Just like natural lakes, big reservoirs occur close to main rivers. Figures 7 and 8 show the association between major reservoirs and key wetlands of sub-Saharan Africa.
The biogeography of sub-Saharan fauna and flora shows patterns that can be reconstructed from the sequence of events which commenced with drifting apart of the continents. Within the sub-Saharan region, there are some keystone species that evidently connect water catchments. From the Nile to West Africa and southward to Zaire, Sarotherodon galile-laeus, Lates niloticus, Hydrocynus forskalii, and Tilapia zillii are represented. These species occur in reservoirs as far apart as Kossou in Cote d'Ivoire, Ladgo in Cameroun, and Jebel Aulia in Sudan. The connection between Nile and the western reservoirs that excludes the south is suggested by occurrence of
Alestes dentex, Citharinus citharus, Oreochromis aureus and Oreochromi niloticus. The Chad basin's connection to West Africa and Zaire is indicated by presence of Hepseus odoe, Labeo parvus and Gnathonemus petersii.
Fish are also good indicators of environmental conditions because they cannot move overland from one aquatic system to another, hence they have to adapt to changes within a given system or face extinction. In addition, they impact on the distribution and abundance of other aquatic organisms through the food chain or competition.
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