commercial traps also catch two other smaller non-exploited competing species, Baptozius vinosus and Thalamita crenata that are discarded by fishers. The relative abundance of these three species was used to separate the effects of habitat from fishing pressure and recruitment limitation. Four sites on Panay Island, central Philippines were selected to represent different types of mangrove habitat; a replanted fringing area predominantly of Rhizophora spp., a natural fringing area predominantly of Sonneratia spp., a diverse natural basin mangrove area and a degraded mangrove site. The relative abundance of mud crabs was found to be equivalent in the natural fringing mangrove (1.89 crabs trap⁻¹ day⁻¹) and the replanted mangrove area (1.71 and 0.81 crabs trap⁻¹ day⁻¹). Lower densities of S. olivacea in the basin mangrove area (0.33 crabs trap⁻¹ day⁻¹) appear to be due to limited recruitment, and at this site there was instead a higher abundance of the other non-commercial crab species. No mud crabs were caught in the degraded mangrove area and CPUE for other crab species was also low. Overall, the study suggests that replanting of mangroves even in mono-genus stands was effective in restoring mud crab populations, indicating recovery of an ecological function to a level equivalent to that of natural mangrove environments. The use of CPUE as an indictor of relative abundance of S. olivacea was supported by single release mark–recapture studies and a multiple release mark–recapture study in the replanted mangrove site.