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

island nations and groups of the Indian Ocean, although mangroves in the Andaman and NicobarIslands appear to be among the least disturbed.In the Arabian Gulf relatively few stands of mangrove survive, but in the Red Sea patches ofmangrove are scattered along both coasts, being more abundant in the south and thinning out to thenorth. The Saudi Red Sea coast, for example, is thought to support approximately 5000 hectares ofmangrove (IUCN, 1983a).The character and development of the mangal changes across the region in a similar way(UNEP, 1983a). In the east many species contribute to the mangal, for example 38 in Indonesia(SOEGIARTO and POLUNIN, 1982), and well-developed forests incorporate four or more zonesdominated by different genera, particularly Avicennia, Sonneratia, Rhizophora and Bruguiera (GONGet al., 1980). The mangroves of eastern Bangladesh are essentially similar to those of Malaysia(SNEDAKER, 1984), whereas on the west coast of India the mangrove areas are dominated byAvicennia marina and Rhizophora mucronata, although a total of about twenty genera are reputed to befound in the well developed mangals of the Gulfs of Cambay and Kutch (UNTAWALE, 1984). InPakistan the Indus River delta is dominated by Avicennia oflicinalis, often in poor stands, with onlyoccasional trees of Ceriops (SALM, 1975).Avicennia marina is the only mangrove recorded from the Arabian Gulf and the Gulf of Oman(BASSON et al., 1977), and this is also the principal species in the Red Sea, although very occasionalstands of Rhizophra mucronata also occur (IUCN, 1984), mainly towards the south.STATUSDespite their enormous value as a renewable resource, mangrove forests within the IndianOcean are being destroyed at an alarming and accelerating rate. They are being lost by increasedpressure of cutting for fuel and timber, related to the rapid growth of human populations. They arecleared, drained and/or felled for agriculture, residential or commercial development. They are clearedfor conversion into fishponds. And in particular in recent years large areas of mangrove forest havebeen felled for conversion to chipboard or paper, generally for export to industrial nations outside theregion (MacNAE, 1974; SOEGIARTO, 1980). In the past, forest managers frequently have croppedmangroves on a 16 to 30 year silvicultural cycle, but increasingly timber companies (particularlyJapanese) appear to be felling large areas with no regard for either traditional or recommendedsilvicultural practices.Thus in Indonesia an estimated 700,000 hectares were converted to agricultural land between1969 and 1979 (SOEGIARTO, 1980). In India, BLASCO (1977) estimated that only 365,000hectares remained, roughly half the official estimate for 1963. Total destruction is even reported overlarge areas of the massive and relatively inaccessible stands in the Bay of Bengal (MUKHERJEE andTIWARI, 1984). In western and southern India, much of the originally extensive mangrove has beenremoved (UNTAWALE, 1984; KRISHNAMURTHY and JEYASEELAN, 1984). And even wheremangroves represent a scarce resource they are being lost without regard to their value; over half of themangrove stands on the Saudi Arabian Gulf coast have been infilled for housing or industrialdevelopment in the past ten to fifteen years (IUCN, 1982).In addition to such direct destruction, mangrove forests are particularly susceptible to impactsthat affect their leaves or aerial roots, since both organs are essential to the plants' survival, even forshort periods (see ODUM and JOHANNES, 1975). The leaves, in addition to meeting the energyrequirements of the plant through photosynthesis, are the main organs of salt secretion that allow theplants to survive in saline waters. And the aerial roots are essential in allowing oxygen access to theunderground system which is frequently growing in anaerobic conditions; respiration takes placethrough small pores (lenticels) which are susceptible to smothering and clogging. The most significantadditional impacts from the point of view of Indian Ocean mangroves may be summarised as follows:174