Fish faunas of the Chamo-Abaya and Chew Bahir basins in southern portion of the Ethiopian Rift Valley: origin and prospects for survival

Fish faunas of the Chamo-Abaya (21 species) and Chew Bahir (12-14 species) basins are more diverse than in other basins of the Ethiopian Rift Valley. Their increased diversity is determined by past connections to the Omo-Turkana basin, while the latter was connected to the White Nile system seven thousand years BP. The vast majority of fish species inhabiting the Chamo-Abaya and Chew Bahir basins are of Nilo-Sudan origin; however, some elements of the East-African ichthyofauna (Barbus kerstenii and Labeo cylindricus) are present. In contrast to other basins in the Ethiopian Rift Valley, the number of endemic fish species in the Chamo-Abaya and Chew Bahir basins is low. As in other Rift Valley basins, the fish diversity depends on the balance between extinction, driven mostly by episodic desiccation, and dispersal from adjacent water systems. The higher diversity of the Chamo-Abaya ichthyofauna, compared to that of the Chew Bahir basin, is apparently caused by the larger volume of water in the former basin and, consequently, by more stable hydrological conditions. However, the dispersal routes of most fish species now inhabiting the Chamo-Abaya basin were via the Chew Bahir basin. During the last twenty years intensive development of fisheries in Chamo and Abaya lakes has driven local populations of commercial fishes (e. g. Mormyrus caschive, Labeo horie and L. niloticus) to the verge of extinction. Currently, the main threat to the Chew Bahir ichthyofauna is the damming of tributaries for irrigation.