Marine Flora and Fauna of Hong Kong and Southern China IV

The impacts of dredging on the epibenthic molluscan community of the southeastern waters of Hong Kong: a comparison of the 1992 and 1995 trawl programmes

The Marine Flora and Fauna ofHong Kong and Southern China IV (ed. B. Morton). Proceedings of the Eighth International Marine Biological Workshop: The Marine Flora and Fauna of Hong Kong and Southern China, Hong Kong, 2-20 April 1995. Hong Kong: Hong Kong University Press, 1997. THE IMPACTS OF DREDGING ON THE EPIBENTHIC MOLLUSCAN COMMUNITY OF THE SOUTHEASTERN WATERS OF HONG KONG: A COMPARISON OF THE 1992 AND 1995 TRAWL PROGRAMMES K. F. Leung and Brian Morton The Swire Institute of Marine Science, The University of Hong Kong, Cape D'Aguilar, Shek 0, Hong Kong ABSTRACT Between 1992 and 1995, extensive areas of the sea bed in the southeastern waters of Hong Kong were suction dredged for marine sand to build the new airport and associated infrastructure works at Chek Lap Kok on Lantau Island. The designated areas of sea bed were surveyed, pre- and post-dredging, for evidence of any impacts upon the resident molluscan epibenthos. In areas immediately adjacent to and surrounding the dredging activities, settling and dispersing sediment plumes reduced species richness, abundance and diversity of both the Gastropoda and Bivalvia. The resident communities are now dominated by generalist, often scavenging, gastropods and a suite of highly tolerant ~ivalves. Representatives of the once common specialist polychaete feeding, Turridae (Gastropoda), were noticeably adversely affected. Such a community is, in many ways, typical of most of Hong Kong's territorial sea bed and, it is now realised, represents the result of many pertubations including trawl fishing and spoil dumping, both legal and illegal. Because of the continuous re-working of the dredging-impacted areas by trawlers, it is unlikely that the molluscan community will ever revert to a pre-dredged state that, though in itself highly perturbated, it is now known, was much more diverse. INTRODUCTION The territorial waters of Hong Kong have an area of some 1,827 km2, i.e., about one and a half times that of the land (1,150 km2). Since its founding in 1841, Hong Kong has grown to become one of the most important and densely populated cities on the 402 K.F. LEUNG & B. MORTON southern coast of China. It has a population of 6.2 million and is the world's eighth largest trading economy. Success is, now, based largely on its infrastructural and economic value to China so that its port is now, arguably, the busiest in the world. To keep ahead of the growing local and Chinese economy, Hong Kong is building a new airport at Chek Lap Kok and new container terminals on Lantau Island at a projected cost, to give an indication of the project's size, of US$12.6 billion (in 1993). Because of its mountainous terrain, most development in Hong Kong has been by coastal reclamation and as it has proceeded, without infrastructural capabilities to treat domestic, agricultural and industrial wastes, so the problems of marine pollution have also grown (Wong 1974; Morton 1976, 1989; Wu 1982, 1988). There are, however, other pressures put upon the sea and its biota. The Hong Kong fishing fleet comprises about 4,800 vessels of which 515 are shrimp trawlers, 586 are pair trawlers and 222 are stern trawlers (Wilson and Wong 1996). The trawlers account for well over half of the total catch of the local fleet and although most larger vessels operate outside Hong Kong waters, many do so within them, at least some of the time. There are, however, no data on fishing impacts upon the local subtidal benthic communities. In the past, coastal reclamation for new development projects has been achieved using land-based fill from 'borrow' areas. So big is the Port and Airport Development Strategy (PADS), however, that an estimated 500 million m3 of fill is required while the estimated volume of mud arising from the entire PADS during the period 1992 to 2000 and requiring disposal is in excess of 330 million m3 • This mud disposal issue is further complicated by the fact that most of this mud, especially at originated from areas around Victoria Harbour, is contaminated by heavy metals and organic pollutants (Lau et ai. 1993) with toxic effects on molluscs (Cheung and Wong 1993). The scale of the project is so large that by its completion it is estimated that ~ 20% of Hong Kong's 800 km of coastline will be reclaimed. Dutton (1987) demonstrated the existence of large volumes of dredgeable sand in the territorial waters of Hong Kong and, subsequently, more than twenty potential marine areas...