Nursery beaches for Horseshoe Crabs in Hong Kong

by Huang Qin, Helen Chiu and Brian Morton

Horseshoe crabs once thrived on many beaches in Hong Kong, but have largely disappeared since the late 1980s. In a recent study, many local fishermen expressed the belief that Hong Kong is no longer a habitat for horseshoe crabs. Apart from fishing, one of the most obvious reasons accounting for such a change is the environmental impacts, including urban development and marine pollution, upon beaches where adult horseshoe crabs breed and juveniles grow.

All three extant Asian horseshoe crab species have been reported from Hong Kong, i.e. Tachypleus tridentatus (Leach, 1819), T. gigas (Müller, 1785) and Carcinoscorpius rotundicauda (Latreille, 1802). T. gigas has not been identified definitively from here, however, and its local occurrence is anecdotal.

Tachypleus tridentatus is the only horseshoe crab species in Japan and is protected as a national treasure, but its regional extinction along the coast of the Seto Inland Sea, Habu Bay and Hakata Bay has been predicted as a consequence of habitat destruction and marine pollution. Similar environmental problems also exist in Hong Kong but their effect on horseshoe crab populations was not fully appreciated until recently as the species is not considered to be 'endangered' according to the 'Red List' of the International Union for the Conservation of Nature (IUCN), 1996.

In North America, a census of horseshoe crabs, Limulus polyphemus (Linnaeus, 1758), is carried out yearly, by monitoring the numbers of breeding individuals. The census showed there to be 1,200,000 horseshoe crabs in Delaware Bay, New Jersey, in 1991. The number dropped to ~400,000 in 1992 and dipped to <200,000 in 1994 and 1995. The situation was controlled in 1996 and regulations have been set up in those states possessing horseshoe crab populations, governing their harvesting and protecting beach environments.

Although no horseshoe crabs have been seen spawning in Hong Kong for many years (the last time was at Lung Kwu Sheung Tan, west of Castle Peak, by BM), a few beaches are still inhabited by a small number of juveniles, for example, at Tung Chung on Lantau Island, and the Pak Nai mudflat in Deep Bay. In the summer of 1998, two newly-born horseshoe crabs, with a carapace diameter of ~5mm, were seen, respectively, on the mudflat at Pak Nai and the mangrove beach at Tai Ho Wan, Lantau Island (Shea and Frew, pers. comms.).

Juveniles of only one species, i.e. Tachypleus tridentatus, have been identified from Shui Hau Wan and San Tau, on Lantau Island. They were estimated to be between 2 and 9 years old. Further investigations are required to locate the nursery beaches of Carcinoscorpius rotundicauda on Lantau, as adults of this species are frequently fished from the subtidal mud along the northwest coast of the island, including the sandy-mud beaches at Tai O, Yi O, Sham Wat Wan, Sha Lo Wan and Tung Chung Bay.

Compared with Lantau beaches, the intertidal mudflat at Pak Nai, in Deep Bay, has a higher density of juvenile horseshoe crabs. Juveniles of two species, i.e., Tachypleus tridentatus and Carcinoscorpius rotundicauda, here occur sympatrically, with the former being more numerous than the latter. Analysis of juveniles from this beach has shown that the smallest specimen of T. tridentatus, with a prosoma width of 20.5mm, could have been 2 years old (a 6th instar). The largest measured 86.0mm and was possibly 10 years old (a 12th instar). Specimens of C. rotundicauda were estimated to be 6th to 10th instars. Using a mark-release-recapture method, the number of juvenile T. tridentatus on the Pak Nai mudflat was estimated to be 200 in 1998, with an age range of between 3 and 9 years.

Researches in different countries suggest that Tachypleus tridentatus and Carcinoscorpius rotundicauda breed in different habitats: sand patches at high tide are essential for the former, while freshwater streams are required by the latter. The extensive Pak Nai intertidal mudflat provides ideal spawning and nursery habitats for both species. The mud stretches for ~4,000m along the shore and is lined by sand at the highest tidal levels. Five streams run across the mudflat. The three freshwater streams have broad sandy beds that extend into the subtidal. The other two streams are formed by water passing through respective mangrove areas. Between the two Ha Pak Nai freshwater streams, the mud is covered with a seagrass Halophila beccarii, that extends for more than 20m down the beach. Among the seagrass bed are small pools of water where juvenile horseshoe crabs feed when the tide goes out [see also next article]. According to local fishermen, the Pak Nai mudflat used to attract large numbers of spawning horseshoe crabs in summer, but few in the last decade. This too is largely unsurprising in view of the fact that they are, locally and elsewhere in China, fished, sold in markets and eaten as a specialist sea food.

The rate of regional species extinction is related to the rate of habitat loss which depends largely on the scale of human enterprises. Although far from the city, the extensive, multi-scaped intertidal mudflat at Pak Nai has not escaped local disturbance. At the southern end of Ha Pak Nai sits a small pier where ship fuel is released and spreads out to form an oily film floating over the nearby mud. A few metres to the north of the southern Ha Pak Nai freshwater stream, an outlet discharges a noxious black effluent onto the beach. Experiments elsewhere have demonstrated that horseshoe crabs are unable to maintain normal embryonic and juvenile developments when exposed to oil and chlorinated hydrocarbons. It is clear, therefore, that both local horseshoe crab species are endangered by the fishing of adults, the similar collection of beached reproductively mature individuals and habitat destruction of the juveniles.

The stream at Sheung Pak Nai once served as an important freshwater resource for the village's pond fishery. In recent years, however, the formerly clear stream has been injected with pig sewage near Sheung Pak Nai. From the pig farm down to the shore the river bed has turned black, carrying black water, a foul smell, and polluting the horseshoe crab beach.

The Pak Nai mudflat is also threatened by planned constructions, including the building of the Lingding Yang Bridge that will link Hong Kong and Zhu Hai across the mouth of the Pearl River on its western estuary. If at all possible, and if it is not destroyed by such developments, habitat restoration is needed at Pak Nai to restore the various components of this complex intertidal ecosystem. Protection of this mudflat is concerned with not only the horseshoe crabs (which are, for it, but a symbol of its health), but with the numerous other floristic and faunistic elements of the shore, including the resident sea grass, and the Deep Bay coastal environment as a whole. It is also time that the two horseshoe crabs be declared locally endangered species and removed from the markets.


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