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

in 6 months when spawning occurs. They occur where the depth exceeds 150 m, which in the presentcontext would be the shelf edge. Vertical migrations occur between 150 to 200 m depth in the dayrising to within 10 m of the surface at night.Stock densities in the Gulf of Aden were estimated at above 100 tons km-2 (GJOSAETER,1984). Based on growth and recruitment studies, SANDERS and BOUHLEL (1982), estimated theannual yield from the Gulf of Aden at about 15 x lo6 tons.GENERAL CHARACTERISTICS OF EXPLOITABLE RESOURCESDemersal fish populations vary in composition and standing stocks with season, depth andlocality on the coast. In the west around Aden the dominant taxa in the non-upwelling season areLeiognathidae, Carangidae, Pomadasyidae, Lethrinidae and Nemipteridae. During upwellingCentrolophidae, Sparidae, Synodontidae, Sepia sp. and Loligo spp. are prominent. In the east aroundRas Fartak the dominant non-upwelling taxa are Sepia, Ariidae, Sparidae, Dasyatidae andSphyraenidae, while during upwelling Callionymidae, Triglidae, Nemipteridae, Clupeidae andS y nodontidae predominate.Standing stocks at 30 m depth have been measured to be 3.6 and 6.0 tons km-2 in the westand east respectively during non-upwelling, falling to 1.9 and 2.5 tons km-2 during upwelling.Maximum standing stocks occur at 30 m depth in October, at 110 m in May and 20 m or less inAugust.During upwelling demersal populations migrate both inshore and vertically in the water, andappear to separate into pockets along the coast. Deeper water species move on to the shelf and intoshallow water at this time. These migrations probably occur in response to oxygen minima.Pelagic fish stocks are concentrated along the coast in the non-upwelling season at Aden,Mukalla and around Ras Fartak. During upwelling part of the stocks in the east migrate offshore,while those at Aden remain inshore. Pelagic trawls have showed Clupeidae (i.e. Surdinella longiceps ,the oil sardine) to be dominant in the non-upwelling period.Large pelagic fish (e.g. tunas, Spanish mackerels) have not yet been assessed, butconcentrations appear mainly in the western two thirds of the Gulf, with highest stocks in the Adenregion. Here substantial stocks of Indian mackerel, Rastrelliger kanagurta, are found.Total demersal stocks may exceed 116 x lo3 tons, while pelagic stocks may be around 300 x103 tons.SUMMARYThis region of the Indian Ocean presents a rather unique, large-size ecosystem of which themost remarkable characteristics are as follows:Oceanographic conditions that reverse in response to the changes between NE and SWmonsoons also respond to variable and poorly understood influences of the SomaliCurrent and inputs from the Red Sea and the North Arabian Sea intermediate - deep watermasses.Upwelling occurs regularly during the SW monsoon season in many areas of this region.However, the most affected areas are in the east where the upwelling is much greater thanin “classic” upwelling regions of the world oceans.Because of the above conditions, the supply of nutrients to euphotic layers is suffici:ntthroughout the annual cycle to support a balanced primary production. During the summerand the subsequent postmonsoon season, however, the production dramatically increases,reaching densities up to 6 million cells dm-3 and 5 pg dm-3 of chlorophyll a. Secondaryproduction of zooplankton is also very high, with average dry weight biomass of about100 mg m3. This suggests that these waters are among the richest not only within theIndian Ocean but also in the world oceans.269