Present population and habitat status of potentially threatened Asian horseshoe crabs Tachypleus tridentatus and Carcinoscorpius rotundicauda in Hong Kong: a proposal for marine protected areas

While there are persistent calls for developing more marine protected areas (MPAs) for Asian horseshoe crab conservation in response to population declines in Asia–Pacific region, most existing horseshoe crab MPAs were designated without prior comprehensive population assessment and habitat characterization. This study collected biological and habitat information in Hong Kong, in order to identify priority sites for conservation and management of Asian horseshoe crab populations. The territory-wide surveys at 18 spawning/nursery beaches displayed a persistently low mean juvenile density from 0.16 to 2.19 ind/100 m² in 2012 and 0.08 to 1.41 ind/100 m² in 2014. These density data were within the same range of that in 2002 and 2007 (0.10–1.97 ind/100 m²), apart from a low survey return of 0.08–0.31 ind/100 m² in 2004. The current population of juvenile T. tridentatus and C. rotundicauda in Hong Kong is estimated about 2100–4300 and 2400–3000 individuals, respectively. From the 2012–2014 data, no new recruitments of 1st–3rd instar juveniles were found, as the shores were occupied mostly by 6th–9th instar juveniles of prosomal width between 23 and 45 mm, in which considerably high mortality rates were noted. The present findings of existing small and discrete juvenile populations, coupled with relatively few recruits, suggest that the status of juvenile horseshoe crabs in Hong Kong is fragile and vulnerable to local extirpation, especially if no urgent protection measures are implemented. Based on available population data and habitat characteristics, three MPAs, ranging from 5 to 7 km², are proposed, so as to conserve over 60 and 70 % of the existing juvenile populations of T. tridentatus and C. rotundicauda, respectively. The approach adopted in this study may serve as a case study for proposing other horseshoe crab MPAs in Asia, where increasing human disturbances and over-harvest are imminent.