Chapter 13 Lake Nasser and other Man-made African Reservoirs

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1 Chapter 13 Lake Nasser and other Man-made African Reservoirs L arge reservoirs are generally used for many purposes, which include flood control, water storage for irrigation; generation of electricity, industrial or domestic use, and regulation of flow for navigation. During the 1950's and 1960's many reservoirs were constructed in Africa, the largest four of them are Lake Volta Ghana ( Fig. 258), Lake Kainji on River Niger ( Fig. 259), Lake Kariba on Zambezi River, Zambia (1958) and the High Dam Lake on the River Nile, Egypt / Sudan (1964) (Table 186). The High Dam differs from other dams in a number of characteristics. It is the biggest rockfill dam in the world. It has an impervious core, with a grout curtain that extends 180 m under the core to meet the rock, and a horizontal upstream impervious blanket. The length of the dam at the top is 3,600 m, while the width is 980 m at the bottom and 40 m at the top and its height above the river bed level is 111 m. The High Dam Lake (6,276 km 2 area) is one of the largest man-made lakes in Africa and second in area after Lake Volta (8,845 km 2 ). Also, Lake Volta has a greater volume (165 km 3 ) than the High Dam Lake (156.9 km 3 ), while the mean depths of both lakes are almost similar (Table 187). Comparing Lake Kainji with the other three lakes, it has the smallest area and volume, being 1280 km 2 and 15.8 km 3 respectively. Moreover, the mean depth of Lake Nasser is twice deeper than Lake Kainji. Lake Kariba has an area of about 5,364 km 2, while its volume is about km 3. On the other hand, the High Dam Lake has a much more irregular shoreline (9,250 km), so its perimeter is much longer than that of Lake Volta (5,300 km). Therefore, "Shoreline Index" values for the High Dam Lake are 2-5 times greater than those for Lake Volta and the other two lakes (Table 187). PHYSICO-CHEMICAL CHARACTERISTICS Water temperature. Comparing the water temperature profile of Lake Nasser with that of other Lakes (Table 188), the depth profiles clearly reveal that the 1

2 Fig. 258 Map of Volta Lake, showing the eight strata. Fig. 259 Map of Lake Kainji, Nigeria, with location map insert, showing the original path of the River Niger and the three basins. 2

3 Table 186 General comparison among the four African reservoirs. Characteristics L.Kariba L. Nasser/ Nubia L. Kainji L. Volta Geography Country Rhodesia-Zambia Egypt/Sudan Nigeria Ghana Latitude 16 28' S' 23 58'-20 27' N 9 30'-10 35' N N Longitude 26 42'-29 03' E 30 35'-33 15' E 4 20'- 4 40' E 1 40 W-0 20 E Altitude, m a.s.l Vegetation type forest - wooden desert forest+guinean forest-guinean savannah Savannah savannah Meteorology Climate (Köppen) BSwh BWhs Aw Amw-Aw Mean air temp. ( C) (24) (26.5) Abs. min temp. ( C) (mean) (mean 25.7) (Dec. Feb.) Abs. max temp. ( C) (mean) 39.0 (mean) 28.0 (Mar. - May) Rainfall year (mm) (May - Sep.) Gross evapor. (mm) Dam Site Kariba Aswan Kainji Akosombo Closed in: December 1958 May 1964 August 1968 May 1964 Normal water level in Hydroelectric max. capacity (Kw) Table 187 Main morphometric features and potential yield of the four African Lakes. Parameter Kariba Nasser/Nubia Volta Kainji Length (km) Width, mean (km) Width, maximum (km) Depth, mean (m) Depth, maximum (m) Shoreline length (km) Shoreline development Area (km 2 ) Volume (km 3 ) Approx. outflow : volume ratio 1:3 1:2 1:4 4: 1 Morphoedaphic Index (MEI) Potential fish yield from MEI * (kg/ha/year) Ref. Marshall (1984), Braimah (1995) * Actual for Lake Nasser: kg/ha/year based on catch statistics. 3

4 Table 188 Hydrology and physico-chemical characteristics of the four African Lakes. Characteristics L. Kariba L.Nasser/Nubia L. Kainji L. Volta Hydrology River catchment Zambezi Nile Niger Volta (Black - White) Area catchment ,400,000 1, ,000 above dam (km 2 ) River flood season Mar. Apr. Aug. Oct. white flood (July - Sep.) Black flood (Dec. Feb.) Jul.-Sep. Ann. Gross evaporation, (km 3 ) Water level max. (m) Water level normal (m) Water level fluctuations (m) Water exchange ratio (7.35 mm/day) :4.0 (1:3.1) 1:2 4:1 1:3.7 Temp. range, ºC Water Physics Thermal cycle Warm monomictic Temp. homothermy (ºC) Thermal strat. period Depth of mixed layer, (m) (June July) Warm monomictic (monomictic) Warm polymictic (Jan Mar) (July Oct.) Apr. June and May Nov. Dec. Oct. - June May Aug. Feb. mid Secchi D.V., (cm) (405) Depth euphotic zone, (m) 2 24 (10 16) 1 10 ab. 1 8 ab. 10 Water Chemistry ph Conductivity ( mhos/cm) Total solids (mg/l) Diss. oxygen surf. (mg/l) Oxygen stratification (72) (35 40) Oct. - June May Aug. Mar. mid May Oct. Nov. Nov. Jan. & Apr. July 4

5 Lake is generally warm, monomictic with a single circulation period during winter. Thus, it has thermal stratification from May to August and more uniform temperature distribution between March and November (Entz 1976). Contrary to Lake Nasser, Lake Volta is warm polymictic, as the stratification is fairly stable in April - June, but frequently broken down during the rest of the year by southerly and harmattan winds and the annual floods (Braimah 1995). Water level. Lake Volta reached its maximum level of 85.3 m in 1968 after the closure of the Dam in Seasonal variations were relatively moderate until 1975, when the water level started to drop, until June 1984, when the Lake reached an unprecedentedly low level of 71.9 m. At this level the Lake had shrunk to roughly half of its maximum size. Since then, Lake level fluctuated, but has been generally rising. The Lake reached its maximum level again in 1989, and again in 1991 (Braimah 1995). In Lake Nasser water level reaches its maximum in November and December of each year, then it decreases gradually till the second half of July. The annual minimum level dropped from a maximum of m in 1979 to a least minimum of m in 1988, after which it increased to an average of m in The maximum operation level of 175 m above MSL was reached in October 1975, but recently, in 1999 water level reached a maximum of m above sea level, which is considered as the highest record since the construction of AHD. Thus, the Lake's expected maximum level of 183 m, has not yet been reached. In Lake Kainji, water level increases from August until it reaches to about 140 m in February, then decreases from March to July each year (Balogun & Ibeun 1995). Apart from the regular inflow of water from the River Niger, the reservoir experiences two major floods, namely the "white" flood and the "black" flood. The white flood, characterized by high turbidity, is a result of rainfall in the catchment area of the River Niger up to Mali, and it enters the Lake in late August. The black flood is caused by rainfall at the source of the Rive Niger, in Guinea, and enters the Lake in November and is characterized by a high water transparency. Electrical conductivity. Electrical conductivity in Lake Nasser exhibits seasonal, vertical, horizontal and local variations ranging from 160 to 350 mhos/cm, but the highest values were recorded just in front of the flood water ( mhos/cm) and the lowest at the end of the flood ( mhos/cm). Generally, conductivity values show a decreasing trend southwards in the different seasons, except for the summer, when a reverse picture is generally seen (Latif 1984a). Comparing electrical conductivity of Lake Nasser with that of Lake Volta (Table 188), the latter has very low values, ranging between 50.0 and 80.0 mhos/cm. This may be due to the low values of total dissolved solids 5

6 which range between and mg/1 (Braimah 1995), contrary to Lake Nasser that has TDS 4-5 times higher than the other lakes. ph. Generally, the ph values of Lake Nasser always lie on the alkaline side. Thus, the ph values range between 6.8 and 9.5 and decreases in general with depth. The lowest values were recorded in deep anoxic layers of the Lake. In Volta Lake, Obeng-Asamoah (1984) found also that the ph value of the surface water was about 7.0, declining with depth to 6.45 in the anoxic water near the bottom of the lake. In Lake Nasser, bicarbonate alkalinity is common in most of its water and ranges from 58 to 146 mg/1 in the main channel of the Lake. Carbonate (0-32 mg/1) shows a decreasing pattern from year to year and in recent years has not constituted more than 1% of the total alkalinity (Aly 1992). In Lake Volta total alkalinity increased during the filling and post impoundment phases (Biswas 1966) and a total alkalinity of 41 mg CaCO3/1 was recorded in 1989 (Braimah 1995). On the other hand, Lake Kariba is a slightly alkaline, oligotrophic lake, and this may be due to its relatively low photosynthesis rate and phytoplankton population (Machena 1995). Generally, the total alkalinity in Lake Nasser is 3-4 times higher than in the other three African lakes. Nutrient salts. In Lake Nasser the total nutrient salts, PO4, and NO3 concentrations range between , and mg/l respectively, being generally higher in the bottom than in the surface layers. Lowest concentrations were recorded in February, while the highest ones were observed in November (Zaghloul 1985). Comparing Lake Nasser with Lake Volta, the latter has generally very low nutrient content and only traces of phosphate, nitrate, nitrite, ammonia and sulphate have been recorded in the upper 40 m of the Lake, while measurable quantities of these ions were recorded in the bottom waters (Braimah 1995). The low nutrient content was attributed to the catchment area itself being poor in nutrients, and also due to the low solubility of the Precambrian rock granites found in the upper catchment area (Antwi 1990). In Lake Kariba, both nitrogen and phosphorus are limiting (Marshall 1984). The latter author attributed this to the largely steeply sloping of the Lake, which limits the abundance and distribution of submerged macrophytes that affects the nutrient cycles and consequently the fish production. PHYTOPLANKTON Phytoplankton community in Lake Nasser is rich, both in species diversity and density. 107 species belonging to four divisions have been recorded. Numerically, the standing crop increases southwards (Mohammed et al. 1989) from 3.40 x 10 6 to x 10 6 algal unit/l at Adindan. In surface water, the average density of phytoplankton is 6.3 x 10 6 algal unit/l, while it is only 2.08 x 10 6 at 20 m depth. Diatoms, green alage and blue-green algae are the dominant groups which exhibit seasonal fluctuation, strongly affected by the flood (Latif 1984a). Flood waters push great amounts of phytoplankton to the 6

7 northern region of the Lake. So, at the southernmost area (Adindan) the lowest densities are recorded during flood and post-flood seasons. When comparing Lake Nasser with Lake Volta, the phytoplankton of the latter lake is quantitatively poor, especially in the southern region, and algal blooms are only observed occasionally in some limited areas (Obeng - Asamoah 1984). The number of the species is just over twenty and confined to not more than 10 genera. The most abundant species are Synedra and Melosira for the main channel, while Oscillatoria dominates the shallow arms and inshore areas. Eudorina and Volvox also occurred in relatively large numbers (Braimah 1995). MACROPHYTES Macrophyte communities play an important role in providing habitat complexity, food to aquatic animals, in nutrient recycling, etc. In Lake Nasser, eleven species were recorded since its filling, including 4 cosmopolitan species : Potamogeton crispus, P. pectinatus, Najas marina sub sp. armata, and Zannichellia palustris, as well as 4 subcosmopolitan species: Potamogeton trichoides, P. lucens, Vallisneria spiralis and Najas horrida. Subsequent to the construction of the Aswan High Dam, two euhydrophyte species seem to have disappeared from the region, Alisma gramineum and Damasonium alisma. Najas marina dominates the deep water zones at most sites and to a lesser extent in shallow waters, while Vallisneria spiralis dominates the submerged macrophytes, especially at Amada. In recent years Myriophyllum spicatum appeared in the Lake. It is worth mentioning that Eichhornia crassipes and Pistia stratiotes - which are found in some sections of the Nile River both upstream and downstream - have not been recorded in Lake Nasser and it is highly recommended to prevent infestation of the Lake with these pests. On the other hand, the emergent aquatic macrophytes in Lake Kainji include Echinochloa spp., Cyperus sp., Pistia stratiotes and Ceratophyllum demersum, of which Echinochloa stagnina forms the major component (Balogun & Ibeun 1995). In 1971 emergent macrophytes were estimated to cover only 0.5% of the Lake surface area, but by 1977, 8.9% of the surface area was covered, in particular by E. stagnina (Obot 1984). The macrophytes in Lake Kainji are most productive in the dry season, when the Lake water level is highest, and are used as livestock fodder during the dry season. In Lake Kariba the various types of shores have been colonized by macrophytes of different species and different growth forms. Exposed and rocky shores are not colonized by plants. Vallisneria sp. and Potamogeton sp. occupy shore areas with fine to sandy sediments. Lagarosiphon sp. occupies areas where sediments are rich in nutrients, with high light levels and low disturbance (Machena 1995). 7

8 Lagarosiphon sp. is fast growing, canopy forming and competes effectively. It forms more mixed groupings than the other species, and in fact mixed with all other species. Vallisneria forms the most monospecific group, while Potamogeton is mixed with neither Ceratophyllum nor Najas. There is evidence that Lagarosiphon sp. translocates phosphorus and ammonium from the sediments to littoral water (Machena 1989). Moreover, the mean net production rate of one individual of Lagarosiphon sp. shoots is about 7.5 mg/g (dw) / day (Machena 1995). EPIPHYTES Epiphytic algae are among the most important food items for the tilapiine species which constitute the major fish species of African Lakes. In Lake Nasser 28 predominant algal genera were recorded belonging to four major groups; Chlorophyta, Bacillariophyta, Cyanophyta and Pyrrophyta (Habib 1997). The predominant genera are Oedogonium, Stigeoclonium and Spirogyra. The maximum amount of chlorophyll a fluctuated from 10.9 in September to 78 mg/m 2 in November Generally low values of chlorophyll a were found in April or May. The calculated total amount of attached microalgae for the whole shoreline at 160 m water level was ton in November 1989 (Habib, 1997), being the average value of Abu Simbel and Tushka areas. In Lake Volta, the flooded trees provide a substrate for attached algae in the epilimnion of the inshore and offshore areas (Vanderpuye 1984). The most abundant species are Synedra, Melosira and Oscillatoria spp. In Lake Kariba, the dominant attached algae species are Gleotrichi, Oscillatoria and Lyngbya spp. The important substrata are the dead trees and macrophytes such as Lagarosiphon and Najas, while some species seem to be avoided (Ramberg et al. 1987). The attached algae have a mean biomass (dw) of 60 g/m 2 in the 0-5 m depth zone, where they are evenly distributed and contributed about 30% of macrophyte biomass (Ramberg et al. 1987). PRIMARY PRODUCTIVITY Primary productivity values of Lake Nasser show that it is eutrophic with highest production at 1-2 m depth. Samaan (1971) estimated primary production at different localities of the Lake and reported that it ranged between g C/m 2 /day. Abdel-Monem (1995) recorded values ranging from mg C/m 3 /h. Gradual eutrophication of Lake Nasser may be a result of continuous sedimentation of organic matter which accumulates annually with flood water rich in nutrients that flourish phytoplankton growth. Comparing Lake Nasser with Lake Volta, the latter has very low values of primary productivity, ranging from g C/m 3 /day. This is primarily due to the limited supply of nutrients (Antwi 1990). 8

9 EVOLUTION AND HISTORICAL TRENDS OF DIFFERENT FISH GROUP CATCHES A list of commercial fish species from the largest four African Lakes is presented in Table 189. It may be said that, in Lake Nasser there are 9 fish families and 23 species, in Lake Kainji - 11 families and 38 species, in Lake Kariba - 9 families and 22 species and in Lake Volta 12 families and 37 species (Table 189). Table 189 List of commercial fishes (Families and Species of the four African Lakes). Lake Nasser 1 Kainji 2 Kariba 3 Volta 4 Cichlidae Oreochromis niloticus Sarotherodon galilaeus Sarotherodon galilaeus Centropomidae Lates niloticus Bagridae Bagrus bajad Bagrus docmak Cyprinidae Labeo niloticus Labeo coubie Labeo horie Barbus bynni Clariidae Clarias anguillaris Clarias gariepinus Heterobranchus bidorsalis Heterobranchus longifilis Characidae Hydrocynus forskalii Brycinus nurse Alestes dentex Alestes baremoze Mormyridae Mormyrus kannume Mormyrus caschive Shilbeidae Eutropius niloticus Schilbe mystus Cichlidae Oreochromis niloticus Tilapia zillii Hemichromis fasciatus Tilapia monodi Centropomidae Lates niloticus Bagridae Chrysichthys nigrodigitatus Clarias gariepinus Chrysichthys auratus Clarotes laticeps Bagrus bajad Bagrus docmak Auchenoglanis biscutatushydrocynus forskalii Auchenoglanis occidentalis Micralestes acutidens Cyprinidae Labeo senegalensis Labeo coubie Labeo parvus Clariidae Clarias anguillaris Clarias gariepinus Characidae Alestes macrolepidotus Hydrocynus forskalii Alestes dentex Alestes baremoze Alestes nurse Cichlidae Oreochromis mortimeri Pharyngochromis darlingi Pseudocrenilabrus philander Serranochromis codrington Tilapia rendalii Cyprinidae Labeo altivelis Clariidae Heterobranchus longifilis Characidae Alests imberi Brycinus lateralis Mormyridae Hippopotamyrus discorhynchus Marcusenius macrolepidotus Mormyrops deliciosus Mormyrus longirostris Schilbeidae Schilbe depressirostris Schilbe mystus Mochokidae Synodontis nebulosus Synodontis zambezensis Clupeidae Limnothrissa miodon Cichlidae Sarotherodon galilaeus Oreochromis niloticus Tilapia zillii Chromidotilapia guentheri Leptotilapia irvinei Hemichromis bimaculatus Centropomidae Lates niloticus Bagridae Bagrus docmak Bagrus bajad Chrysichthys nigrodigitatus Chrysichthys walkeri Chrysichthys auratus Auchenoglanis occidentalis Cyprinidae Labeo coubie Raiamas senegalensis Characidae Brycinus nurse Brycinus leuciscus Brycinus macrolepidotus Hydrocynus forskalii Hydrocynus lineatus Mormyridae Mormyrus deliciosus Mormyrus rume Hippopotamus pictus 9

10 Schilbe uranscopus Mochokidae Synodontis spp. 9 Families 23 Species Hydrocynus brevis Mormyridae Mormyrus rume Hyperopsus bebe Schilbeidae Eutropius niloticus Physallia pellucida Mochokidae Synodontis gambiensis Synodontis schall Synodontis nigrita Synodontis filamentosus Synodontis gobrani Synodontis sorex Distichodontidae Distichodus rostratus Citharinus citharas Distichodus engycephalus Osteoglossidae Heterotis niloticus 11 Families 38 Species Malapteruridae Malapterurus electricus 9 Families 22 Species Pollimyrus isidori Schilbeidae Schilbe mystus Silurandon auritus Mochokidae Synodontis gambiensis Synodontis ocellifer Synodontis schall Synodontis eupterus Synodontis velifer Clupeidae Pellonula afzeliusi Cynothrissa mento Sierrathrissa leonensis Osteoglossidae Heterotis niloticus Malapteruridae Malapterurus electricus Anabantidae Ctenopoma kingsleyae 12 Families 37 Species 1: Latif (1974a and b and 1977), 2: Balogun (1986), 3: Machena (1995) 4: Braimah (1995). 10

11 COMPOSITION OF FISH LANDINGS AND CHANGES IN FISH POPULATIONS Lake Nasser: Tilapiine species mainly Sarotherodon galilaeus and Oreochromis niloticus are the most dominant fish species, particularly during the period , as their percentage by weight were high and ranged between and 95.29%, with an average of 88.48%. It is worth mentioning that, with the progressive increase of mean water level from m MSL in 1991 to m in 1999, there was a continuous decrease in tilapiine landings from 29,383 ton in 1991 to 8606 ton in This may be mainly attributed to that, a large portion of tilapiine catch is sold by the fishermen illegally in the black market with high prices, hence not recorded in the official catches and d'not represent the actual tilapiine catch from the Lake. Furthermore, it is probable that, with the increase of water level in the Lake, tilapiines are spread in very large areas, and hence are difficult to capture by the fishing gear in use. The composition of fish landings in Lake Nasser is in favour of periphyton -plankton feeders. Thus, during the first period of inundation ( ) the average percentage of the annual catch of periphyton- plankton feeders was only 39.92% (Table 106). This percentage increased gradually during the following periods, being and 88.27% during the second ( ) and third ( ) periods (Table 106 and Fig. 150). On the other hand, the average percentage of the annual catch of carnivorous and zooplanktoninsect feeders was 42.04% during the first period ( ), which decreased gradually to and 10.76%, during the second and third periods respectively. Furthermore, the average percentage of the annual catch of omnivorous fishes was 18.04% during the first period ( ), followed by a sharp decrease to 0.77 and 0.97% during the second and third periods respectively (Table 106) and Fig. (150). Lake Kainji : The pre-and post-impoundment fish species composition was described by Ita (1984). Two gill net surveys were carried out in 1976 (Ita 1984, Balogun 1986). Comparison of these two surveys reveals changes in species composition. In 1976, 58 species were recorded, while only 43 species were caught in The major difference between the catches in 1976 and 1984 was that, the Cichlidae, ranking third in terms of abundance in 1976, dominated the catches in The Bagridae and Cyprinidae, which formed low catches in the gill nets during the early post impoundment period (Lelek & El-Zarka 1973, Lewis, 1974) and ranked sixth and fourth respectively in 1976, became more important in 1984, ranking second and fourth respectively. Major changes in species composition and the relative importance of the fish species and families were observed after impoundment of Lake Kainji. The Mormyridae and Distichodontidae were abundant in the River Niger, but after 11

12 impoundment, the Distichodontidae dominated the catches in the first two years and thereafter decreased in number. The Mormyridae did not feature well after the impoundment and remained very low, whereas the Cichlidae, Cyprinidae and Bagridae, which formed low catches in the River Niger and shortly after the impoundment, have significantly increased in number. The Characidae, which featured prominently in the River Niger, became dominant shortly after the impoundment and continued to remain prominent after the Lake became a tilapia lake in A comparison of percentage composition by number of fish families in the regular gill net sampling in the period 1970 to 1973 and 1984 to 1985 is shown in Table 190. Lake Kariba : Kenmuir (1984) analyzed experimental gill netting data collected by the Lake Kariba Fisheries Research Institute at two stations between 1960 and 1975, and Karenge (1992) analysed data collected at one of those stations between 1969 and Table 190 Comparison of percentage composition by number of fish families in the regular gill nets at Lake Kainji (Balogun & Ibeun 1995). Family Characidae Mochokidae Cichlidae Cyprinidae Distichodontidae Bagridae Schilbeidae Mormyridae Centropomidae Others Number of fish Number of fishing days Average no. of fish caught/day Note: Results for were based on surface catches, while those, for were based on shore, surface and bottom catches. After closure of the Dam in 1958 the fish population was similar to the pre-impoundment riverine population, with Labeo spp., Distichodus spp. Clarias 12

13 Shannon's diversity index gariepinus and two characid species dominating gill net catches, while the fish biomass increased. In the early 1960's some of the early abundant species in the catch declined rapidly, e.g. Clarias gariepinus, Labeo spp. and Distichodus spp. At the same time, mormyrids (e.g. Mormyrus longirostris), cichlids (e.g., Serranochromis codringtoni S. macrocephalus), and silurids (e.g. Synodontis zambezensis) increased significantly in the catches. Hydrocynus vittatus increased significantly following the introduction and establishment of Limnothrissa miodon. Using the Shannon Index, Karenge (1992) observed an increase in fish diversity at the Lakeside station between 1969 and 1991 (Fig. 260). Fish diversity and abundance followed increased abundance of macrophytes and benthic fauna, which was, for example, the case with Serranochromis macrocephalus (Karenge 1992). Changes in fish population are still taking place (Sanyanga 1990). The catfish Synodontis zambezensis now seems to be the most abundant fish in the lake in terms of catch per unit effort. Synodontis is abundant in fished areas and occurs deeper than the other species. Increases in the population of benthic species is a common trend in the biological development of reservoirs. Studies have been undertaken to develop appropriate gear to commercially exploit this species (Songore 1992). Fig. 260 Increase in fish diversity (Shannon Index) at the Lakeside Station, Lake Kariba, over the period 1969 to 1991 (Karenge 1992). Lake Volta: Commercial catches comprise only 26 genera. The record does not include a breakdown of different species. In the absence of experimental catch data for the Lake, commercial fish landings might give an insight into the size of the components of the eco-groups described by Vanderpuye (1984). Using his categorization, the insect-aufwuchs and detritus-herbivore feeders predominate (52.9%), with Tilapia spp. alone contributing 25.9%, while the piscivores were the second eco-group (37.8%), followed by the benthic-omnivores (5.7%) and the semipelagic omnivores (3.6%) (Table 191). Table 191 Main feeding group categories as percentage composition based on Year 13

14 fresh weight equivalent (FWE) fish landings 1991 in Lake Volta (Braimah 1995). Insect-aufwuchs, detritus feeders and herbivores Piscivores Semi-pelagic Benthic - omnivores omnivores Species % Species % Species % Species % Heterotis niloticus 2.8 Polypterus senegalus Clupeidae 0.5 Mormyridae 2.7 Distichodus spp. 0.5 Gymnarchus niloticus Schilbeidae 1.7 Auchanoglanis 0.6 Citharinus spp. 4.7 Hydrocynus spp. 4.5 Alestes spp. 0.9 Clarias spp. 2.4 Labeo spp. 3.2 Chrysichlhys spp Distichodus sp. 0.5 Synodontis spp Bagrus spp. 6.4 Tilapia spp Lates niloticus 3.2 Tetraodon fahaka Total Table 192 Estimated fish yield of four African Lakes and their trophic states. Lake Nasser Kainji* Kariba** Volta*** Estimated fish yield (kg/ha/yr) Trophic state (av.73.70) Eutrophic Mesotrophiceutrophic Mesotrophiceutrophic * = Braimah (1995); **=Machena(1995); *** =Ita (1984). ESTIMATED FISH YIELD Mesotrophiceutrophic Table 192 presents the estimated fish yield of the four largest African Lakes. It is clear that the fish yield of Lake Nasser ranged between and kg/ha/yr, with an average of kg/ha/yr during the period (Table 192). This means that Lake Nasser may be categorized as an eutrophic Lake. On the other hand, the other three lakes (i.e. Kainji, Kariba and Volta) had fish yield less than 60 kg/ha/yr. Hence, Lakes Kainji, Kariba and Volta may be considered as mesotrophic - eutrophic (Table 192). 14