Ichthyofaunal diversity of the Omo-Turkana basin , East Africa , with specific reference to fish diversity within the limits of Ethiopian waters

The freshwaters of the East African nation of Ethiopia are divided into nine main drainage basins. One of these, the Omo-Turkana basin, spans a large part of southwestern Ethiopian highlands and northern Kenya, and consists of the Omo-Gibe (or simply, Omo) River and a northern portion of Lake Turkana. Despite some development activities, including proposed dam construction with potential impacts on ichthyofaunal diversity, the Ethiopian part of the basin generally lacks comprehensive study or full scientific documentation. During the current surveys 31 species were identified from the lower Omo River and Ethiopian part of Lake Turkana, with some new records for the basin. The Omo River system was found to be richer in species while Lake Turkana has a higher abundance. Ichthyofaunal diversity within Ethiopian waters is specifically addressed, and an annotated checklist for native species of the basin is provided.


INTRODUCTION
Ethiopia's inland waters (lakes, rivers and streams) are in nine main drainage basins situated within major physiogeographic units.One of these, the Omo-Turkana basin, spans a large part of the southwestern Ethiopian highlands and northern Kenya (Feibel 2011;Velpuri et al. 2012) (Figure 1).

Historical overview
The basin was first explored for its ichthyofaunal diversity in the second half of the 19 th century during Dr. Donald Smith's Lake Turkana expedition (1894)(1895).Eight species, with one new description, were identified from the Smith Lake Turkana collection (Günther 1896).In the early 20 th century, five species were identified from the Omo River collections of R. Neumann and C. von Erlanger (Boulenger 1903a).A few years later, 14 species from the middle to upper Omo River, and five species from northeastern Lake Turkana were identified from the collections of W. N. McMillan and Zaphiro (Boulenger 1906).At the same time, Pellegrin (1905) identified four species from Lake Turkana in the collections of Maurice de Rothschild from [1904][1905].The first organized expedition to the Omo-Turkana basin was undertaken by Cambridge University in 1930-31 (Worthington 1931(Worthington , 1932;;Worthington & Richardo 1936) as part of its East African Great Lakes expedition.This was followed by a French multidisciplinary scientific expedition to the lower Omo River valley (La mission scientifique de l 'Omo, 1932'Omo, -1933)).Pellegrin (1935)  13 genera and seven families from that expedition's fish collections, of which two species, Aplocheilichthys jeanneli and Barbus arambourgi (now Enteromius arambourgi), and one sub species, Brycinus nurse nana, were described.Of these 15 species, 12 species were identified from the Omo River (Delta) while three species were from Lake Turkana.The East African Freshwater Fisheries Research Organization (EAFFRO), based in Uganda, also visited Lake Turkana and identified ten species (Hamblyn 1962).A review by Tedla (1973) listed 23 species for the Omo-Turkana system, the majority of which were from the Omo River system.The British Overseas Development Administration's Lake Turkana Project (1972Project ( -1975) ) laid the foundation for much contemporary knowledge of Lake Turkana's ichthyofaunal diversity (Hopson 1982).That comprehensive study also generated information on the lake's limnology and fisheries.The project report, combining original data and review of past work, listed 48 species for the lake with the descriptions of three new ones (Hopson & Hopson 1982).A subsequent Norwegian project (1985)(1986)(1987)(1988), focused mainly on the lake's fisheries (Kolding 1989) and limnology (Källqvist et al. 1988).The Joint Ethio-Russian Biological Expedition (JERBE) listed 53 species in the Omo River basin and 72 species in the entire Omo-Turkana system (Baron et al. 1997).A combination of these preceding studies and other reviews (Roberts 1975;Kolding 1989;Lévêque et al. 1991) results in 74 valid species for the entire Omo-Turkana system.Golubtsov & Darkov (2008) later increased the number of species for the Omo-Turkana system to 76-79, with some pending taxonomic and identification uncertainities of a few species.

Diversity measurements
Global biological diversity is measured at various scales.Alpha diversity measures species richness at a single locality or in a community, whereas beta diversity is a measure of the amount of turnover or heterogeneity in species composition along environmental gradients between localities or communities (Whittaker 1972).Alpha diversity can be used to reflect how finely species are dividing ecological resources, whereas beta diversity can reflect the extent of habitat selection or specialization (Jost et al. 2011).Measures of taxonomic composition and variation as one or more of the components of global diversity have important applications in setting conservation priorities or evaluating regional conservation plans.In ecology, they can also be used to study the homogenizing or diversifying effects of human activities, natural disturbance, or spatial variability of environmental conditions (Olden 2006;Vellend et al. 2007).Studies pertaining to these diversity parameters do not exist for the Omo-Turkana basin except for the scant reports on species occurrences.Moreover, many previous assessments of fish diversity in the basin largely dealt with the main lake in Kenya.Despite some development activities and proposed dams in the Omo basin (Avery 2010) and their potential impacts on biodiversity, the Ethiopian part of the basin generally lacks a comprehensive study or scientific documentation of its ichthyofauna.Such a lack of scientific documentation renders recommendations and/or implementation of appropriate management and conservation measures, as well as future appraisal of impacts, highly problematical.Therefore, the purposes of this study are to: • Assess alpha and beta diversity indices as well as the species richness of the ichthyofauna of the lower Omo River and Ethiopian part of Lake Turkana, • Develop a comprehensive ichthyofaunal checklist for the region and an artificial identification key in an effort to document the basin's freshwater fish diversity.
This study forms part of a broader research project on the ichthyofaunal community structure, feeding ecology of dominant fish species, and fisheries socioeconomics of these water bodies.As such, it is hoped that this study will contribute to a comprehensive understanding of the basin's fish fauna as a baseline for its conservation and sustainable fishery.

Study sites
Fish specimens were obtained from the Ethiopian part of the Omo-Turkana basin (Figure 2).The Omo basin is part of the large Turkana system with a catchment area of 131,000-145,500 km 2 covering southwestern Ethiopia and northern Kenya (Feibel 2011;Velpuri et al. 2012).The Turkana system essentially consists of the Omo-Gibe (or simply, Omo) River and Lake Turkana.However, the basin also encompasses the smaller Turkwell and Kerio River basins on the southwestern shore of the lake in Kenya, and intermittent streams such as the Kalakol and Kataboi on the western Kenyan shore, and the Kibbish on the northwestern shore in Ethiopia (Ferguson & Harbott 1982).In Ethiopia, the basin consists of the Omo River extending north upstream from the shore of the lake to the southwestern Ethiopian highlands, and a portion of Lake Turkana.The Ethiopian part of Lake Turkana is estimated at 1.3% (98 km 2 ) of the entire lake area (≈ 7560 km 2 ) (FAO 2003).The Omo River basin makes up 52-58% of the catchment area of the entire Turkana system and supplies 80-90% of the total inflow to the lake (Ferguson & Harbott 1982;Avery, 2010;UNEP 2010).It receives an annual precipitation of up to 2,000 mm (UNEP 2010) although the mean annual rainfall could be as low as 350 mm in the lower Omo River Valley near the lake (EEPCO 2009).The principal stream, the Omo River, has its sources in the southwestern Ethiopian highlands at an elevation of about 2,200 m above sea level (a.s.l.).Its major tributaries, in upstream to downstream order, include Gibe, Gilgel-Gibe, Gojeb, Amara, Alanga, Denchiya, Mui, Zigina-Shoshuma, Mantsa and Usno (with sub-tributaries Mago and Neri).It is 760 km long, traverses Oromia and Southern Nations Nationalities and Peoples (SNNP) regions of Ethiopia, and ultimately flows into Lake Turkana at 365 at a subsurface level at each sampling site.Hook and line, traps, and cast nets supplemented the gillnet sampling especially in inshore areas.Some additional specimens were provided by local artisanal fishers.Fish specimens were fixed in 10% formalin.Identification was made to species level following relevant literature (e.g., Boulenger 1909Boulenger , 1911Boulenger , 1915Boulenger , 1916;;Tedla 1973;Hopson & Hopson 1982;Seegers et al. 2003;Habteselassie 2012).Voucher specimens were deposited in the Zoological Natural History Museum (ZNHM) of Addis Ababa University, Ethiopia (Table 2).Unless noted otherwise, the classification of species follows Nelson (2006).

Data analysis
Shannon diversity (H′) and evenness (J′) indices (Maurer & McGill 2011) were computed as measures of the alpha diversity for the lower Omo River and the Ethiopian part of Lake Turkana using the following formula: Where, Pi = Proportion of abundance of species i S = Species richness Rank order abundance plots were generated as the natural logarithm of abundance values versus the abundance rank order for further assessment of evenness.Differences in the parameters of alpha diversity measures between the river and the lake samples were compared using a randomization (permutation) test (Magurran 1988;Solow 1993).Whittaker's beta diversity index (β w ) (Whittaker 1972), a robust index (Wilson & Shmida 1984), was used to assess the rate of species turnover between the lower Omo River and Lake Turkana samples, and is calculated as: Where, S = total number of species of the habitats α = average species richness per habitat An individual-based rarefaction analysis was performed to standardize diversity comparison between the two systems (Magurran 2004;Gotelli & Colwell 2011).All statistical analyses were carried out in PAST version 3.08 (Hammer et al. 2001).Ultimately, data obtained during the m a.s.l.(CSA 2009).Biogeographically, the Omo-Turkana basin belongs to the Nilo-Sudanic freshwater ichthyofaunal province (Roberts 1975;Paugy 2010).
Four sampling sites were established at each of two major sampling localities: the lower reaches of the Omo River 50-60 km upstream of its delta, and the Ethiopian part of Lake Turkana (Figure 2).Site selection was based on accessibility and sampling safety.The Omo River sites consisted predominantly of pools with sand or mud substrates whereas the Lake Turkana sites had a largely muddy substrate.Geographic coordinates and related features of the sampling sites are summarized in Table 1.

Data collection
Permissions to sample in the study areas were obtained from local government administrations and fish samples were collected during two dry and two wet seasons between January 2013 and September 2014 (Table 1).Dry season sampling was conducted during periods of the year when there was no rainfall whereas wet season sampling was undertaken immediately following decline of a prolonged rainfall.A combination of monofilament and multifilament gillnets were used for sampling.The gillnets had stretched mesh sizes of 8-44 cm, a panel length of 100 m and a width of 1 m per mesh size.Four sets of gillnets, two parallel and two perpendicular to the shore, were set   present study was combined with a thorough review of past studies to develop an annotated checklist of valid native fish species for the entire Omo-Turkana Basin, and an artificial identification key to the species within the Ethiopian part of the basin is provided.

Species richness
During the present sampling, 4,386 fish specimens (lower Omo River, n = 823; Lake Turkana, n = 3,563) were collected.Altogether, 31 fish species (26 from river sites and 24 from the lake) in 22 genera, 17 families and seven orders were identified from the collections of both localities (Figures 3  and 4; Table 2).
Three species, Mormyrus caschive, Labeo coubie, and Auchenoglanis biscutatus, are new records for Lake Turkana, and two species, Hydrocynus vittatus and Schilbe uranoscopus, are new records for the Omo River.One species, Synodontis filamentosus, is a new record for both the Omo River and Lake Turkana.

Material examined: Table 2
Nine dorsal finlets; 57 scales in the lateral line; terminal mouth with small unicuspid teeth on jaws; body grayish; sampled in the lower Omo River.

Material examined: Table 2
Terminal mouth with unicuspid teeth; body scales large and strong; dorsal fin long, with more than 30 soft rays, positioned on the posterior body close to caudal; caudal fin round; sampled in the Ethiopian part of Lake Turkana.Linnaeus, 1758: Figure 3d Mormyrus caschive Linnaeus (1758) Material examined: Table 2 Mouth terminal; proboscis-like snout slightly curved downward; dorsal fin rays fewer than 75; sampled in the lower Omo River.(Linnaeus, 1758): Figure 3f Mormyrus anguilloides Linnaeus (1758) Material examined: Table 2 Mouth terminal; body laterally compressed; snout moderately long; thin white sheath covering snout including eye; head depressed; body not deep, anterior body with dark reddish wash; dorsal fin about as long as anal fin, both located near caudal; dark violet in color; sampled in the lower Omo River.(Lacepède, 1803): Figure 3g Mormyrus bebe Lacepède ( 1803 Material examined: Table 2 Mouth terminal; snout round and short; thin white sheath covering snout including eyes; body laterally compressed; dorsal fin much shorter than anal fin; dorsal fin located posteriorly, near caudal; dark brown in color; sampled in the lower Omo River.Material examined: Table 2 Snout relatively short; dorsal fin originating well anterior of anal fin origin, relatively long with up to 36 rays; both dorsal and anal fins located near caudal; dorsal profile tends to be concave; sampled in the lower Omo River.

Material examined: Table 2
Teeth unicuspid, in a single row on each jaw, visible when mouth is closed; adipose eyelid present; two rows of scales between the scaly process at pelvic fins and lateral line; dorsal fin tip, adipose fin and caudal fin fork edges black; body silvery with light black stripes; sampled in the lower Omo River and the Ethiopian part of Lake Turkana; a new record for the former.(Cuvier, 1819): Figure 3j Hydrocyon forskahlii Cuvier ( 1819 Material examined: Table 2 Teeth unicuspid, in a single row on each jaw, visible when mouth is closed; adipose eyelid present; two rows of scales between the scaly process at pelvic fins and lateral line; tip of dorsal fin, inner edges of caudal fin, and adipose fin uniformly grayish; upper dorsal part silvery; lower caudal lobe orange-red; sampled in the lower Omo River and the Ethiopian part of Lake Turkana.

Material examined: Table 2
Dorsal fin equidistant between pelvic and anal fins or nearer to the latter; anal fin moderately long, with 25-30 branched rays; edges of caudal fin finely black; adipose eyelid present; more than 38 scales in the lateral line; body slightly laterally compressed; dorsal surface dark silver; lower caudal lobe orange red; pelvic and anal fins with orange red wash; sampled in the lower Omo River and the Ethiopian part of Lake Turkana.(Rüppell, 1832): Figure 3l Myletes nurse Rüppell (1832)

Material examined: Table 2
Dorsal fin above or only slightly anterior to pelvic fin; head slightly flattened; teeth in outer row of premaxilla 8; sides silvery; unpaired fins bright red; paired fins colorless to light orange; dorsal and sides of the body silvery; sampled in the lower Omo River and the Ethiopian part of Lake Turkana.Worthington (1932): Figure 3m Citharinus citharus intermedius Worthington (1932)

Material examined: Table 2
Mouth terminal, with unicuspid teeth on lip; body laterally compressed, covered with cycloid scales; dorsal and lateral upper half dark olive, lateral lower half white; pelvic, anal and lower lobe of caudal fin pinkish; sampled in the lower Omo River and the Ethiopian part of Lake Turkana.

Material examined: Table 2
Mouth subinferior, with small bicuspid teeth in two rows in each jaw; more than 20 dorsal fin rays; more than 90 lateral line scales; more than 15 scales between lateral line and pelvic fin; body slightly laterally compressed, covered with ctenoid scales; dorsal surface dark silver, lateral lower half light grey; sampled in the lower Omo River and the Ethiopian part of Lake Turkana.

Order Cypriniformes Family Cyprinidae
Labeo horie Heckel, 1847: Figure 3o Labeo horie Heckel (1847) Material examined: Table 2 No teeth on the jaws; inferior mouth with 1 pair of minute barbels present; maximum 14 branched dorsal fin rays; upper edge of dorsal fin straight or slightly convex; labial folds rather poorly developed; 40-44 scales in the lateral line; no transverse plicae of papillae on the inner sides of the lips; dorsal surface dark olive; sampled in the lower Omo River and the Ethiopian part of Lake Turkana.(Linnaeus, 1758): Figure 3p Cyprinus niloticus Linnaeus (1758) Material examined: Table 2 No teeth on the jaws; inferior mouth with one pair of minute barbels; more than 14 branched dorsal fin rays; upper edge of dorsal fin often concave; 41-45 scales in the lateral line; no transverse plicae of papillae on the inner sides of the lips; dorsal surface dark olive; sampled in the lower Omo River and the Ethiopian part of Lake Turkana.Rüppell, 1832:

Material examined: Table 2
Scales absent; inferior mouth yellowish, with three pairs of barbels; maxillary barbels short, not reaching the posterior edge of eye; caudal truncate; moderate-sized black spots on body, small black spots on caudal fin; caudal fin orange red; sampled in the lower Omo River and the Ethiopian part of Lake Turkana.

Material examined: Table 2
Scales absent; head dorsally flattened; mouth subinferior with four pairs of barbels; caudal fin forked, upper and lower lobes extending into long filaments; the first branched dorsal fin ray extending into short filament; long fleshy adipose fin present; dorsally brown, laterally silver; some specimens dark brown; body gray; sampled in the lower Omo River and the Ethiopian part of Lake Turkana.(Forsskål, 1775): Figure 4f Silurus docmak Forsskål (1775) Material examined: Table 2 Scales absent; head dorsally flat; mouth subinferior with four pairs of barbels; caudal fin forked, lower lobe not extending into long filament; the first branched dorsal fin ray not extending into short filament; long fleshy adipose fin present; body pale red in fresh specimens; sampled in the lower Omo River and the Ethiopian part of Lake Turkana.

Material examined: Table 2
Scales absent; head flat; mouth subinferior with four pairs of short barbels, not extending beyond head; head golden, dorsally golden dark, laterally golden white; dorsal fin with six unbranched rays; caudal fin forked; specimens of Chrysichthys collected from both the Omo River and Lake Turkana during the present study are identified as Chrysichthys turkana sensu Hardman (2008).Sagua, 1987:  Material examined: Table 2 Scales absent; three pairs of unbranched barbels; rayed dorsal fin absent; fleshy adipose fin present; pectoral fins placed low on the body; anal fin with 10 soft rays; caudal fin round or slightly truncate; sampled in the lower Omo River.The occurrence of this species in the Omo-Turkana system was also noted by Golubtsov and Berendzen (1999); sampled in the lower Omo River during this study.Boulenger, 1901: Figure 4i Synodontis filamentosus Boulenger (1901)

Material examined: Table 2
Scales absent; dorsal fin spine extends into very long filament approximately half the length of the spine; soft rays not extending into filament; body dark olive; sampled in the lower Omo River and the Ethiopian part of Lake Turkana; a new record for both localities.(Bloch & Schneider, 1801): Figure 4j Silurus schall Bloch & Schneider (1801)  Material examined: Table 2 Scales absent; dorsal fin spine not extending into filament, feebly serrated posteriorly, smooth anteriorly, except for a few apical spines, also weak serration on lower anterior half in large individuals; first soft dorsal fin ray extends into short (rarely long) filament; no basal marginal membrane on maxillary barbel; body dark brown; sampled in the lower Omo River and the Ethiopian part of Lake Turkana.

Material examined: Table 2.
Scales absent; mouth terminal with four pairs of barbels; small fleshy adipose fin present far behind the small rayed dorsal fin; anal fin long, extending from pelvic fins to caudal, with more than 63 rays; body laterally compressed; head profile rises gradually to dorsal fin; dorsal surface light brown, lateral side silvery white; fins with reddish wash; sampled in the lower Omo River.Rüppell, 1832: Figure 4l Schilbe uranoscopus Rüppell (1832) Material examined: Table 2 Scales absent; mouth terminal with four pairs of barbels; adipose fin absent; anal fin long extending from pelvics to caudal, with more than 63 rays; body laterally compressed; head dorsally horizontal, with nape rising abruptly from occiput to the dorsal fin; dorsal surface light brown, lateral side silvery white; fins with reddish wash; sampled in the lower Omo River and the Ethiopian part of Lake Turkana; a new record for the former.

Order Perciformes Family Cichlidae
Oreochromis niloticus (Linnaeus, 1758): Figure 4m Perca nilotica Linnaeus ( 1758 Material examined: Table 2 Mouth terminal with bicuspid teeth on the outer jaws; dark vertical bands on flank, caudal peduncle and caudal fin; scales between pelvic and pectoral fins distinctly smaller than those on the rest of the body; body light grey, each scale with a black spot; black opercular spot; pectoral fin red; sampled in the lower Omo River and the Ethiopian part of Lake Turkana.

Material examined: Table 2
Terminal mouth with villiform teeth; dorsal fin long, deeply notched into anterior and posterior regions; seven spines in the anterior dorsal fin; caudal fin rounded; body scales ctenoid; single complete lateral line present; body silvery; sampled in the lower Omo River and the Ethiopian part of Lake Turkana.

Material examined: Table 2
Dorsal fin short, located posteriorly; body scaleless, head and body covered with small thin spines; lateral line absent; a pair of fused teeth at the front of each jaw; yellow longitudinal bands on the body; caudal fin truncate to round, yellow in color; yellow longitudinal bands on body; ventral fins absent; sampled in the Ethiopian part of Lake Turkana.

Diversity indices
The values for the Shannon diversity, evenness indices and the permutation p are summarized in Table 3.The differences in the number of specimens and Shannon diversity index between the lower Omo River and the Ethiopian part of Lake Turkana were statistically significant (p < 0.05; Table 3).The number of specimens was higher for the lake (n = 3,563) whereas the Shannon diversity index was higher for the lower Omo River (H′ = 2.43) than for the Ethiopian part of Lake Turkana (H′ = 2.20).The expected species richness for the lower Omo River (i.e., the smaller sample) rarefied from the Ethiopian part of Lake Turkana (i.e., the larger sample) at a subsample size of n = 811 was 19 ± 1.5 at the 95% Confidence Interval (CI) (Figure 5). Figure 6 indicates the rank order abundance plot of the two habitats.The Whittaker's beta diversity index (β w ) for the lower Omo River and the Ethiopian part of Lake Turkana system was 0.24.

DISCUSSION
Analyses of fish sampled indicate that diversity was higher in the sampled section of the Omo River than in the lake, but relative abundance was higher in the lake.Species richness in the lower Omo River (i.e., 26 species) was above the 95% CI of its expected (rarefied) richness of 19 ± 1.5 species.Thus, differences in richness between the two environments could not be ascribed to differences in abundance or sampling effort (Magurran 2004;Gotelli & Colwell 2011).Both the Shannon evenness indices (Table 3) and the rank order abundance plots (Figure 6) showed that species abundance distribution was more even for the lower Omo River fishes.Therefore, the higher value of Shannon diversity index for the lower Omo River (Table 3), despite its lower overall relative abundance, should be accounted for by its greater evenness coupled with its relatively higher species richness (S).
Sites sampled during the present study represented only a small portion of the basin.Therefore, an analysis combining the present data and past studies is valuable for an understanding of the basin's ichthyofaunal diversity.Based on such analysis, 79 valid native fish species referable to 44 genera, 22 families and nine orders can be recognized for the entire Omo-Turkana basin, for which an annotated checklist is provided in Table 4.Besides the valid native species, the Omo-Turkana system also harbors one exotic subspecies i.e.Oreochromis spilurus spilurus and three potentially undescribed species, i.e., Chiloglanis sp."Kerio", Marcusenius sp."Turkwell" and 'Enteromius' spec."Baringo", in the Kerio and Turkwell basins (Seegers et al. 2003).In this basin-wide analysis, the Omo River still retains a greater richness at 63 (79.75%) fish species while Lake Turkana harbors only 55 (69.62%).Within Omo-Turkan basin, 24 species are restricted to the Omo River system (Table 4), of which five (Enteromius arambourgi, Garra chebera, Neobola bottegoi, Afronemacheilus kaffa and Aplocheilichthys jeanneli) are endemic to the river basin.However, the status of N. bottegoi as endemic to the Omo River (Lévêque et al. 1991) is not supported as the species also occurs in Somaliland (Auata River, tributary of Daua River), which is its type locality.Additionally, the current status of Andersonia leptura in the Omo River is uncertain as it has not been recorded since its first capture there in the 1930s (Table 4).Garra makiensis, cited as occurring in the Omo-Gibe River by Stiassny & Getahun (2007), is apparently endemic to the Awash River and Lake Zeway basins in the central portion of the Ethiopian Rift Valley (Boulenger 1909;Golubtsov et al. 2012).Sixteen species are restricted to Lake Turkana, nine of these are endemic to the lake (Table 4).Citharinus citharus inter medius and Chrysichthys turkana are endemic to the Lake Turkana and Omo River, thus increasing the total number of endemic species or subspecies for the Lake Turkana and Omo River (excluding N. bottegoi ) to 11 and six, respectively (Table 4).It is, however, worth noting that the present status of some species reported from Lake Turkana is also uncertain.In particular Brycinus macrolepidotus, Labeobar bus nedgia, L. intermedius, Labeo cylindricus and Synodontis frontosus have not been recently sampled in the lake (Table 4).In contrast, these species have recently been found in the Omo River system, suggesting their riverine occurrence (Baron et al. 1997;Mohammed 2014).As a result, the actual number of valid fish species for Lake Turkana could decrease from 55 to 51 (63.75% of the basin's total richness).Overall, compared to the great African lakes such as Victoria, Tanganyika and Malawi, which are dominated by endemic cichlids, Lake Turkana retains a Nilotic riverine fauna with low diversity (Kolding 1989).The lake is also characterized by a low endemism with a maximum of 11 (13.75%)endemic species and subspecies (Table 4).The low endemism of the lake, and the basin as a whole, is apparently due to its historical connections to the White Nile River system, the recent connection being between 10,000-5,000 years ago, when the lake achieved its historical high levels (+77 to +80 m its present water level of 365±5 m a.s.l.) (Harvey & Grove 1982).
At the family level, the Cyprinidae are the largest group in the entire Omo-Turkana system comprising 22 species (27.85% of the basin's total richness), despite their poor presence in the present study.They occur in both the lacustrine and riverine habitats.In the basin, Nemacheilidae and Amphiliidae are known from the Omo River while three families (Protopteridae, Gymnarchidae and Tetraodontidae) are known from Lake Turkana.Mormyridae remains largely a riverine family with most of its species confined to the Omo River system.Confinement of this family to a riverine habitat could likely be attributed to the high conductivity of Lake Turkana's waters (2,342.47µS/ cm, recorded during the present study) which interferes with the current generating capacity of the electric organs of these fishes (Greenwood 1994;Hopkins 1999).
Previous studies addressing the Ethiopian drainages provided ichthyofaunal diversity for the entire Omo-Turkana Basin but lacked data on the extent of fish diversity in the Ethiopian part of the lake (Golubtsov & Mina 2003;Golubtsov & Darkov 2008).Of all valid native fish species reported from Lake Turkana (51 species), only 24 species could be verified for the Ethiopian part of the lake based on the present collections (Table 2).Accordingly, 65 species in 39 genera, 20 families and seven orders can be recognized for the Omo-Turkana basin within the limits of Ethiopia pending further sampling efforts.Thus, the present study puts the Ethiopian Omo-Turkana basin in third place, Table 3. Summary of specimen numbers (n) and diversity indices for the lower Omo River (OR) and the Ethiopian part of Lake Turkana (LT) based on the present data; *represents statistically significant p values (i.e, < 0.005).

Number of specimens Species richness (95% Cl)
after the Baro-Akobo (Ethiopian White Nile, 113 species) and Abay (Ethiopian Blue, 77 species) basins, in terms of ichthyofaunal diversity.Similar to most other basins of Ethiopia, with the exception of the Blue Nile (Golubtsov & Mina 2003;Getahun 2007), it is characterized by a low endemism.So far, of the 10 species and one subspecies endemic to Lake Turkana, only C. citharus intermedius and Chrysichthys turkana are known from the Ethiopian waters.The Ethiopian parts of the Omo-Turkana basin house five species and one subspecies endemic to the Omo River system and one species (Labeobarbus nedgia) endemic to Ethiopia.An artificial identification key for the fish species occurring within the limit of Ethiopia is provided in the Appendix.
Based on the present collections, the rate of turnover in fish species composition between the lower Omo River and the Ethiopian part of Lake Turkana, measured as Whittaker's beta diversity index, was reasonably high (βw = 0.24).Seven species, i.e., Polypterus senegalus, Mormyrus kannume, Mormyrops anguilloides, Hyperopisus bebe, Polli myrus petherici, Malapterurus minjiriya and Schilbe mystus, were recorded only from the lower Omo River.Correspondingly, five species, i.e., Polypterus bichir, Heterotis niloticus, Labeo coubie, Clarias gariepinus and Tetraodon lineatus were recorded only from Lake Turkana.The turnover in species composition between the two systems is attributable Table 4. Annotated checklist of the native freshwater fishes of the Omo-Turkana system with local names (in Daasanach language) where available, historical and present records, notes on valid scientific names and synonyms, brief distribution account, and maximum length (total length TL, standard length SL or fork length FL) reported for the basin or elsewhere; family-group name follows Laan et al. (2014) for Arapaimidae and Nemacheilidae.

Order/family/species/local name(s) where available
Annotations on occurrence in the basin, scientific names, distribution elsewhere, and maximum length (TL/SL/FL)

Protopteridae (1 species)
Protopterus aethiopicus Heckel, 1851 Known only from three records from Lake Turkana by the Kenyan Marine and Fisheries Research Institute (KMFRI) station on the western shore of the lake at Kalakol; reported as Protopterus aethiopicus aethiopicus (SeegerS et al. 2003); not known from the Ethiopian part of the lake; Nilo-Sudanic in distribution; 200 cm TL (SeegerS et al. 2003).

Mormyrus kannume
Hyperopisus bebe (Lacepède, 1803) Omo River Delta (hopSon & hopSon 1982), lower Omo River (Baron et al. 1997); no scientific collections of the species exist from Lake Turkana, only verbal reports by fishermen of its occurrence in the northern end of the lake (SeegerS et al. 2003); thus, past and present records tend to indicate the species to be riverine and probably confined to the Omo River; Nilo-Sudanic in distribution; 51 cm SL (SeegerS et al. 2003).

CYPRINIFORMES Cyprinidae (22 species)
Enteromius arambourgi (Pellegrin, 1935) Collected during the "La mission scientifique mission de l'Omo" in the Omo River system at a marsh about 50 km northwest of Sergoit, on the plateau Uasin Gishu, Ethiopia, and described by pellegrin (1935); also subsequently recorded in the lower Omo River by Baron et al. (1997); previously reported as Barbus arambourgi; endemic to the Omo River system; 7.8 cm SL (léVêque & Daget 1984).
Labeo cylindricus Peters, 1852 Karitach Middle and upper Omo River system (Boulenger 1906(Boulenger , 1909)); occurs in Lake Turkana probably as stragglers (hopSon & hopSon 1982); recorded in the lower Omo River (Baron et al. 1997), and in the Kerio-Turkwell system in Kenya (SeegerS et al. 2003); not reported from the Ethiopian part of the lake; Nilotic in distribution, also occurs in Congo and Zambezi provinces; 40 cm TL (SeegerS et al. 2003).
Labeo niloticus (Linnaeus, 1758) Karitach Upper Omo River (Boulenger 1906(Boulenger , 1909)); in lower Omo River (Baron et al. 1997);despite reiD's (1985) reference to its occurrence in Lake Turkana, SeegerS et al. ( 2003) considered the species' record from the lake (from the collections of the Natural History Museum, London) as a misidentification of Labeo horie Heckel, 1847; however, its present collection in both the lower Omo River and the Ethiopian part of Lake Turkana supports its occurrence in Lake Turkana; restricted to the Nile basin; 48 cm SL (this study).
Neobola bottegoi Vinciguerra, 1895 Recorded in the Omo River system by Boulenger (1903aBoulenger ( , 1906Boulenger ( , 1911) ) who also reported its presence in Lake Turkana as Engraulicypris bottegi (Vinciguerra, 1895); however, its presence in Lake Turkana has been suspected by subsequent workers to be a misidentification of Neobola stellae; lower Omo River (Baron et al. 1997); likely a riverine species restricted to the Omo River; mentioned as endemic to the Omo River (léVêque et al. 1991), but the species also occurs in Somaliland (Auata River, tributary of Daua River), the type locality; 7.3 cm TL (léVêque & Daget 1984).
particularly to members of the family Mormyridae, most of which are restricted to the Omo River system.

Figure 2 .
Figure 2. Sampling sites on the lower Omo River (OR1-OR4) and the Ethiopian part of Lake Turkana (LT1-LT4); the reference datum used for geographic coordinates is WGS84.

Figure 5 .
Figure 5. Individual based rarefaction analysis for the lower Omo River (n = 823) and the Ethiopian part of Lake Turkana (n = 3,563); CI = Confidence Interval.

Figure 6 .
Figure 6.Rank order abundance plots for fish communities of the lower Omo River (OR) and the Ethiopian part of Lake Turkana (LT).

Table 1 .
Sampling sites and sampling seasons on the lower Omo River (OR1-OR4) and the Ethiopian part of Lake Turkana (LT1-LT4), including altitude above sea level (Alt, meters), water depth (Depth, meters), and geographic coordinates.

Table 2 .
Laan et al. (2014)es collected in the lower Omo River (OR1-OR4) and the Ethiopian part of Lake Turkana (LT1-LT4); family-group name for Arapaimidae is according toLaan et al. (2014); "X" stands for species sampled in a particular locality; voucher numbers of the specimens deposited in the Zoological Natural History Museum (ZNHM) of Addis Ababa University are in the last column.

Table 2
No teeth on the jaws; inferior mouth with one pair of minute barbels; maximum 14 branched dorsal fin rays; upper edge of dorsal fin convex or straight; labial folds relatively well developed; 36-40 scales in the lateral line; transverse plicae of papillae present on the inner sides of the lips; sampled in the Ethiopian part of Lake Turkana (a new record).Scales absent; inferior mouth with three pairs of barbels; maxillary barbels long, reaching the posterior edge of eye, in contrast to A. occidentalis; caudal truncate; body medium brown with dark brown spots; lower lateral body part with reddish wash; specimens from the lower Omo River with reddish fins; sampled in the lower Omo River and the Ethiopian part of Lake Turkana; a new record for the latter.