The differing composition of the modern Nile River system fish faunas reflects the tectonic and climatic changes which occurred throughout the Cenozoic, particularly the late Cenozoic. The pre-Pliocene fish taxa were widespread (‘ nilotic’) and fairly uniform in composition, with some of the earliest records of several families recorded in the Nile Valley. However geomorphological and climatic changes in much of Plio-Pleistocene northern and eastern Africa severely changed the hydrology of these areas, and the fish faunas of the Nile’s southern basins experienced considerable immigration and disappearances. Exchange of fish between basins apparently occurred, particularly between the Nile and Lakes Albert and Turkana. The high humidity of the early Holocene resulted in expanded lakes and rivers, leading to exchanges between the Nile and the saharan and sahelian zones. The Nile and two of its large basins now have a fish fauna with a primarily ‘nilotic’ distribution, while three other basins whose faunas were isolated in the Holocene have largely endemic faunas.
Stewart, K. M. (2009). Fossil Fish from the Nile River and Its Southern Basins (pp. 677–704). https://doi.org/10.1007/978-1-4020-9726-3_32