2. TOPOGRAPHY AND OCEANOGRAPHIC CONDITIONS

The topography and oceanographic conditions of the West African Continental Shelf affect the distribution and composition of fish communities. Longhurst (1958) considers the absence of reef/building corals of many tropical waters and accounts for the predominance of sand, silt and shells in bottom deposits. Oceanographic surveys indicate that the prevailing hydrographic conditions are of biological significance to fisheries. This study gives a summary of topographic and hydrographic factors which influence fish productivity on the continental shelf of Cameroon.

2.1 TOPOGRAPHY

The Cameroonian coastline stretches for about 360 km, extending from the border with Equatorial Guinea, south of the Campo River estuary to the Nigerian border north of Akwayafe River (Figure 1 ). The present-day coastline of Cameroon shows a number of depositional and erosional features ( that is, sandy beaches and spits; lagoons; estuaries; corals; mangrove swamps, etc.). The general topography of the coastal sector is shown in Figure 2. The continental shelf area (up to 200 m depth) is about 12 900 km².

2.1.1 Bottom Types and Littoral Morphology

The bottom deposits consist of sand, coastal hard deposits, coral, muddy sand, sandy mud and mud. There is useful information concerning bottom deposits of the continental shelf (Figure 2). The sedimentary sequence of the coastline is a result of interaction of fluvial and marine processes. The series of beds deposited reflect a seaward advance of the shoreline beginning with mud in quiet waters outside the estuary, overlaid with muddy sand and sandy mud. There are considerable deposits of mud and sandy mud caused by tidal action in the intertidal flats on which mangroves grow. Extensive mangrove swamps are located around the estuaries of Rivers Manyu (Cross River), Rio del Rey, Bimbia, Cameroon, Sanaga and Nyong. It is observed that the distribution of the estuarine sciaenid sub-community is associated with the type of bottom deposit and hydrographic conditions. This type of bottom deposit affects fishing performance, rocky bottoms and areas with corals being difficult to trawl.

2.2 OCEANOGRAPHIC CONDITIONS

The prevailing temperatures, humidity and rainfall influence the oceanographic conditions and the fisheries of Cameroon. The mean annual temperature is 25°C (= 79°F) at Douala. The climate is generally characterized by two distinct seasons: the dry season (November-March) and the rainy season (April-October) (Njock, 1985). The heaviest rainfall usually occurs during August-September. Trends in the rainfall pattern are shown in Table 1.

The hydrographic regime of Cameroonian waters is characterized by: the relatively stable shallow thermocline, steep temperature gradient and stable oceanographic conditions below the mixed layer throughout the year. The Eastern Tropical Zone (ETZ) of the Gulf of Guinea from Cotonou (Benin) to Cape Lopez (Gabon) is not affected by seasonal upwelling. However, even in this sector, the surface water temperature fluctuates between 25° and 30°C (Williams, 1968). The hydrographic conditions in the coastal area are greatly affected by the affluent rivers. The effect of the affluents on the oceanographic condition depends on the average annual discharge. It was greatest for River Sanaga (1896 m³/sec) at Edea in 1980. Generally, the annual discharge is greater for rivers north of Nyong River estuary. A summary of the annual discharges of some coastal rivers is given in Appendix 1.

2.2.1 Temperature, Salinity and Thermal Stratification

During the dry season (November-March), the southwesterly winds (southwest monsoon) are weak in the Gulf of Guinea. In this period, the equatorial under-current (Lomonosov current) attains maximum velocity and penetrates the Bight of Biafra. The oceanographic conditions are more stable during the dry season than during the rains.

The Gulf of Guinea is a region where the surface waters remain warm in all seasons. The surface temperatures are generally above 25°C and the salinity is always low, less than 35°/oo. These two factors characterize the surface waters of Cameroon and the entire Gulf of Guinea. The zone of warm and less saline waters is between 0 and 30 m. Below the warm layer lies the thermocline which extends from 30 m to 50 m. The thermocline is marked by low temperatures varying from 26°C (at the upper end of the thermocline) to 18°C (at the lower end). The salinity of thermocline is higher (from 30 to 35 °/oo). Thermal stratification is of significance in the distribution of fish on the continental shelf as illustrated in Figure 3. The warm surface and less saline waters are dominated by white fish consisting of croakers, bonga, sardine, Caranx, threadfins, catfish, soles, grunters, etc. The zone of cold waters below the thermocline is dominated by demersal red fish (croakers, snappers, red seabreams). The thermocline has a mixture of white and red fish.

2.2.2 Water Masses

One of the important factors influencing the productivity of Cameroonian waters is water movement. The thermocline oscillates vertically in accordance with the oscillation of the equatorial current system. There are also some effects of Lomonosov current (equatorial under-current) which is the most productive part of the equatorial area (Herbland and Le Bouteiller, 1982). The continental shelf of Cameroon is under the influence of the following water masses:

1. Guinea surface water; uniformly warm and of low salinity (ca 28°C, 32 °/ooS)

2. Benguela surface water, cooler and of higher salinity (23–27°C, 34 °/ooS)