Freshwater Ecoregions of the World
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525: Ethiopian Highlands

Author

Abebe Getahun, Department of Biology, Addis Ababa University, Addis Ababa, Ethiopia

Reviewers

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Biodiversity
Species Richness
# of Endemic Species
Threats
Description

Major Habitat Type

Montane freshwaters

Countries

Eritrea, Ethiopia, Somalia

Boundaries

This high-altitude ecoregion is defined by the two blocks of highlands in Ethiopia, separated by the rift valley and distinguished by a freshwater fauna adapted to the ecoregion’s swift-flowing rivers. The highlands extend from Eritrea in the north to Kenya in the south. With a long history of isolation, the Ethiopian Highlands are known to harbor a highly endemic biota.

Drainages flowing into

Rivers of the western highlands generally flow towards Sudan, whereas those of the eastern highlands tend to flow towards the Indian Ocean. 

Main rivers or other water bodies

In the northwestern part of the highlands, the deep, steep-sided valleys of the major rivers separate blocks of mountains, and the upper courses of the big rivers such as the Tekezze and Abay (Blue Nile) plunge through deep gorges. This part of the Ethiopian highlands is also the source of the headwaters for the Blue Nile. The Blue Nile watershed is the largest basin in Ethiopia. Rivers of this basin drain the great central plateau and the Blue Nile descends about 1,450 m in a distance of only 350 km from its source to Khartoum. The Blue Nile flows to the north and forms a large floodplain in Egypt. The rich sediments carried by the Blue Nile are deposited downstream, greatly increasing the soil fertility of floodplains along the course of the Nile. Historically, nearly all of the sediments in the Nile Delta came from Ethiopia via the Blue Nile and Atbara Rivers (Beadle 1981).

The westward flowing rivers (the Tekezze, Angereb, Atbara, Abay, Baro and Akobo) form part of the Nile drainage basin. Three major highland lakes, Lakes Hayq, Ardebo and Ashengie, lie near the edge of the western escarpment of the rift valley at altitudes between 2,000 and 2,500 m. Lake Hayq, located in northern Ethiopia’s Wollo region, has an area of 5 km2 and a maximum depth of 23 m, and is noteworthy for its extremely clear water (Kebede et al. 1992). Lake Ardebo is located about 5-km southeast of Lake Hayq. This lake is smaller in size than Lake Hayq and flows into Hayq via the Anchercah River (Kebede et al. 1992). Lake Ashengie is located north of Lake Hayq in the Tigray region, and sits at an altitude of 2,460 m. The lake covers an area of 25 km2 with a maximum depth of 20 m and a mean depth of 14 m (Wood & Talling 1988). The lake is fed by a number of small streams from the surrounding areas and there is no drainage out of the lake (Ethiopian Wildlife and Natural History Society 1996).

 

Topography

According to Westphall (1975), uplift of the Ethiopian highlands together with Arabia occurred on an extensive scale after the regression of the Red Sea towards the southeast in the late Mesozoic to early Tertiary. The Great Rift Valley bisects the highlands into the eastern and western massifs, which are surrounded by escarpments. This ecoregion contains about 70% of Africa’s highlands. The highest peak, Ras Dejen (Dashan) at 4,620 meters, is in northern Ethiopia’s Gondar region. The southeastern portion of the Ethiopian Highlands includes the Sidamo, Bale, Arsi and Harerge Mountains. The highlands in this region are made up of volcanic rocks, and deep river cuts expose crystalline rocks (Ethiopian Mapping 1988)

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Climate

The Ethiopian highlands receive about 950 mm or more of rainfall due to a double passage of the intertropical convergence zone. The high mountains east of Lake Tana and the southwestern mountains stand out as places of higher rainfall. They receive 2,000 mm or more of rainfall each year (Westphal 1975). A rainfall regime that peaks in March-May and June-August is typical for the Ethiopian Highlands.

Fish Fauna

The fishes of the high mountain torrential streams are largely cyprinids (Harrison 1995; Getahun & Stiassny 1998) adapted to the swiftly flowing floodwaters that occur seasonally. Two genera of fishes (Barbus and Garra) dominate the fish fauna of these streams. Clarias gariepinus, Varicorhinus beso and Labeo spp. are also found in high numbers. The Baro-Akobo basin is apparently particularly rich in fish diversity (Golubtsov et al. 1995). The fauna is Nilo-Sudanic and is dominated by Alestes, Bagrus, Barilius, Citharinus, Hydrocynus, Hyperopisus, Labeo, Malapterurus, and Mormyrus genera. 

Description of endemic fishes

Endemism appears to be high among fish, but the fish fauna is not well known. Endemic fishes of the genus Garra (e.g. G. dembecha, G. duobarbis and G. ignestii) have recently been described (Getahun 2000). Lake Hayq is believed to have no indigenous fish species, although the presence of Clarias gariepinus has been reported (Beckingham and Huntingford in Kebede et al. 1992). Nemacheilus abyssinicus is an endemic species found in the Baro-Akobo drainage basin, the Omo-Gibe drainage basin, and Lake Tana.

Other noteworthy fishes


Other noteworthy aquatic biotic elements

The invertebrate fauna is less well known than the fishes and it is difficult to estimate endemism among the aquatic invertebrates. Harrison and Hynes (Harrison & Hynes 1988)(Harrison and Hynes 198 indicated that Dugesia spp., Baetis harrisoni, Pseudocloeon sp., Centroptilum sudafricanus, Afronorus peringueyi, Neoperla spp., Hydropsyche sp., Simulium spp., nymphs of Aeschna, and chironomid larvae dominate the benthic communities in the stony runs and torrents of the Ethiopian highlands. Compared to other highland ecoregions, the Ethiopian Highlands support a rich aquatic mollusc fauna with over 20 species described.

Justification for delineation

Despite the fact that the Ethiopian highlands are presently separated from both the East African and the South Arabian mountains, the riverine fauna resembles that of east and southern Africa (Tudorancea et al. 1999), along with some elements of the Arabian Peninsula. Cyprinids are the dominant fish in the rivers of this ecoregion. For example, it is known that some small Barbus species (e.g., Barbus paludinosus, B. trimaculatus and B. radiatus) have widespread distributions extending from South Africa to East Africa and into the highlands (Skelton et al. 1991). There are also fish groups (e.g. Garra) common to the Ethiopian highlands and the Arabian Peninsula. These fish groups are estimated to have originated in the Lower Tertiary or late Cretaceous, before the separation of India and the Arabian Peninsula from continental Africa (Briggs 1987); whereas the Red Sea is believed to have separated the African continent from the Arabian Peninsula in the early Tertiary, between the Eocene and Oligocene epochs (Getahun 1998).

Level of taxonomic exploration

Fair. Historically, few scientific studies have been made on the fauna of the river systems of Ethiopia; however, two recent studies have elevated the level of data available for the fish of this ecoregion (Getahun & Stiassny 1998; Golubstov et al. 2002). River systems in the Tekezze-Angereb basin have not been studied at all due to security problems in the past. Preliminary reports indicate that the large river bodies of this basin support a rich fish fauna and research is needed to confirm this. Some information on the benthic fauna of Ethiopian mountain streams and rivers is available in Harrison and Hynes (1988).

References

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