Ichthyogeography of the Guinea–Congo rain forest, West Africa

Abstract

SynopsisIchthyogeography is the section of biogeography which seeks to interpret fish biodiversity in terms of present and past distributions and abundance. It can also have practical applications in fisheries management and conservation. There are thought to be more than one thousand species of fish in the 40 or so major rivers of the Guinea–Congo rain forest region, with correspondingly high levels of endemicity. In the Congo or Zaire river basin alone, the proportion of endemic fish species may exceed 80% (from a total of >690), but many more taxonomic and distributional data remain to be gathered.Unlike terrestrial vertebrates, the diverse freshwater fishes of Guinea–Congo evidently have distributions which are closely confined by hydrography. Hence, fish distributions may be of particular help in corroborating or refuting postulated geological events, patterns and processes and in explaining associated aspects of rainforest evolution. From the 19th century until now, the ichthyogeography of Guinea–Congo has been diagnosed largely in terms of presumed post-Miocene geological sequences of fish taxa and their past and recent dispersal in relation to particular hydrological conditions: mainly riverine volume discharge, salinity and temperature. From this, the fish fauna is, by convention, divided into ‘provinces’ established on endemism, palaeogeography and supposed physical or ecological barriers to dispersal. However, in this paper it is argued that the traditional ichthyogeographical accounts which highlight endemism and dispersal are generally flawed. It is argued here that historical patterns of fish distribution can only be fully understood if phyletic (cladistic) data are taken into consideration. While Upper and Lower Guinea and the Zaïre basin may be defined in part on the basis of endemism there is a lack of taxonomic and distributional evidence to show that Guinea–Congo is itself a cohesive ichthyogeographical unit.There is clearly a need for comparisons with fish distributions outside the rain forest zone of Guinea–Congo. African inter-provincial, trans-continental and inter-continental comparisons reveal distribution patterns which may relate more to pre-Miocene rather than post-Miocene geology or present-day rain forest ecology. Continental drift, notably between Africa and South America, probably led to the separation 85 million years ago of previously united fish populations. This may account for recent higher-level phyletic correspondences between the separate rain forest fish faunas of Africa and the neotropics.Last, the so-called ‘marine intrusive’ fishes – which are normally excluded from zoogeographical consideration – merit a careful re-evaluation. While they may be regarded as an inconvenience in developing scientific hypotheses, such intrusives can comprise a remarkable 30% or more of the riverine fish fauna in Guinea–Congo. It seems that the widely accepted ecological divisions between marine and ‘primary freshwater’ fishes are not as clear-cut as has been supposed. In addition, zoogeographically critical marine, trans-Atlantic, phyletic relationships apparently exist. These are probably best interpreted by using area cladograms in the context of ocean basin development, rather than by referring solely to marine fish dispersal and the traditional continental and provincial ichthyogeography of Guinea–Congo.