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The Rift Valley Lakes are world-renowned for the diversity of cichlid fish species that have evolved in their waters. About 800 species of cichlids live in the lakes, with many more species yet to be discovered. The Great Rift Valley was created by the moving of tectonic plates beneath the crust of Africa about 40 million years ago. Since then, land has shifted, volcanoes have erupted, and channels to the sea have closed. All these geological events helped create numerous lakes along the Great Rift Valley. Some, such as Lakes Malawi and Tanganyika, have formed in the rifts, and the vast Lake Victoria is located in a shallow depression between the two rifts.
At more than 5,200 feet (1,600 m) in depth, Tanganyika is the second-deepest freshwater lake in the world (after Lake Baikal in eastern Asia). Victoria is the second-largest in surface area, after Lake Superior in North America. Lake Malawi is also very deep (about 2,600 feet or 800 m) and is the third largest lake in Africa. In the eastern rift, volcanic soils -- along with high rates of evaporation -- allow for the creation of a group of soda lakes. Algal productivity is very high in these lakes, and millions of lesser flamingoes congregate to feed here. Lake Tana, located in the Ethiopian highlands, is the source of the mighty Blue Nile River. The Rift Valley Lakes (particularly Malawi, Tanganyika, and Victoria) are known for the approximately 800 species of tropical cichlids that have evolved in their waters. Large numbers of cichlid species live adjacent to one another along the edges of the lakes. For example, one researcher caught 7,000 fish representing 38 families in one 4,300 square foot (400 sq. m) sampling area in Lake Tanganyika. The cichlids have evolved specializations to take advantage of their environment and to limit competition for resources. For example, one unusual group of fish, the scale-eaters, feed exclusively on the scales of other fish. Most cichlids spend a lot of time caring for their young, but the young of mouth-brooders swim into their parent’s mouth for protection in the face of imminent danger. It is not only the fish that are unique to these lakes. Copepods, ostracods, shrimps, crabs, and mollusks are also represented by high numbers of endemic species. Lake Tana supports a group of cyprinid fish that are all descended from a common ancestor (called a "species flock"). This cyprinid species flock is one of only two known in the world, and the only one that is still intact.
The introduction of exotic fishes (tilapia and Nile perch) has decimated the native cichlids of Lake Victoria, with nearly 200 species of cichlids estimated to have been lost. Run-off from urban areas, pollution from industrial activities, overfishing, and deforestation also threaten the health of the lakes. Harvesting of species for the aquarium trade is a potential threat to fish populations, although the impact is unknown.
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