The course lines and stations are shown in Fig. 3.1. The transects across the shelf were generally 10-15 miles apart and stretched to about 5 miles beyond the shelf edge. The positions of the bottom trawl stations were decided according to the bottom conditions.
The bottom was often too rough to allow trawling. Mid-water trawling was carried out following the echo-sounder recordings and/or surface observations. The mid-water trawl was operated with success only during night time because the pelagic species easily avoided the trawl during daytime.
Hydrography was observed in two sections, one across the channel between Ras Asir and Abd al Kuri Island and one eastward from Ras Hafun.
The section off Ras Asir is shown in Fig. 3.2. The upper 50 m layer is characterized by mixed surface waters with temperatures over 24°C and salinities exceeding 36 units. The main thermocline was observed at 100 to 150 m depth but was rather moderately developed. At greater depths the temperature decreased from about 17°C, at the bottom of the thermocline, to approximately 11°C at 900 m depth, close to the bottom.
The salinity section shows an intermediate minimum with salinities of about 36.6 units. This minimum was observed at about 100 m depth off the Somali coast and tilted towards greater depths as distance from the coast increased. Near the eastern slope of the channel this minimum was found at 600 m depth.
It seems likely that the high salinities well above 36 units found in the surface layer are due to local evaporation. The salinities below 36 units in mid-water indicate Indian Ocean water masses while the salinity increase toward the bottom indicates admixture of Red Sea waters.
The oxygen distribution shows fairly-well aerated waters down to 100 m depth on the Somali shelf, increasing to approximately 200 m on the eastern side of the channel. An intermediate oxygen minimum with contents below 0.5 ml/l was observed at around 400 m depth on the Somali slope, its approximate depth increasing to 700-800 m on the eastern side of the channel. Towards greater depths there was again a slight increase to approximately 1 ml/l near the bottom.
The short section off Ras Hafun (Fig. 3.3) was worked with the main intention to observe the depth of the water with high oxygen content on the shelf.
The distributions of temperature and salinity were very similar to those observed in the previous section (Fig. 3.2). The thermocline was situated at about 100 m depth and this coincided with the oxycline. Oxygen contents were above 1 ml/l at about 125 m depth.
Pelagic fish: Fig. 3.4 shows the distribution of pelagic fish. Fairly dense concentrations were observed in an area stretching southwards from Ras Asir to about 11°N. The fish occurred mainly in schools, but during night time dispersed layers were also observed. Integrator values above 50 units were obtained in three areas and in the southern of these there were two 5-mile readings with values above 100.
The southern area with recordings of pelagic fish, stretching from just north of Ras Hafun to Ras Mabber, had considerably lower concentrations. Here the fish occurred in small schools and no areas with concentrations of schools were observed. The integrator values were generally between 1 and 5.
The schools of pelagic fish were in many cases observed at the surface, particularly in the area where the densest concentrations were observed.
As indicated by the catches taken with the mid-water trawl, the pelagic resources consisted of various species of sardines, scads and mackerel. Sardinella longiceps and Dussumieria acuta seemed to be the dominating sardines while Decapterus russelli, D. macrosoma and Scomber japonicus were important components as regards the other types. More details on species composition can be found in the attached fishing logs.
Demersal fish: The distribution of demersal resources is shown in Fig. 3.5. Dense concentrations were observed in an area just south of Ras Asir, while bottom fish in more scattered concentrations were recorded on most of the shelf south to Ras Mabber. Fairly good catches were obtained with bottom trawl down to almost 100 m depth.
At greater depths the species compostion in the catches changed to species of smaller commercial importance. The reason for this is the oxygen content in the water which at depths greater than about 100 m seems to be insufficient for most of the demersal species.
Catches up to 9700 Kg. per trawling hour were obtained and a variety of excellent food fishes were caught. Various species of seabreams, snappers and groupers were important components. The composition of the various catches is given in the fishing log. Here species like Lethrinus nebulosus, local name Dowani, Diagramma picta represent important higher quality components. Smaller species like Pagellus affinis and Carangoides equula represented important components in the catches due to their quantity.
