Apart from the rocky bottom of the eastern and southern subtidal regions of Hong Kong, the majority of Hong Kong's seabed area is soft and comprises a mixture of sand and mud which varies in species composition from place to place and under differing degrees of current scour (Morton et al 1996). The northwestern waters of Hong Kong are subject to regular deposition of large quantities of silt from the Pearl River creating, in many cases, an unstable and inhospitable environment for seabed organisms. In contrast, the seabed of the eastern waters of Hong Kong is generally composed of materials that are more coarse in nature, and consequently form a more stable environment for seabed organisms.

Over the past decade, the soft bottom seabed has been subject to numerous sand dredging, dredged material disposal and backfilling (ie of empty sand dredging pits) activities. Sand dredging has occurred at inter alia, Outer Deep Bay, the Brothers, East Sha Chau, North Lantau, Sokos, South Tsing Yi, East Lamma Channel, Po Toi, and East Tung Lung Chau (see Paragraph 3.6.1.4 - Minerals, Aggregates and Energy for a discussion of remaining sand resources). On at least one occasion smothering by fine sediments from dredging activities at the East Tung Lung Chau Marine Borrow Area resulted in mortality to gorgonians and hard corals near the Ninepins (Hodgson 1994). It is also known that the intensity of trawl activities in Hong Kong waters has a considerable influence on the soft seabed environment (Selby and Evans 1997) and a detrimental effect on benthic fauna (Morton 1996).

Marine habitats which are of particular importance in maintaining the marine ecological natural capital stock of Hong Kong can be considered to lie within the following categories:

Areas which support the highest known marine biodiversity values for Hong Kong: These areas are considered to comprise areas which are documented to provide habitats for supporting high biodiversity assemblages and/or designated, or considered for designation as Marine Protected Areas (eg Marine Parks and Marine Reserves) because of high biodiversity. Therefore the subtidal areas of the designated Marine Protected Areas, including Hoi Ha Wan Marine Park, Yan Chau Tong Marine Park and Cape d'Aguilar Marine Reserve, which are discussed under Paragraphs 3.2.1.9 - 3.2.1.11 - Land Use and Land Supply are considered to be of special concern. The Sha Chau and Lung Kwu Chau Marine Park is not included in this category because it was designated on the basis of habitat for Chinese White Dolphin (Sousa chinensis) rather than the presence of high biodiversity. It is, however, included in the category concerning habitat for [rare] species (see below).

The potential Marine Parks/Marine Reserves at Shelter Island, South Lamma, South-west Lantau, and Ping Chau, also support a rich, biologically important, marine community and can be regarded as having the highest biodiversity values among the Hong Kong marine areas. The potential marine park at Shelter Island contains a high diversity of subtidal hermatypic coral assemblages (Binnie Consultants Ltd 1995c) and benthic infaunal communities (Shin and Thompson 1982). The subtidal areas of South Lamma Island support high abundances of soft corals (ERM 1998e) and the soft bottom benthic areas at Sham Wan are reported to contain high abundance of benthic fauna (ERM 1997b). Also, the southwestern waters within the potential South Lamma Marine Park have the highest sighting frequency of Finless Porpoise among the Hong Kong marine areas (ERM 1998f). For the potential Marine Park at Southwest Lantau, the presence of a rich infaunal community in the benthic areas (ERM 1997c) and a relatively high abundance of the coral reef butterflyfish (Chateodon modestus) near the Soko Islands (Binnie Consultants Ltd 1997g) together with a documented utilization of the marine areas by both the Chinese White Dolphin and the Finless Porpoise, provide evidence for the high biodiversity nature of the area. The subtidal areas of the potential Ping Chau Marine Reserve provide a unique seabed configuration which supports a rich associated fauna of corals (D McCorry pers comm) and reef building corals (Morton and Morton 1983).

Areas which are documented to function as important marine spawning, nursery or key foraging habitats: Marine areas which serve as spawning and nursery grounds for commercially important species were identified by the AFCD Study on Fisheries Resources and Fishing Operations in Hong Kong Waters (ERM 1998a). Critical spawning areas were found to include eastern, southern and northeastern waters (Figure 4.2e). Southern waters, Port Shelter and the northeast New Territories (Figure 4.2e) were found to be important nursery habitats for commercial fisheries in Hong Kong.

Areas which contain marine habitats that are regionally rare or threatened or provide documented critical habitat for rare species: Species which are of special concern are discussed in the following sections. Key habitats for marine mammals, turtles, horseshoe crabs, corals and seagrasses, which are protected under the Wild Animals Protection Ordinance (Cap 170) and/or Marine Parks Ordinance (Cap 476), and/or documented to be rare/threatened are briefly identified here. All areas are shown in Figure 4.2e.