Influence of environmental conditions on the distribution of burrows of the mud crab, Scylla serrata, in a fringing mangrove ecosystem

Mud crab, Scylla serrata, is a common mangrove species in the Indian Ocean. Over the last decades, fishing pressure has greatly increased especially in Madagascar. Therefore, improving the knowledge of mud crab biology and ecology is crucial to guide conservation and restoration plans. This study aimed at describing the distribution of burrows of the mud crab in a mangrove in southwestern Madagascar. Burrows fulfill an essential functional role by limiting the exposure of crab to environmental stress, such as desiccation during low tide, and predation. A total of 140 burrows were observed and available conditions were described at 168 random sampling points at low tide in three different mangrove zones, during the dry season (July 2014). Twelve environmental factors were recorded to reflect the position within the mangrove, the substrate composition and the surrounding floristic structure. A multivariate ordination analysis highlighted an overall association between burrows and specific habitat conditions. Boosted regression trees then indicated that the burrows distribution was predominantly explained by the avoidance of hard substrates. The occurrence of burrows was positively related to the upper intertidal zone of the mangrove where the water level was below 100 cm. Although the tree density had a positive effect, the occurrence of burrows was higher for low percentage of shade (<40%). These results provide insight for identifying mangrove areas that have an essential functional role for the conservation of mud crab populations.