3.1 Cruise narrative
3.2 Hydrography
3.3 Bottom conditions
3.4 Pelagic fish
3.5 Demersal fish
3.6 Shrimp
3.7 Mesopelagic fish, plankton, swimming crab, cephalopods
The cruise commenced on the 2 March 1981 at 1920 hours. The inner part of the Gulf of Tadjura was surveyed during the night. Three pelagic trawl hauls were made here, and a bathythermograph station was taken in the centre of the gulf.
At daybreak on the 3 March, the vessel entered the plateau between Djibouti harbour and Musha Islands. A bottom trawl haul was made here, mainly to look for large shrimp.
The entrance to the Gulf of Tadjura was searched until midday. The rest of the day and the 4 March until dusk was spent surveying the shelf and the deeps between Ras Bir and the country’s northern border. One bottom trawl and six pelagic trawl hauls were made in this area. A bathythermograph station was made off Ras Bir.
The eastern triangle of the EEZ to the east of Ras Bir was not surveyed.
North of Ras Si-Ane fishing was restricted to the coastal plateau, as it was not advisable to trawl in the heavily trafficked ships’ lanes in the Straits of Bab-el-Mandeb.
For comparison with the daylight trawl haul on the plateau off Djibouti, another haul was made here around midnight.
The vessel anchored in the roads off Djibouti at 0115 on the 5 March. The local participants disembarked in the morning.
The weather conditions were favourable with calm in the Gulf of Tadjura and southeasterly winds up to force 4 in the northern region.
The survey courses and stations are shown in Figure 1.
Temperature was recorded continuously at 5 m depth. Temperatures varied between 26.5 and 27.5°C in the whole area.
Two bathythermograph stations were made to determine the thermocline. In the Gulf of Tadjura a distinct thermocline was found at 60 m. The upper layer held about 26.5°C, temperature falling to about 22.5°C between 60 and 110 m. Off Ras Bir the temperature fell more evenly from 26.7°C at the surface to about 24.8°C at 85 m, rising slightly and falling again grom 95 m to 24°C at 130 m (Figure 5).
Most of the shelf area that was covered, off Djibouti and between Ras Bir and the northern border (see Figure 1) had bottom suitable for demersal trawling.
Depths in the inner part of the Gulf of Tadjura are uncertain, and the charts cannot be trusted. Bottom topography has changed in recent years because of volcanic activity.
The main objective of the survey was to find whether migratory small pelagic fish species (Carangidae, Clupeidae, Engraulidae and Scombridae) had arrived in Djibouti waters. Therefore the instruments were set to record the upper 200 m.
No significant schools were recorded, and the trawl catches were poor. As the survey was made rather early in the season, this was no surprise.
Complete details of fishing stations are given in Tables 1 and 2, and density distribution chart is given in Figure 2. Some of the sought for small pelagic species are mentioned below:
Decapterus macrosoma (Lajang scad)One specimen only (TP 112), over the shelf north of Ras Bir.
Decapterus maruadsi (Round scad)
A few speciment caught in bottom trawl (TB 116) on the plateau off Djibouti harbour.
Selar crumenophthalmus (Bigeye scad)
Present in small number in catches over the shelf from Tadjura towards Ras Si-Ane (TP 107, TP 109, TP 112). About 15 kg caught in bottom trawl off Djibouti harbour (TB 116). Length frequency distributions presented in Table 3.
Dussumieria acuta (Rainbow sardine)
Caught in small number in the two bottom hauls (TB 108, TB 116) off Djibouti harbour.
Sardinella sp.
Present in bottom trawl catches (TB 108, TB 116) on the plateau off Djibouti harbour. Also two specimens caught at TP 105 at the head of the Gulf of Tadjura. The species probably S. gibbosa.
Stolephorus sp.
One specimen only (TP 107).
Rastrelliger kanagurta (Indian mackerel)
Three specimens caught at TB 116 off Djibouti harbour.
The two bottom trawl hauls (TB 108, TB 116) on the plateau off Djibouti harbour yielded a rich variety of species (Table 2). Areas where demersal fish were recorded are shown in Figure 3.
The Leiognathidae, which may be labelled “semi-pelagic” species, constituted 80% of the catch at TB 108 and 13% of TB 116. Other species that were fairly numerous were Japanese threadfin bream (Nemipterus japonicus), Great lizardfish (Saurida tumbil) and Therapon perches (Therapon jarbua, T. theraps).
In the north, recordings at the bottom were poor.
The bottom trawl hauls on the plateau off Djibouti harbour were made primarily to look for large shrimp.
A naval exercise with several moored installations over the plateau permitted trawling only in a very narrow area where one day and one night haul were made.
The two fishing stations confirmed the presence of commercially valuable large shrimp (Penaeus sp.), but only a few specimens were caught. The two hauls were made at the same position.
Most of the integrated echo intensities were ascribed to mesopelagic fish and plankton. The contribution from swimming crabs is somewhat uncertain. The density distribution of these organisms plus squid is shown in Figure 4.
The most abundant mesopelagic species were Myctophids, mainly Benthosema pterotum, and also Lampanyctus macropterus. Other mesopelagic species that were taken in numbers, but at fewer stations were Stomias affinis and Lestrolepis intermedia.
Eel larvae were not taken in the inner part of the Gulf of Tadjura, but were quite numerous outside from Ras Bir northwards.
Small squids and swimming crabs were present in most pelagic hauls, and cuttlefish were caught in the bottom trawl off Djibouti.
No plankton hauls were made, and the planktonic organisms caught by the trawl were not identified (“krill”).
Samples of mesopelagic fish were preserved and taken to the University of Bergen, Norway for further study. The results will be presented in a separate publication.
