Angola has a coastline of 800 nm of which that off Cabinda is 45 nm. The total shelf area is about 17.000 nm2 and the mean width is thus about 20 nm. The extension of the shelf by depth ranges in the different regions is shown in Table 2. The southernmost region from Cunene up to Porto Alexandre has a relatively broad and shallow shelf, but with a steep slope except in the extreme southern part. From Porto Alexandre to Benguela the shelf is very narrow and the slope too steep for any deep water trawling. From Benguela northwards the shelf is generally wide all the way up to Cabinda, but with a narrow part off Luanda and a steep “canyon” outside the Congo River.
| Bottom depth range | Cunene River Porto Alexandre n.m.2 | Porto Alexandre Pta. Salinas n.m.2 | Pta. Salinas Pta. Palmeirinhas n.m.2 | Pta.Palmeirinhas Cabinda n.m.2 | Total n.m.2 |
|---|---|---|---|---|---|
| 0– 50m | 473 | 318 | 1746 | 2500 | 5037 |
| 50–100m | 713 | 382 | 1684 | 2123 | 4902 |
| 100–200m | 164 | 416 | 1284 | 1830 | 3694 |
| 200–300m | 486 | 656 | (1142) | ||
| 300–400m | 395 | 474 | (869) | ||
| 400–500m | 441 | 443 | (884) | ||
| 500–600m | 410 | 524 | (934) | ||
| 0–200m | 1350 | 1116 | 4714 | 6453 | 13633 |
| 0–600m | 1788 | 1604 | 6446 | 8550 | 18388 |
| Coast line(nm) | 85 | 190 | 275 | 250 | 800 |
The slope down to 600m and beyond has a gradient which over wide distances is sufficiently low and smooth to permit bottom trawling.
From two of the surveys records are available of the character of the bottom with reference to its suitability for trawling based on analysis of the echo sounder diagrams. These observations are set out in Figure 1 and show that most of the shelf and the slope can be fished with trawl, but some areas may have too rough ground especially along the narrow shelf from Porto Alexandre to Benguela and off Ambrizete. In the northern areas offshore oil exploration and production may prevent or endanger trawling operations, not only near the installations, but also in areas of previous exploration and along supply vessel routes where debris may have been dumped.
The litterature describes the main circulation off the Angolan coast as being dominated by the Angolan current flowing mainly southward and apparently formed by the southeast branch of the South Equatorial Countercurrent and southward-turning waters from the north branch of the Benguela current.
A characterization of the Angolan current is a generally southward moving stream from 9 to 16°S, that penetrates to depths of 250–300 meters and covers not only the shelf regions, but also the continental slope, with surface velocities exceeding 50cm/sec.
The Benguela current, part of the Anticyclonic subtropical gyre of the South Atlantic, and forming its eastern boundary, flows parallel to the southern African coast in a north to north-westward direction. Streams of the Benguela current proceed northward along the shore and reach 13–14°S, and they form a divergence zone along 11°E from 17° to 13°S, as a result of an interaction within the Angolan current.
The southern part of the Angolan coast, up to approximately 13–14°S is influenced by the Benguela current, and the hydrographic regime therefore differs significantly from that further north along the coast particularly in the low temperature of the inshore surface waters. This leads to a division of the coast into two sub-areas: a northern area covering the region 6–14°S and a southern area from 14–17°S. In the following these regimes are described in a summer situation (data from Survey 1) and a winter situation (data from Survey 3).
The northern area
January-February 1985. The highest surface temperatures 28– 30°C were observed off Ponta da Moita Seca and decreased to about 26°C in Cabeca da Baleia. The lowest surface salinities were observed off the mouth of the Congo river due to the freshwater outflow from the river. These surface waters are transported by the surface current southwards past Ponta das Palmeirinhas. Further south, the salinities increase.
In all sections see Figure 1, there is a distinct thermocline between about 30 and 50 meters depth, which can be considered as characteristic for the season. From this, there is a gradual decrease in temperature with depth to about 8°C at 500 meters.
The oxygen content of the surface waters in the northern area is high (almost 5ml/l) and we find this high concentration down to dephts of 20m, but there is then a rapid decrease through the thermocline to about 2ml/l. The upper boundary of the layer of low oxygen content (less than 2ml/l) lies between 150– 200m in the north and slightly below 50m in the south, off Lobito.
August-September 1985. (See Figure 2). The surface temperature ranges from 19 to 23°C which is 5–8°C lower than that found in February, demonstrating the change from a summer to a winter situation. As the surface layer is not well heated, the thermocline, located in 30-50m depth, is now less developed. The distribution of temperature and oxygen towards the coast, as well as analysis of the TS relationships of the sections off Lobito and Ponta das Palmeirinhas demonstrate that active upwelling had been taking place. The salinity of the surface water show an increas as a consequence of the reduction of the freshwater outflow from the Congo river, and a shift of the currents. A section worked off Cabinda demonstrated a northward flow of the low-salinity surface water.
The Southern area
January-February 1985. The surface temperature off Baia dos Tigres (Figure 1) was 20–22°C. The lowest temperatures are observed in the inshore waters. The thermocline is well defined at about 15–30m depth. The salinity at the surface in all the stations is higher than 35°/oo. There is a rapid decrease of oxygen with depth, and the boundary layer of low-oxygen content ( <2ml/l) lies at about 30m depth.
August-September 1985. (See Figure 2) The surface temperatures range from 15 to 18°C which is some 5-7°C lower than found in February, demonstrating the change from a summer to a winter situation. The distribution of temperature and oxygen towards the coast and analyses of the TS relationships indicate that upwelling in the area was taking place.
The northern area
In winter, the surface temperature was 5–8° lower than summer. The upper layer was not well heated and the thermocline was less pronounced. There seemed to be a decrease in temperature from north to south throughout the year. Usually the temperature increased while moving away from the coast. The highest seasonal variations were found off Lobito and the lowest off the northern part of the coast. In summer, the surface waters off the Congo river and along almost all the northern coast, were extremely mixed with fresh water originating from the runoff of the river. The observation period during the rainy season seemed to correspond to a runoff maximum. Low saline surface water with salinities less than 27‰ were observed in some stations close to the river mouth, and a large proportion of the area southward along the coast had surface salinities below 34‰ indicating a southward transport of the uppermost coastal watermasses.
During winter, when the river runoff is at a minimum, the conditions were slightly different. The surface salinity increased by 5–7‰ and no low-salinity water was observed in the southern region. The section off Cabinda demonstrated a northward transport of surface water. Thus, the salinity distribution of the surface layers over the continental shelf showed large seasonal variations depending on the freshwater outflow from the Congo river, and the shift of the current.
In winter the minimum oxygen concentration ( <1ml/l) is found in the layer between 240–400m, and between 320–400m during summer. In the northernmost areas this oxygen minimum concentration was not found and all the water column down to 500m had oxygen values higher than 1ml/l.
The Southern area
The seasonal variations in surface temperature seemed to be between 5–8°C in the southern area of the Angolan coast. During August-September, the water along the coast was well mixed in the upper 50m, with temperatures of 15°. The thermocline structure is generally weak and the stratification of the upper layer only appears with summer warming. Intensive upwelling is observed during this period.
Analysis of the SST satellite charts for different seasons, show that a thermal front at approximately 16°S is a permanent feature of the southern area. The horizontal gradient of temperature is less pronounced during winter and a thermal front of 18–20°C is normally found. This front tends to be more pronounced during summer, when the transport of warm water by the Angolan current southward increases, and interact with the streams of the Benguela current moving northward.
During summer, the surface layer near the coast is normally rich in oxygen content, and values of 6–8ml/l are found. These values decrease rapidly through the thermocline to values below 2ml/l. The situation changes during winter, when the low-oxygen content subsurface water reaches the surface by upwelling.
Thus, the observations indicated seasonal variations in hydrographic conditions both in surface and subsurface layers on the continental shelf which may cause fluctuations in fish concentration patterns. In deeper waters, at depths greater than 150–200m, the hydrographical conditions were more stable.
