Course tracks and stations are shown in Fig. 2.1. Most of the shelf is very narrow and the survey work was therefore confined to this narrow zone along the shore.
Hydrography: A hydrographic section was worked southwards from Aden stretching approximately half way across the gulf.
Temperature, salinity and dissolved oxygen was observed at standard depths to 1000 meters. The distribution of these variables and the density distribution are shown in Fig. 2.2.
The mixed waters in the upper layer had temperatures between 24° and 25°C. A fairly well developed thermocline was observed at 100 m depth. The temperature at the bottom of the thermocline was 17° to 18° C decreasing gently towards greater depths. Over the continental slope there was a maximum with temperatures above 18° C between 500 and 650 m depth and temperatures above 15°C were observed down to 900 m depth.
Below this depth there was a relatively sharp transition layer were temperatures decreased to approximately 11.5°C at 1000 m. At 1000 m depth there was however a southward temperature increase to 14.6°C at the most southern station. The temperature maximum over the slope as well as the high temperatures at 1000 m on the southern part of the section is due to the warm and saline waters flowing out of the Red Sea. This is also indicated by salinity maxima with values exceeding 37 units in the same localities. An intermediate salinity minimum stretching from the bottom of the thermocline to 300 to 500 m depth may indicate inflowing waters from the Indian Ocean. This assumption is supported by the low oxygen content in this layer. Below this intermediate minimum the oxygen content increased slightly with depth. The oxycline, i.e. the transition layer of the oxygen, coincided with the thermocline and the surface waters, to about 100 m depth, were well aerated with contents exceeding 4 ml/l.
The density structure was suggestive of fairly moderate currents in the upper and intermediate layers. In the deeper layer there were indications of higher velocities. This is in agreement with the outflow of the Red Sea water which is suggested by the temperature and salinity structure.
Fig. 2.3 shows the section which was worked across the Gulf of Aden between Ras Fartak and Socotra. Here the thermocline was observed slightly deeper than off Aden and it was somewhat less developed, particularly in the southern part. The salinities in the upper layers were higher than 36 units while there was a transition layer at the depth of the thermocline with salinities below 35.75. This water filled the water column to 1000 m depth.
Its oxygen content was very low, mainly between 0.5 and 0.75 ml/l. From about 900 m depth there was a slight increase in the oxygen content towards greater depths. These properties indicate that water from the Indian. Ocean occupied the whole water column. Red Sea waters flow along the bottom and were not observed above 1000 m depth.
Pelagic fish: The distribution of pelagic fish is shown in Fig. 2.4. Only very small and scattered schools of pelagic fish were recorded. The largest concentration was recorded off Aden. Here, integrator deflections above 10 units were Obtained in the densest school. These resources consisted mainly of scad, Decapterus russelli which counted for more than 90% of the catch in the pelagic trawl. Rastrelliger kanagurta and Sardinella longiceps were also present.
The trawl catch in an equally dense, but smaller concentration further west consisted mainly of Sardinella gibbosa with some admixture of Gazza minuta and Rastrelliger kanagurta.
Along the coast, east of Aden the pelagic resources were quite small. Only a few very scattered recordings were made. The only exception was a small concentration mainly consisting of Scomberomorus commerson in position 13°39' N and 47°30' E (Fishing station 113).
Demersal fishes: The distribution of demersal fish as recorded by the acoustic system is shown in Fig. 2.5. Again only very scattered recordings were obtained. Only at the western end of the investigated area there was a very small concentration being dense enough to give integrator deflections above 10 units. A trawl catch in this area (Fishing station 96) consisted of a wide variety of demersal, pelagic and shallow water species. Species of the family Carangidae, Trachinotus blochii and Megalaspis cordyla, made up the main component.
Eastwards along the coast the catch rates in the bottom trawl were rather low. The greatest catches consisted mainly of species of Leiognathidae, Carangidae and Nemipteridae. The species composition of the catches was however quite diversified and reference should be made to the attached trawl log.
In the eastern part of the survey area, juvenile fish counted for a considerable proportion of the catches and most of the demersal species in the area were represented (see for example stations 125 and 130).
Socotra: The shelf area around Socotra was covered from 25 to 28 February. Course tracks and stations are shown in Fig. 2.6.
The. recordings of pelagic fish is shown in Fig. 2.7. Only. very scattered recordings were obtained in a few limited areas mainly along the shelf edge.
The recordings of demersal fish are shown in Fig. 2.8. Also these were at a quite low level. The highest integrator deflections and the largest catches were obtained on the northeast coast. The shelf was however very narrow in this area and the possibilities for demersal trawling were quite few due to rough bottom. Two catches (Fishing stations 131 and 132) consisted of many species mainly of the families Sparidae, Lethrinidae and Lutjanidae as the dominating component.
On the shelf south of Socotra the catch rates were generally low and often consisting of small fish. Only in the western area (Fishing stations 144 and 145), commercial important species counted for the greater proportion in the catches.
The survey work in South Yemen was terminated on 28 February.
