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STATUS OF CRITICAL MARINE HABITATS IN THE INDIAN OCEANR. ORMONDTMRU, Department of BiologyYork UniversityYork YO1 5DDUnited KingdomINTRODUCTIONThis paper summarises the nature, occurrence and status within the Indian Ocean of severaltypes of habitat which, because of their significance in supporting marine life have, in recent years,been considered as Critical Murine Hubituts (RAY, 1976). Critical marine habitats include the feeding,resting, breeding or nursery areas of marine animals, or are major sources of numents for feedingareas elsewhere (for example, mangroves), or are particularly rich in species (such as coral reefs), orare highly productive (such as seagrass beds) or of special scientific interest (such as coral islands).The contribution of such habitats is frequently essential for the survival and productivity of commercialspecies, as well as of rare and threatened ones.Thus, for example, although commercial shrimp may be caught over a wide area of sea bed,these shrimp may depend to a very large extent on food originating from more limited areas of highlyproductive seagrass bed and mangrove; even smaller spawning areas and nursery areas may beessential to the survival of the stock (see FAO, 1981; PRICE, 1982). All these latter areas are criticalto the maintenance of the resource, in a way that the much larger areas of open seabed are not.In the Indian Ocean three types of critical habitat are especially important - coral reefs,mangrove forests and seagrass beds. Their social and economic importance arises in particularbecause of the very high primary productivity which these habitats are able to sustain: 500 to 5,000 gC m-2 yr-1. B contrast, the open ocean in the tropics generally has a very low productivity, around20-50 g C m- I yr-l (STEEMAN-NIELSEN, 1954; DOT1 and OGURI, 1956; PLA" and SUBBARAO, 1975). This is largely because of the way in which, in tropical seas, essential numents tend tobecome trapped at depth below a permanent body of warmer surface water. Thus, although most fishmay be caught in the open sea offshore of reefs or mangroves, frequently a significant proportion ofthe food that sustains the fish stock wil have originated directly or indirectly from one or more criticalhabitats.In addition to these three highly productive marine habitats, three other habitats, small islands,kelp forests and enclosed soft-bottomed lagoons and mud-flats, also occur locally within the IndianOcean, and may be regarded as critical in some degree. Each of these is considered more briefly.SIGNIFICANCECORAL REEFSCoral reefs and communities are perhaps the most economically significant coastal marinehabitat dver large parts of the Indian Ocean region, notably within the Red Sea and along the EastAfrican Coast. Handlining, principally for demersal and deep water reef species, is the most commonmethod of fishing in these areas, and a majority of the species caught are mainly or frequentlyassociated with the reef environment (see, e.g. WRAY, 1979). In the Red Sea, for example, five ofthe six most important groups of commercial fish, as judged by percentage contribution by weight ofthe total fish catch, are principally associated with the coral and reef environments. These are snappers(Lutjanidae), groupers (Serranidae), jacks (Carangidae), parrotfish (Scaridae), and emperors(Le thrinidae).The significance of the reefs for fisheries relates in part to their very high primaryproductivity. Gross primary productivities of between 1,800 g C m-2 yr1 (SARGENT and AUSTIN,167