The topography and hydrographic regimes of the West African continental shelf affect the distribution and constituents of the benthic animal communities. Longhurst (1958) considered the absence of reef-building corals of many tropical waters and accounted for the predominance of sand, silt and shells in the bottom deposits. Previous oceanographic surveys also indicates that the prevailing hydrographic conditions are of biological significance to fisheries.
The surface waters of the Gulf of Guinea are basically warm (temperature greater than 24°C) and with low salinity of less than 35 °/00. The sector is not affected by the seasonal upwelling occurring between Cape Palmas and Cotonou, which brings cold, high-salinity and nutrient-rich waters to the surface between June and October.
Liberia has an area of 111 400 km2. The continental shelf area up to 200 m is about 18 400 km2 whereas the length of the coastline is about 590 km. The continental shelf is narrower in the coastal sector between Monrovia and Robertsport/Cape Mount as shown in Figures 1a, 1b and 1c, and based on Rancurel (1965) and Villegas and Garcia (1983). The continental shelf is wider in the southern sector between Monrovia and the border with Côte d'Ivoire, than the north as evidenced by the depth contours in Figures 1a, 1b and 1c. The present-day coastline of Liberia exhibits depositional and erosional features (sandy beaches, lagoons, estuaries, mangrove swamps, rocky bottoms, etc.) as shown in Figures 1a, 1b and 1c. Liberia has a number of coastal rivers (see, Figure 2) which influence the depositional and erosional features. The areas surrounding the river estuaries are good spawning and fishing grounds.
The surface wind system of the equatorial zone affects the oceanographic conditions. Three zone systems can be recognized in the intertropical region: a central doldrum zone with some clouds, precipitation, light wind and low atmospheric pressure corresponding to the heat equator; a zone of the northeast trade winds free of cloud and rain, also referred to as the Harmattan; and a zone of the southeast trade winds (with more clouds and rain) which gives rise to the southwest onshore monsoon north of the equator.
The hydrographic regime of Liberian waters is affected by a relatively stable, shallow thermocline lying at mid-shelf and affecting the distribution of fish. The average depth of the thermocline is between 20 and 35 m in most areas of West Africa, but in Liberia as well as in the Bight of Biafra, the thermocline can be as shallow as 12 to 14 m (Williams, 1969). As a consequence there is lack of renewal of water, low productivity and dominance of river influence as opposed to upwelling.
High temperatures, high humidity and heavy rainfall in the coastal sector of Guinea, Sierra Leone and Liberia affect the various fisheries. Trends in yearly maximum and minimum air temperatures are shown in Figures 3 and 4 respectively. The Liberian surface waters (part of the Guinea surface water) is uniformly warm (26–28°C) and of low salinity, mostly less than 32% S (Longhurst, 1962). The relatively low salinity is due to heavy rainfall and high river discharge (see Figure 2 and Appendixes 1a, 1b, 1c, 1d). In Liberia, highest river discharge occurs during the period June-November which is also the rainy season. It should also be noted that there are important demersal finfish and shrimp fisheries around the estuaries of these rivers.
One of the important factors influencing the productivity of Liberian waters and fishing activities is water movement. There is seasonal oscillation of the thermocline and nutrients according to the oscillation of the equatorial undercurrent. Liberia is situated in the area of permanence of the “Guiean waters” which are more or less permanently warm and with low salinity levels and also with low nutrient content.
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 this sector.
The basic features of the surface current system of the Central Atlantic Ocean is given in Figure 5.
