3.1 Cruise narrative
3.2 Hydrography
3.3 Bottom conditions
3.4 Surface observations
3.5 Distribution and abundance of pelagic fish
3.6 Demersal fish
3.7 Plankton
According to the plan, work should proceed on the north coast and continue southwards along the east coast. A severe northwesterly gale led to reversal of the plan.
The vessel sailed from La Goulette on 19 March 1981 at 0800 hours and commenced work east of Djerba Island on 20 March in the morning. By then the sea was calm, and the cruise was run under favourable weather conditions throughout.
The sailing track was laid with course-lines perpendicular on the coast between the 20 and 200 m isobaths, with 10 nautical miles intervals on the east coast and 6 nautical miles intervals on the north coast. The chart in Fig. 1 shows the sailing route, and the fishing and hydrographic stations.
In the northern area fishing was impeded by heavy traffic in the ships’ lanes off the coast, and some trawling stations were cancelled for navigational safety reasons.
The cruise was terminated on 30 March in the afternoon when the vessel entered Algerian waters. The Tunisian scientific staff left the expedition at Alger.
Hydrographic work was restricted to one temperature section at 35°20’N, between Chebba and Lampedusa, and one temperature/salinity section north-south off the Gulf of Tunis at 10°30’E.
The east coast section (Fig. 2) shows temperatures just above 14°C in the surface layer, slightly rising seawards to above 14.5°C at the edge of the shelf. A minimum of 13.7°C was recorded at about 125 m depth with a slight increase towards the maximum recording depth of 140 m.
The section off the Gulf of Tunis (Fig. 3) records the highest temperature, above 14.5°C towards the coast, and further offshore above 14 C at the surface. A minimum below 13.5°C is recorded between 100-200 m, increasing slightly towards the bottom.
Salinity minimum below 37.25 per mille was found near the coast. Values above 30.00 per mille were recorded in the trench between the coastal plateau and the Skerki Bank area deeper than 160 m, and up to 50 m depth on the northern side of the bank.
Continuous temperature recording at the cooling water intake (at 5 m depth) gave temperatures between 14°-15°C in the whole area.
Most of the coastal bank plateau on the east coast offered good trawling ground. The shallows around the Kerkennah Islands had rough bottom unsuitable for trawling.
Also the Gulf of Tunis, sloping into deeper waters had good trawling grounds. Further west the ground was more uneven and rocky near the coast and also at the offshore banks. Good trawling ground was found in the extreme west between the coast and the Galite Islands, while the banks to the northeast of these islands were rocky.
On the eastward leg at 34°20’N, east of 12°E many schools of small pelagic fish were observed at the surface. In this area large flocks of dolphins and some larger whales were also spotted.
Dolphins were also spotted near the coast north of Mahdia, and further east, half way to Lampedusa surface schools of pelagic fish were found.
Fig. 4 shows positions of surface observations.
Species defined as pelagic fish in this report are: Trachurus spp. Sardina pilchardus, Sardinella aurita, Engraulis encrasicolus and Scomber spp. Boops boops and Maena spp., in previous reports defined as “semi-pelagic” species are also included in the group of pelagic species here, as it would be difficult to distinguish between these and the “pure” pelagic species in the echo recordings.
Fish registrations were generally confined to the shelf and the outlying bank plateaus.
Complete details of fishing stations are given in Tables 1-4. Table 5 presents length frequency distributions of the more important species.
Pelagic trawl hauls were generally poor, even when echo recordings were good. On turning back for trawling on recordings in midwater, the echogramme often showed the schools diving to the bottom. This was further substantiated by the net sonde echogramme. The fish was then too close to the bottom for the pelagic trawl to reach it. Bottom trawl would usually catch more. Some echogrammes that demonstrates this phenomenon are presented in the ANNEX.
Trachurus spp.
Trachurus mediterraneus was caught at one fishing station only (Stn No. 156).
In the southeastern part, Djerba - Kerkennah region, mostly juvenile Trachurus trachurus of length less than 10 cm were caught. A few fishes were up to 22 cm length. Large fish were caught only in deeper waters (108 m and 160 m) along the slope of the shelf east of Sousse.
In the north most of the horse mackerel in the catches were of 15-25 cm size. A good bottom trawl haul (725 kg) was made north of the Gulf of Tunis at 190 m depth. Here the fish were standing close to the bottom and the echogramme showed only a thickening of the bottom line. (see ANNEX).
Sardina pilchardus
Sardine was caught over the whole area, but never in large quantities. Size varied between 11-20 cm.
Data on parasites on sardine, as well as sex and maturity data on this species and Trachurus trachurus were collected for the La Goulette institute, and is not presented in this report.
Sardinella aurita
The round sardinella were encountered only in the southern part, south of Kerkennah. The length varied between 10-20 cm. A single large specimen was caught on the north coast.
Engraulis encrasicolus
Anchovy joined the catches in small numbers in the Gulf of Gabes, and were present in fair numbers off Nabeul and Kelibia. Three fair catches were made on the north coast.
Scomber spp.
Small Scomber japonicus were caught in bottom trawl in the southern region, south of Kerkennah, while a few Scomber scombrus were taken in the north.
Boops boops
The bogue appeard in catches with the sardine, and may prefer the same environment, even if it stays closer to the bottom.
Maena maena and Maena smaris
These species were distributed all over the eastern region, while only Maena smaris was taken off the north coast. A particularly good catch was made in the east, northeast of Kerkennah (Stn No 145).
Pelagic fish biomass estimated from echo integrator values is shown in the following table:
|
|
Area of fish distribution |
Abundance index |
Fish biomass |
|
|
n. mile2 |
|
tonnes |
|
East coast |
11,800+ |
45,030 |
540,400 |
|
North coast |
2,100 |
5,520 |
66,200 |
|
Total |
13,900 |
|
606,600 |
+) The uncovered area around the Kerkennah Islands is included.
Demersal fish were caught only in small quantities throughout the whole area. Mullus barbatus, Merluccius merluccius and Pagellus erythrinus were the most frequent species, present in most catches on the east coast and also in several catches on the north coast. The family Sparidae was the most varied with ten species on the east coast, but only four on the north coast. Length frequency distributions of some demersal species are given in Table 5.
Rather few bottom trawl hauls were made. Trawling was, moreover, often directed towards the best recordings. Therefore it is not possible to make any useful biomass estimate from swept area calculations.
Very little plankton was recorded over the coastal plateau. But strong recordings were made in deeper waters east of the Gulf of Hammamet. Trawl hauls through this layer gave krill (Euphauciacea). Large Trachurus trachurus and Lepidopus caudatus were feeding in this layer.
Similar recordings were made in the north, especially north of the Galite Islands plateau. Because of heavy traffic, no haul was made here. A deep haul north of the Gulf of Tunis, on a thinner layer close to the bottom yielded only a few krill and mesopelagic species (Myctophidae and Sternoptychidae), and mainly “jellyfish”.
