The trawl bottom conditions of the waters adjacent to Mozambique are shown in Fig. 3.1. The map is a compilation of the information from the “Dr. Fridtjof Nansen”, “Kattegat” and “Aelita” expeditions. Off the northern part of the coast from Cabo Delgado to Angoche including the St. Lazarus Bank, it is, in general, impossible to use bottom trawl. This is due to several submarine canyons and the narrow shelf with fringing corals. The coral extends south along the coast approximately to Angoche, where it spreads out to sea. It continues southward to about 17°30’ S as isolated submarine platforms forming a chain of small islands on the shelf some 70 km offshore (TINLEY, 1971).
Fig. 3.2 is an echo recording from the depth range 0-250 m showing a transsection across the St. Lazarus Bank. The distance between the vertical lines is one nautical mile, and the depths of the bank between 20 and 30 m.
The Sofala-Bank area south to Beira gives only small problems in relation to bottom trawling at depths less than 100 m. An area of corals occurs at depths between 40 and 100 m between the Zambezi delta and Quelimane. The continental slope is very precipitious and partially uneven with sharp edges, so trawling has to be carried out with caution.
The shelf area south of Beira between 19°30’ S and 21°30’ S is unfishable with bottom trawls at depths less than 50 m. This is caused by the undulating character of the bottom, as shown by the echo recording in Fig. 3.3. The distance between the vertical lines is one nautical mile and the depth is between 20 and 40 m. The waves are most likely sandwaves generated by the strong tidal currents in the area. The wavelength is between 200 and 400 m and the amplitude mainly 10-15 m, in some cases even 20 m.
The area between Bazaruto Island and Ponta Zavora is partly unfishable at depths less than 150 m due to bottom irregularities, rocky outcrops and canyons. In deeper water the bottom is muddy or sandy and characterized as a good trawling area.
Fig. 3.2. Echo recording across the St. Lazarus Bank.
In the Delagoa Bay, some areas at depths less than 150 m are unfishable, especially south-east of the Quissico lighthouse at 40-50 m, and off the Limpopo River. At greater depths there are usually no problems in using a bottom trawl.
On the stretch Ponta do Ouro to Inhaca Island it is difficult to trawl due to coral and rocks at depths less than 100 m. Generally there are no problems in deeper waters, but some rocky outcrops and canyons have also been observed here.
The area of the shelf zone has been calculated using British Admiralty charts. The results are shown in Table 3.1. The total area of the shelf between 10 and 200 m is about 70 000 km2. The frequently cited area of 120 000 km2 from MOISEEV (1969) for the upper 200 m is obviously too large. The splitting of the Mozambican coast into five different areas appears in Fig. 2.2.
Fig. 3.3. Echo recording from Sofala Bank.
Table 3.1. Area of the shelf of Mozambique (km2).
|
Area\Depth |
Northern |
Sofala |
Bazaruto |
Delagoa |
Inhaca |
Total |
|
10-50 m |
Including St. Lazarus Bank = 133 km2 |
38020 |
4240 |
4570 |
320 |
|
|
51-100 m |
7380 |
1350 |
3020 |
290 |
|
|
|
101-150 m |
490 |
960 |
1280 |
160 |
|
|
|
151-200 m |
490 |
960 |
1280 |
160 |
|
|
|
10-200 m |
6190 |
46380 |
7510 |
10150 |
930 |
71160 |
|
200-400 m |
|
1960 |
4120 |
7900 |
950 |
|
