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

R/V Dr. F. Nansen trawl of 41 m headline with a vertical opening of 6 m. Thecodend contained a fine mesh liner of 1-m mesh, Trawling speed was 2.8 knots andtrawls were of 0.5 hour duration. Demersal fish stocks were assessed by the trawlswept area method, based on the distance between the wings of the 28 m headline trawlof 20 m and the 41 m headline trawl of 25 m. The area covered by the former wasaround 12.8 ha per hour and the latter 15.5 ha per hour. Catches were separated intospecies which were then counted and weighed, or estimates of catch weight made frombaskets. Random samples were taken for length and weight measurements.Pelagic FishThese were surveyed by 3 scientific echo sounders (120,50 and 38 Wz), 2 echo integratorsand 1 net sonde (50 kHz). The pelagic trawl had a 48 m headline and an opening of around 10 m. Afine mesh codend liner was used. The trawl catch was used to identify species in fish schools.Pelagic fish studies on the coast were made from purse seine landings where catch, effort,speciation and age composition were recorded. These data were used for virtual population analysis(VPA).RESULTSThe aim of this paper is not to present the results of our recent investigations in the Gulf ofAden in any detail, which will be done elsewhere, but rather to show selected data in order to give anoverall view on the most crucial oceanographic conditions and processes as related in particular topelagic productivity and living resources of this region.HYDROGRAPHIC CONDITIONSWater MassesThe upper 1,000 m layer of the Gulf of Aden appears to have a 3-layer structure (Table 1) asfollows:Surface Water (SW) occupies the uppermost layer, ranging in depth from 20 m down to125 m. Its lower boundary usually coincides with the thermocline. It forms locally by intensiveheating and evaporation, thus having high temperature and salinity, and is relatively well saturated withoxygen except during the summer in upwelling zones. The volume and physical properties of SWhave pronounced spatial and temporal changes (Figs. 2-8). Usually the lower boundary lies deeper inthe eastern and western parts of the Gulf and is shallower in the center as well as on the shelf.Temperature and salinity decrease slightly from west to east and from shores towards the center.However, temporal changes are much more pronounced than spatial ones. During summer, due towind-driven water circulation (southwest monsoon) and upwelling, the surface layer is the thinnest; itoccupies only the upper 20 m of waters along the Arabian coast and up to 70-80 m in the central part.Upwelling of the subsurface water raises the thermocline, which also leads to low oxygencontent, about 1 ml l-1, in the lower layers of the surface water. The highest temperature (26-32°C)and salinity (35.8 - 36.8 x 10-3) of the SW are observed, however, a bit earlier: in May-June, whenthe heating and evaporation are most intensive. The thickest SW, 100 - 150 m, is observed du-ingwinter time, when surface cooling and density mixing depress the thermocline. Usually at this timetemperature and salinity reach their minimum values. The lower boundary of SW can be placed alongthe isopycnal surfaces of 24 - 25.259