The status and distribution of freshwater fishes in the Lake Victoria basin

In terms of hydrology, the Lake Victoria Basin is part of the Nile system, most of which is within the Nilo-Sudan ichthyofaunal province. However, the ichthyofauna of the Lake Victoria Basin differs substantially from most of the downstream Nile basin. It has traditionally been assigned to the East Coast ichthyofaunal province together with the faunas of the systems of Lakes Kyoga, Edward and Kivu, and the coastal rivers of Eastern Africa (Greenwood, 1983; Roberts, 1975; Snoeks et al., 1997). This viewpoint has been challenged and inclusion of the Lake Victoria ichthyofauna into the Nilo-Sudan province has been suggested (Lévêque, 1997;
Witte et al., 2009). More recent biogeographic and genetic studies have revealed that Lake Victoria and the smaller lakes in the region harbour a mosaic ichthyofauna composed predominantly of Nilotic and Congolese elements with quite balanced contributions from the two (Meier et al., 2017; Seehausen, 2002). Most of the non-endemic species of Lake
Victoria are shared either with the Nile or with the Malagarasi (Congo) system, and very few are shared with coastal rivers. Most of the endemic species have their closest relatives either in the Nile or Congo systems. The large endemic cichlid species radiation evolved from a population of hybrid origins between cichlids from the Nile and the Congo (Meier et al., 2017). The endemic Nothobranchius killifish too derive from two lineages that have their nearest relatives in the Sahel and the Congo respectively (Dorn et al., 2014).
Cichlids form the major component of the fish fauna of the Lake Victoria Basin. Other impor tant families are the Cyprinidae, Mormyridae, Clariidae and Poeciliidae. Prior to major anthropogenic disturbances, including the introduction of the Nile Perch (Lates niloticus) and severe habitat deterioration, the system harboured between 600 and 1,000 species of cichl ids, al l but four of them endemic haplochromines (Kaufman et al., 1997; Seehausen, 2002, 2015; Witte et al., 2007). Since the 1980s, an estimated 200
haplochromine species have, however, likely gone extinct and other species have become severely threatened (Seehausen et al., 1997b; Witte et al., 1992, 2007). Unfortunately, several hundred of the haplochromine species remain undescribed, and this includes both extant species and those likely to now be extinct. The numbers of non-cichlids reported in the past appear to have been underestimations, with 69 non-cichlid species listed from the area (excluding the Lake Kyoga region) by this project compared to Snoeks (2000) reporting 45 for Lake Victoria, and Witte et al. (2009) reporting 46 for Lakes Victoria and Kyoga together.