On the size of african riverine fish faunas

Abstract

SYNOPSIS. An apparent anomaly exists in the size of riverine fish faunas of the Nile and Zaire; the Nile while considerably longer, appears depauperate when compared with the Zaire. This paradox is explained when discharge, not length, is used as a measure of river size. Indeed, the number of freshwater fish species in African rivers is more closely related to discharge than to length or catchment area. Discharge is directly proportional to terrestrial productivity of a river basin, which in turn affects total biomass of fish and number of species. Changes in the size of rivers during the geologic past affected their capacity to sustain diverse fish faunas. Rivers flowing through especially arid lands during the late- Pleistocene were reduced in discharge, and concomitantly, fish faunas. Immigration of fish from refuge rivers during the Holocene partially restored these diminished faunas. We propose that fish are more mobile than they seem, and that the distinctiveness of riverine fish faunas may be maintained by competitive pressure from established residents, rather than by limited dispersal abilities of fish. Theories of the distribution of fish in Africa are considered, and we suggest that discharge as affected by climatic stability is largely responsible for the size of African riverine fish faunas. © 1982 by the American Society of Zoologists.