Population and habitat recovery of mangrove horseshoe crab after a major cyclonic event in the coastal Northern Indian Ocean

Sundarbans, the world’s largest continuous mangrove forest, an UNESCO World Heritage Site and a RAMSAR Site, formed at the Ganga Brahmaputra Meghna riverine system of the Bay of Bengal of Northern Indian Ocean, is home to a large population of mangrove horseshoe crab (Carcinoscorpius rotundicauda) , along with Indo Pacific horseshoe crab (Tachypleus gigas). In 2020, a category 5 very severe cyclonic storm ‘Amphan’ hit and badly affected population and habitat of mangrove horseshoe crabs acrossSunda rbans including Sagar Island. Immediately following cyclone, intense field work was initiated involving local communities to determine population status of mangrove horseshoe crab. The habitat assessment was undertaken by assessing status of mangrove vegetation, quantifying the nature of sediment grain size patterns, status of benthic biodiversity and using stable isotope based measurements. Besides, with the help of indigenous and local communities including fisherfolks, marine litter was removed systematically during the study period. Targeted attempts were undertaken to rescue and release mangrove horseshoe crab from other affected islands gradually into the restored habitat of Sagar Island through involvement of park managers. Immediately following cyclone, population status showed a sharp decline of 1-2 individuals per km (compared to 12-15 individuals per km) in Sagar Island and there was large scale destruction of habitat attributes including that of mangrove vegetation. The silty-clay nature of sediment (35-40% of texture proportions) was replaced by coarse and fine sand particles across habitats of mangrove horseshoe crab. Besides, marine litter dominated by plastics, ghost nets, among others were found across the habitats of mangrove horseshoe crab. More than 500 kgs of plastic and ghost nets were removed between June, 2020 and January, 2021 with the active involvement of local fisherfolk communities to restore the habitats of mangrove horseshoe crab. The sediment texture analysis revealed gradual transition to silt-clay nature of sediment. However, microplastic abundance in sediment and surrounding intertidal water column were found to be higher. During this period, there was occasional presence of adult mangrove horseshoe crabs; however juvenile forms were not encountered. From February, 2021 the occurrence of mangrove horseshoe crab showed an increase (3-5 individuals per km). The overall number of mangrove horseshoe crab presently shows an increasing trend and restoration activities led by coastal fisherfolk communities have led to population and habitat recovery of mangrove horseshoe crabs in Sundarbans through a nexus of science, society and policy.