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

It is logical to expect that such high productivity supports very large stocks of exploitableresources. This expectation is true for the largely underexploited clupeid, carangid andscombrid fishes and an immense potential (est. 15 million tons) of mesopelagicmyctophids. However, stocks of demersal fish (which are at present the target ofcommercial fisheries) appear to be limited and also heavily exploited.Unfortunately the overall high pelagic productivity in an “immature” upwelling ecosystemindirectly contributes to the disequilibrium of its oxygen balance, as a large part ofproduced organic matter is “wasted” by bacterial decomposition. In the case of the Gulf ofAden this is combined with the general deficit of the oxygen in the North Arabian Sea.Consequently, in the largest part of the Gulf the semi-anoxic conditions (0.5 ml 1-1 or lessof dissolved oxygen) appear throughout the upper water column and below 150 - 200 mdepth. During the summer this anoxia is generally pushed into the surface layers, and canbe extreme in upwelling centres where semi-anoxic conditions prevail below a depth of 20m or even less.Anoxia, which can be considered as an ecological catastrophy, affects the ecosystem inmany poorly understood ways. It certainly contributes to an extraordinarily low standingstock of the benthic communities on the shelf bottoms, and consequently to the missing orvery weak relationship of the benthos with the demersal fish populations. This in turnalso means the trophic dependence of demersal fish on pelagic food potentials that leads togreater degree of overall instability. The appearance of anoxia in shelf waters also causeslarge migrations and structural recombinations in demersal fish communities, and mostlikely the migrations of epipelagic fish also. Although anoxia-induced fishmass-mortalities have not been observed in the Gulf of Aden, “hidden” mortalities areassumed to occur.REGIONAL COOPERATIVE INVESTIGATIONS OF THE NW ARABIAN SEAThe unique oceanographic and marine biological phenomena in the Gulf of Aden and adjacentparts of the Arabian Sea are still poorly documented, and therefore deserve greater attention.Moreover, the crucial economic and nutrition needs of bordering nations the Gulf of Aden providemore practical reasons for increased research. The development of a rational and economically healthyexploitation of the living resources in this region, however, cannot work properly without an adequatescientific basis. To achieve such a scientific basis requires a rather complex knowledge on theecosystem and living resources (Fig. 19). Using BEHRMANs (1981) words, “finding fish is notenough”; it leads to the tasks as shown in the lower part of Figure 19.Since the Gulf of Aden and adjacent parts of the Arabian Sea inevitably must be considered asa functional unit, at least from the standpoint of bioproductivity and oceanographic processes,investigations must cover the whole region. However, the area is extensive and the seas, particularlyduring the summer monsoon, are heavy. Therefore the sea-going research requires a sizeable wellequippedresearch vessel, operating continuously year around with an interdisciplinary scientific teamaboard, covering the area as shown in Figure 20. Probably no single country bordering this area(Djibouti, Somalia, P.D.R. Yemen, A.R. Yemen and Oman) alone can support the costs of such astudy, and even so, this would lead to an uneconomic duplication of efforts. Therefore it is proposedherewith to set up a regionally operating organization, shared by all bordering countries and supportedby an UN interagency group (UNDP/FAO/UNESCO-IOC/UNEP). A tentative outline of scientificprogramme of interdisciplinary oceanographic and fisheries investigations for this area is proposed inthe Appendix.270