The subsidiary impacts of dredging (and trawling) on a subtidal benthic Molluscan community in the Southern Waters of Hong Kong

Abstract The macrobenthic fauna of the southern waters of Hong Kong were surveyed in April 1992, notably with regard to the Mollusca. Subsequently, parts of the area were extensively suction dredged for major construction projects. Commercial trawling continued alongside the dredging. In October 1994, with dredging close to finishing, six of the original 50 stations were resurveyed using the same gear, and the Mollusca again re-examined. This study demonstrates that close to dredged sites, i.e. within 2 km, species and individual numbers of both the Gastropoda and Bivalvia had declined by approximately two thirds in the intervening period. With regard to the Gastropoda, most of the species losses were of specialist neogastropod predators. Post-dredging, the gastropod fauna was virtually dominated by opportunistic scavengers, notably Nassarius siquijorensis, Bursa rana and Murex trapa. These, however, were also dominant predredging and this lends support to an earlier argument that disturbed inshore marine sediments favour the presence of such species. The bivalve fauna was dominated by a few species that are resistant to disturbance, such as Placamen calophylla, Corbula crassa and Minnivola pyxidatus. These species are of no commercial value and the former two have solid shells that are resistant to trawl damage and which are, actually, adaptations to avoid predation. Possibly, Veremolpa micra and Paphia undulata are new colonizers of the perturbated sea-bed, but this remains to be substantiated. This study postulates that setting silt plumes associated with dredging activity have exacerbated the problems of a sea-bed already disturbed as a result of trawling and pollution.