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

significant numbers in the Red Sea, in the Arabian Gulf, and on the island groups of the western andcentral Indian Ocean.As is well-known, sea turtles are threatened worldwide, and all of the above species are nowregarded as endangered in the IUCN Amphibia and Reptile Red Data Book (HONNEGER, 1975) andalso are listed in the CITES Convention of Trade in Endangered Species. The precipitous decline ofthe turtle in the Indian Ocean (as elsewhere) stems in part from increasing human access to andexploitation of small remote islands that previously had harboured unimpacted breeding colonies. Theadults are taken on the beaches for their meat, or in the case of the Hawksbill for their shell(tortoiseshell). At an increasing number of sites eggs are collected to excess; at some sites in India andMalaysia over 90% of eggs are thought to be taken. The situation is further exacerbated by the generaldisturbance caused by visitors and by residential and other developments. There is often an increasednumber of feral or domestic dogs digging up the nests. And the adults also may be hunted by net orharpoon in the immediate neighbourhood of their breeding grounds.Such a decline is continuing despite an appreciation that the region’s turtle populationrepresents a valuable renewable resource. For example, the value of turtle products exported fromIndonesia in 1980 was US$ 928,539 and in 1981 US$ 407,542 (SALM and HALIM, 1984).Groombridge (1985) suggests that the Olive Ridley in India could be sustainably exploited, givenproper management, and, once they have been allowed to recover, the same might be said of otherturtle populations in the region. The decline, however, continues despite the laws and regulationswhich in various countries of the region should give adequate protection to all species. Enforcement isdifficult, and exploitation and impact appear continuous and very largely uncontrolled.KELP FORESTSReasonable growth of macroalgae occurs intertidally and subtidally on rocky shores and reefsaround various parts of the Indian Ocean (see, for example, JAASUND, 1976). While these algaemay be relatively productive, they do not form a conspicuous critical habitat except within one area ofthe Arabian Sea, where an unusual and highly productive community recently has been described(IUCN, 1983~).Along parts of the Dhofar coast of Southern Oman a dense algal community is dominatedintertidally by thick growth of the large green Ulva fasciata, and subtidally by the kelp Eckloniaradiata, together with a similar hitherto undescribed plant, probably from a new genus allied toSargassum. Associated with these larger algae are ninety or more ‘understorey’ species, of which themost conspicuous is Amphiroa anceps. It is possible that other areas of kelp forest, or at least of densealgal growth, may occur on parts of the coast of Somalia or of South Yemen, as these also lie adjacentto the southern Arabian upwelling.The occurrence of the kelp forest in Dhofar is associated with the intense upwelling of water insouthern Arabia. This upwelling occurs seasonally during the southwest monsoon when, as a resultof the southwest winds, the surface waters are carried in a southeasterly direction, away from thecoast, and are replaced by the upwelling of water from below (SMITH, 1968; BOTTERO, 1969;CURRIE et al., 1973). According to calculations by BOTTERO (1969), the upwelling extends at least400 kilometers offshore and parallels the coastline for a distance of over 1 ,OOO kilometers; because ofthe breadth of the upwelling zone, water is supplied to the surface from greater depths than is usual inother coastal upwelling regions. However, as the upwelling is seasonal, luxuriant growth of the kelpand of the other algae occurs during the monsoon period (June to September), but decreases rapidlyfollowing the end of the monsoon.Based on studies in other regions, when kelp forests occur in situations where nutrientsupply, illumination and temperature are optimal, they can maintain high productivities of 1,500-3,000g C m-2 yrl, comparable with the most productive of aquatic or terrestrial ecosystems. Not only doesthis productivity sustain large populations of a great variety of invertebrates and fishes, but the plantsthemselves create an environment within which many find shelter. In Oman over 200 species ofmolluscs and 120 species of crustacea have been recorded in association with the algal communities(IUCN, 1983~). Some of the plant material is consumed directly by herbivores, notably the seaurchins Echinometra mathaei and Stomopneustes variolaris, and the abalone, Haliotis rnuriae. But thebulk of the kelp production enters the food web via the detritovore food chain. The fronds of the kelpand other algae are continually eroded by wave action, and the detritus thus formed is utilised by181