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

upstream land and soil erosion. Increased sedimentation is probably the single most destructiveinfluence on coral reefs in the Indian Ocean.Corals are generally intolerant of very heavy sediment loads; not only are the low light levelsin turbid waters insufficient for adequate photosynthesis and growth, but the excessive sedimenteventually clogs or smothers the coral polyps, preventing feeding and interfering with respiration(MARSHALL and ORR, 1931). Corals are able to free themselves of some sediment by secretion ofmucus and the action of cilia, and some coral assemblages are characteristic of turbid waters(CROSSLAND, 1907; MARAGOS, 1972). But they are able to survive only so long as sedimentparticles are small and relatively east to remove; corals die when sediment loads increase, and inextreme cases may be compIeteIy buried (HUBBARD and POCOCK, 1972; MARSH and GORDON,1974).Examples of reef damage from sedimentation have been described from every part of theIndian Ocean. Sedimentation as a result of coastal and marine tin mining is the major cause of damageto reefs in Thailand (CHARSANG et al., 1961). Reef damage off the east coast of PeninsularMalaysia is especially noticeable in bays adjacent to agricultural development (DE SILVA et al., 1980).In the Gulf of Kutch (India) marked deterioration of coral reefs has occurred as a result of dredging of. sand for the cement industry (CHAVAN, 1983). High coral mortality has occurred on Minicoy atoll,the largest in Lakshadweep, from the dredging of the main shipping channel (PILLAI, 198 1). In theComoros siltation of reefs and lagoons has lowered fishery productivity (WORLD BANK, 1979).Sedimentation from inland deforestation is reported to be causing damage in the Andaman Islands(WHITAKER, 1984). In Kenya coral in the area of Malindi Marine Park has been more or less killedas a result of the great increase in sediment input to the sea by the River Sabaki following upriver soilerosion (FINN, 1983). In Bahrain a quarter or more of the main coral area has been lost, and much ofthe remainder noticeably impacted as a result of sedimentation caused by land fill and dredging ofshipping channels (IUCN, 1983b). And in the Saudi Red Sea, fringing reefs along 30 km of coastlinehave been seriously impacted by sedimentation following infill operations to construct the Jeddahcorniche road (IUCN, 1984).Damage by EchinodermsThere is some evidence for an increasing incidence within the region of damage to reefs byendemic populations of sea urchins (echinoids) and of the Crown-of-Thorns starfish (Acanthasterplanci). Crown-of-Thorns starfish, which feeds directly on coral colonies, became notorious in thelate 1960s and early 1970s because of the widespread destruction caused to coral communities on theGreat Barrier Reef and some other parts of the western Pacific Ocean (e.g. Guam). More recentlywithin the Indian Ocean, large populations of A. planci appear to have caused high coral mortality inMinicoy atoll (Lakshadweep) (PILLAI, 198l), in Sri Lanka, in Mauritius, and at locations inPeninsular Malaysia (DE SILVA et al., 1980).Reef urchins normally feed by grazing on the algal turf that colonises rocky surfaces of thereef. However, when the urchins Diadema setosum or Echinometra mathaei are present in largenumbers, the rasping action of their teeth erodes the bases and branches of the corals, causing thecorals to collapse and die. In some cases the urchins appear to erode the living coral tissue or attack itdirectly. Extensive damage of this type, first reported from the Caribbean (LEWIS, 1977;KRISTENSEN, 1978), has now been observed in the Egyptian Red Sea (ORMOND, 1980), inBahrain (BARRATT and ORMOND, 198S), in Kuwait (DOWNING, pers. comm.) and in Kenya(KENDALL, pers. comm.).Whether such population explosions of reef echinodems are an entirely natural phenomenonor are at least in part attributable to man's impact on the environment is as yet uncertain (see ENDEAN,1976; FRANKEL, 1977, for opposing views). However, there is now good evidence that in the RedSea at least high populations of urchins or Crown-of-Thorns are associated with reduced numbers oftheir principal predators, the large trigger fishes (e.g. Balistoides viridescens), the large puffer fishes(e.g. Arothron hispidus), and some of the emperors (Lethrinus spp.) (IUCN, 1984). It seems quitepossible that throughout the western Indo-Pacific increased frequency and size of echinoid and A.planci population outbreaks may be attributable to the reduction of key predators from the markedincrease in commercial and recreational fishing.170