The topography and hydrographic regimes of the West African continental shelf affect the distribution and constituents of benthic fauna. Longhurst (1958) considers the absence of reef-building corals in many tropical waters and explains the predominance of sand, silt and shells in bottom deposits.
In Sierra Leone, the continental shelf is isolated from the seasonal coastal upwelling areas of North-West Africa and central Gulf of Guinea by a complex of shoals (submerged sand banks) that comprises the Bissagos Archipelago, off Guinea-Bissau in the North and the shoals of Saint Ann to the South. The area of the continental shelf up to a depth of 200 m is about 27 500 km2.
The Sierra Leone coastline stretches for about 506 km. The continental shelf reaches a width of 125 km (Longhurst, 1983); it is wider in the northern sector and narrower in the southern part of the coast. In the North, the 50-m isobath occurs at about 100 km from the coastline, whereas in the southern part of the coast the same isobath is located at about 15 km from the shoreline. The present-day coastline of Sierra Leone exhibits a number of depositional and erosional features (that is, sandy beaches, and spits, cliffs, lagoons, estuaries, mangrove swamps, etc.). The general topography of the coastal sector of Sierra Leone is shown in Figure 1.
Figure 1 The West African coastline between Liberia and Guinea with predominantly muddy areas shaded and the predominantly sandy areas of the continental shelf unshaded. Based on Longhurst (1983)
Pleistocene sediments overlie the Rokel estuarine sector of Sierra Leone (Strasser-King, 1979). The sediments consist of a series of sands, clays and gravels with occasional thin beds of argillaceous limestone and calcareous grit and, also, seems of lignite. Tucker (1973) and Strasser-King (1979) recognize the following features:
Sand bars, parallel to the current direction and occurring mainly in lower reaches of estuaries.
Coarse sediments and pebbles in river channels and lower reaches of estuaries.
Intertidal flats containing sediments of grain size varying from clay to medium sand.
Mangrove swamps in upper intertidal and low supratidal zones; with black and dark grey muddy silts and sandy muds.
The sedimentary sequence of the coastline is a response to interaction of fluvial and marine processes. The ‘cyclothem’ (series of beds deposited) in Sierra Leone reflects a seaward advance of the shoreline (progradation) which begins with laminated clays in quiet waters outside the estuary, and this is then overlain by silty sand and silty clays at the mouth of the estuary (Strasser-King, 1979). There is a considerable onshore deposition of clay and silty clays by tidal action (aggradation) in the intertidal flats. The clays deposited in tidal flats are subsequently invaded by mangrove during periods of non-deposition. Then, the accumulated plant material gives rise to deposits of lignite. The distribution of sciaenid community, estuarine sciaenid sub-community, sparid community is associated with the type of bottom deposit and with the hydrographic conditions. A representation of present-day distribution of sediments around the Rokel estuary is given in Figure 2.
The high temperatures, high humidity and heavy rainfall affect the oceanographic conditions and the fisheries of Sierra Leone. The mean annual temperature is 26.6°C (= 80°F) with the range of 5.6°C (42°C). The main daily maximum and minimum of temperatures for the period 1951–80 for Freetown, Lungi and Bonthe are given in Appendix 1. The climate is characterized by two distinct seasons: the dry season (November-April) and the rainy season (May-October). The heaviest rainfall occurs in July, August and September. The average annual rainfall for Freetown for the period 1951–80 was 122.7 in (311.6 cm). The trends in rainfall pattern for Freetown, Lungi and Bonthe are given in Appendix 2.
The hydrographic regime of Sierra Leone waters is characterized by a relatively stable, shallow thermocline lying at ‘mid-shelf’ depth and affecting the distribution of fish. Seasonal changes are due to the following effects of the monsoonal wet season extending from May to October: high river discharges, reduced surface water salinities, lowered solar radiation and a dip in mixed layer temperatures (Longhurst, 1963 and 1983). The oceanographic factors affecting the hydrographic regime, the estuaries and continental shelf of Sierra Leone are given in Table 1.
There are several major rivers which influence the hydrographic conditions along the coast of Sierra Leone. There is marked fluctuation of total river flows along the coast as is indicated in Table 2. A brief account of the characteristics of some of the coastal rivers follows.
Scarcies Rivers: Great Scarcies (Kobuter River) is relatively narrow and its banks are lined with mangroves. The Little Scarcies (Kaba River) has a large mud bank at its entrance south-southeastward of Kortimaw Island. This river is tidal and during the rainy season it rises about 2.7 m.
Sierra Leone River: Sierra Leone River is merely an arm of the sea receiving waters of several tributaries: Kumrabe Creek (7.2 km north-eastward of Tagrin Point), and Bunce River (4.8 km south of Tagrin Point). About 21 km above Tagrin point, Sierra Leone River divides into Port Loko Creek and Rokel River.
Sherbro River: Sherbro River, which is bound on the East and North by the mainland, and on the South and West by Sherbro Island, could be referred to as a sound or strait which extends 48 km eastward. At its entrance Sherbro River is about 32 km wide between Shenge Point (in the North) and Cape Saint Ann (in the South). The estuary of the Sherbro River has strong tidal streams at Sherbro entrance with a rate of 4 kn at half tide during the dry season. During the rainy season it is risky for the canoe fishermen to cross the channel.
The Gallinas and Sulima Rivers: The strong surf around the ‘estuaries’ of the Gallinas and Sulima Rivers and the currents of these two rivers have formed a sand barrier 8–10 km long between the open sea and coast, thus forming a long narrow lagoon. Some shifting openings in this barrier allow water from the Gallinas and Sulima to reach the sea.
Figure 2 Diagramatic representation of present-day distribution of sediments around the Rokel/Sierra Leone estuary - based on Strasser-King (1979)
The Sierra Leone estuaries have salt wedges reaching 5–6 kn in the wet season. During the rainy season, the discharge of Rokel, Scarcies and Sulima Rivers ‘estuarize’ the continental shelf basin between Saint Ann Shoals and the Bissagos Islands (Longhurst, 1983).
The range of salinity values is correlated with amount of rainfall and effluents of rivers. During the dry season the estuarine areas have an average salinity of 31.3°/00, whereas in the wet season the average salinity value is 19.7°/00 (Longhurst, 1983). The muddy and sandy areas of the continental shelf have similar salinity ranges. During the dry season the salinity of the continental shelf reaches a value of 35°/00, whereas it varies from 31°/00 to 30°/00 in the wet season.
|Oceanographic factors||Estuary||Continental shelf|
|Mixed layer temperature||(°C)||27.0||28.6||26–27||28–29||26–27||28–29|
|Mixed layer salinity||(S%∞)||19.7||31.3||31–34||35||31–34||35|
|Surface radiation||(g cal cm-2 day-1)||113||437||113||437||113||437|
|Dissolved phosphate||(g at P1-1)||0.43||0.54||0.38||0.38||0.20||0.20|
|River system||Average total flows m3/S|
|Moa||159 077.91||173 364.26||109 494.30||178 050.10||77 955.60|
|Pampana||287.50||34 956.40||37 703.00||44 810.00||37 430.90|
|Seli (Bumbuna) (Sierra Leone River)||39 815.70||58 299.50||39 904.10||42 153.80||37 724.90|
|Sewa||77 012.10||68 800.10||156 486.10||84 054.70|
One of the important factors influencing the productivity of Sierra Leone waters and the extent of fishing activities is water movement. There is vertical oscillation of the thermocline and nutrients according to the oscillation of the equatorial current system. Additionally, there are effects of the equatorial undercurrent (Lomonosov Current) which is the most productive part of the equatorial area (Herbland and Le Bouteiller, 1982). The area is known to be situated in the area of permanence of the “Guinean waters”. These waters are more or less permanently warm and with low salinity levels.
The cold Canary Current (CC), flowing equatorward, also affects the Sierra Leone waters. As it approaches the equator, it turns westward as North Equatorial Current (NEC).
There is an eastward current, the Equatorial Counter Current (ECC) embedded between the North and South Equatorial Currents. The ECC and the cold CC contribute to the Guinea Current (GC) which runs along the West African coast from Senegal to the Bight of Biafra (Nigeria). The phenomenon of reversal of current is more marked in the coastal sector between Senegal and Liberia and this has biological consequences on fisheries of Sierra Leone.
Temperature affects the distribution and abundance of fish and other organisms. The waters of Sierra Leone are affected by influx of solar energy. Mean monthly temperatures for estuarine waters are 27.0°C and 28.6°C for the wet and dry season, respectively (Watts, 1958). As for the open continental shelf area, the mean monthly temperature range 26°–27°C and 28°–29°C for the wet and dry season, respectively (Williams, 1968). The thermocline and the continental shelf are delimited by the 25°C isothermal and 40-m contour (isobath), respectively. The surface layer is warm and its temperature is isothermal because of wind-induced mixing.
There is a marked difference between solar radiation in the dry and rainy (wet) season. Longhurst (1983) gives values of solar radiation at Freetown as 480 g cal cm-2 day -1 and 69 g cal cm-2 day -1 for the dry and wet season, respectively.