139736eo.pdf (20MB) - Japan Oceanographic Data Center

-resource. Their significance derives from the fact that mangrove communities, like coral reefs, areoften very productive (in terms of the plant material produced by unit area), especially by comparisonwith the productivity of neighbouring areas or of the open ocean. A figure of 350-500 g C m-2 yr-1seems typical for mangrove primary production (GOLLEY et al., 1962; MILLER, 1972; LUG0 andSNEDAKER, 1974).Man exploits this productivity, both directly and indirectly. There is now heavy directutilisation of mangroves in many parts of the Indian Ocean. Coastal people use the wood for fuel,fishing stakes and floats, house posts and thatch, and boat building. The leaves are used for fodderand in the past the bark of Indian mangals has been used for tannin production. One or two speciesproduce edible leaves or fruit. IUCN's review (1983a) on the global status of mangrove ecosystemslists nearly 50 products in all that are obtained from mangrove forest. Most recently mangals havebeen used heavily for chipboard production. In Indonesia, for example, the combined export anddomestic value of all mangrove forestry products was estimated in 1978 at about US$ 26 million.Managed and cropped on a sustainable basis, mangroves can continue to provide such raw materialsfor man's use.But the indirect benefits to be obtained from mangal communities may be more significant.Mangrove is frequently the basis for important marine food chains; the leaves fall into the water wherethey decompose, and the resulting detritus and bacteria provide food for meiofauna and larger molluscsand crustaceans, including some commercial species of shrimp. Food is also provided indirectly forvarious fish, including commercial species such as snapper (Lutjanus spp.) which feed in turn on theinvertebrate fauna (see ODUM and HEALD, 1972; ODUM and JOHANNES, 1975). The roots of themangrove can also provide shelter for the invertebrates and fish which depend on their productivity.And organic and inorganic nutrients may also be exported in the form of leaf litter to adjacent habitatsin deeper water to support yet other animal species (ODUM and HEALD, 1972).Thus the productivity of the marine prawns Penaeus merguiensis, Metapenaeus monocerosand Metapenaeus brevicornis has been clearly related to existing mangrove areas (UNAR andNAAMIN, 1984). In the Philippines the major fishing grounds are generally located near areasbordered by mangrove (GOMEZ, 1980). It was estimated in 1970 (ROBAS, cited in WALSH, 1977)that one acre (0.405 ha) of undisturbed mangal estuary in Florida yields $7,980 worth of commercialfish products in 20 years. And in Indonesia, the combined exports and domestic value ofmangrove-linked fisheries products in 1978 was at least US$194 million (SALM and HALIM, 1984).In addition, mangroves are significant as a habitat or refuge for a wide variety of other speciesof animals and plants of scientific interest or ecological value, including some that are relativelyuncommon or rare (see MacNAE, 1968). For example, IUCN (1983a) indicates that over 300 speciesof plants and over a thousand species of marine invertebrates and vertebrates have been recorded fromAsian mangrove areas. In addition, 177 birds and 36 species of mammals have been reported inassociation with mangroves in this area. More conspicuous species of scientific interest andconservational value include a variety of birds, such as herons and egrets, spoonbills and pelicans,frigate birds and boobies, the Estuarine Crocodile, and the rare Royal Bengal Tiger, found in theSundarban mangroves of Bangladesh and India.Mangroves also have an indirect value in tending to control both coastal erosion and coastalflooding (DAVIS, 1940; CARLTON, 1974). By buffering fresh water run-off into inshore waters,they also protect coastal reefs against variations in salinity to which they are particularly sensitive. Andfinally, mangroves also have aesthetic and landscape value, frequently forming the only dense plantgrowth on many otherwise sparsely vegetated shores around the Indian Ocean.OCCURRENCEThe extent and development of mangrove stands and forests in the Indian Ocean have recentlybeen summarised by UNEP (1985). They reach their fullest extent in southeast Asia, where Indonesiapossesses 3,806,119 hectares (SALM and HALIM, 1984), Malaysia 652,219 hectares(SASEKUMAR, 1980), and Thailand 312,714 hectares (PIYAKARNCHANA, 1980). Mangrovesare moderately well-developed in more restricted areas of the Indian sub-continent and East Africa. Inthe northern Bay of Bengal, the Sundarbans, in the southern Ganges delta, support over 500,000hectares (MUKHARAJEE, 1984), and in Pakistan and western India the Indus delta supports 250,000hectares (SNEDAKER, 1984). Mangrove is less developed in Sri Lanka and patchy or thin in the173