3.1 Upwelling
3.2 Sediments of Land Origin
Annual surface temperature trends in a number of coastal stations of the region (Abidjan, Tema, Lomé, Cotonou, Lagos, Pointe-Noire) are shown in Figure 1.
Seasonal upwelling occurs over the continental shelf of Ivory Coast, Ghana, Togo and Benin (July-September) and off southern Gabon down to Angola (June-September). The hot, less salty surface layer is present year-round in the sector in between (Nigeria, Cameroon, Equatorial Guinea and northern Gabon). The existence of seasonal upwelling is the most important single factor affecting the distribution and abundance of stocks.
Trawler yields for the sparid community are higher during the cool season (such as for Dentex angolensis). It is not presently possible to determine whether the upwelling actually induces a resurgence of the species, which is likely, or whether this is merely a question of availability of species which breed during that period. Certain observations indicate that the croaker community species which live from 0-50 m take refuge along the coast during the cool season. Fontana (personnel communication) indicates the presence in Congo along the shore between two different water levels of large size Pseudotolithus senegalensis chasing the juvenile sardinella. Troadec (1971) has demonstrated the coastal concentrations of Pseudotolithus senegalensis during this season. As a possible explanation of the drop in Ivory Coast yields of Pseudotolithus senegalensis during the cool season, he suggests the possible rise of specimens to a position between two water levels due to a drop in the oxygen content of water close to the bottom during the upwelling.
None of the authors report migrations of any great importance. Tagging operations carried out by Troadec in Ivory Coast (1971) showed that the movements of Pseudotolithus senegalensis were not coast-wise, or at least not particularly so. Beck's (1974) tagging operations in Togo in August 1973, however, which covered over 4 000 trawl fish (including about 2 600 Balistes, 200 Lethrinus and 800 Brachydeuterus), showed an overall west/southwest movement of the fish recovered (Figure 2). This might be explained by the return to Ghana at the close of the cool season of fish carried eastward with the upwelling which has its centre off Ghana.
Known availability fluctuations for numerous species are usually attributed to behavioural variations, which are often linked to the reproductive cycle, which in turn, follows that of the hydrological seasons.
The authors have paid little attention to the influence of floods and riverbourne sediment, except for Le Guen's (1970) study of the dynamics of Pseudotolithus elongatus at the mouth of the Congo. And yet it must be an important factor for estuary zones, particularly in the Nigeria-Cameroon sector.
