19/08/2013
Lake Tana
LakeTana, the source of the Blue Nile, is the largest lake in Ethiopia. It probably was formed during the late Pliocene or early Pliestocene times by volcanic blockage of the Blue Nile River. It now covers an area of about 3150 km 2 and has an average depth of 8m, with a maximum of 14m. It is estimated at an altitude of 1830m absl and can be classified as an oligo-mesotrophic lake. Several rivers, for instance GilgelAbay, Rib, Gumara,Gelda and others join the lake, the Blue Nile being the only outflow. After 30km this river plunges down a 40-meter-high waterfall, isolating Lake Tana and its tributaries from other parts of the Nile Basin.
Remains of ancient Ethiopian emperors and treasures of the Ethiopian Church are kept in the isolated island monasteries (including Kebran Gabriel, Ura Kidane Mehret, Narga Selassie, Daga Estifanos, Medhane Alem of Rema, Kota Maryam and Mertola Maryam). On the island of Tana Qirqos is a rock shown to Paul B. Henze, on which he was told the Virgin Mary had rested on her journey back from Egypt; he was also told that Frumentius, who introduced Christianity to Ethiopia, is "allegedly buried on Tana Cherqos." The body of Yekuno Amlak is interred in the monastery of St. Stephen on Daga Island; other Emperors whose tombs are on Daga include Dawit I, Zara Yaqob, Za Dengel and Fasilides. Other important islands in Lake Tana include Dek, Mitraha, Gelila Zakarias,Halimun, and Briguida.
Four fish species dominate the commercial and artisanal fishery: Tilapia (Oreochromis niloticus), catfish (Clariasgariepinus), the various Barbus species (Nagelkerke, 1997), and to some extent Varicorhinusbeso. The fishery of Lake Tana is expanding through the introduction of modern technology such as the motorised boat fishery with 100m gill net. Traditional fishing activity is also growing as provision of modern gill nets is increasing for small-scale fisheries. Thus fishing is becoming more important both economically and socially especially for the low-income rural population around the lake.
Environmental Impact Assessment Project The research project proposed here uses the Environmental Impact Assessment (EIA) technique that serves as a tool for assessing, estimating, predicting and communicating the environmental effects of proposed policies, development plans and investment projects in relation to the Lake Tana aquatic ecosystem. In the absence of the EIA the conflicting use of the Lake Tana resources system for various benefits can be detrimental to the fisheries of the lake in particular and for the aquatic resources in general
Project Method 1.Describing the existing environment/physical, biological, social economic and others/ 2.Describing and quantifying the existing threats, risks and impacts 3.Determine opportunities and means for reduction in threats, risks and impacts 4.Identify and assess alternative options for control of threats, risks and impacts.
Background and Justification It appears that most inland capture fisheries are currently being exploited at close to or above, their sustainable levels, although catches will vary from seasons, reflecting the qualitative and quantitative fluctuations of the target population. These natural fluctuations result from changing climatic conditions and the complex population dynamics of interacting groups of organisms. This lack of stability is particularly obvious in tropical inland fisheries, which typically exploit mixed species groups (Dunne et al 1996).
LakeTana, situated at an altitude of about 1800 m. above sea level, is a crater lake formed two million years ago due to the volcanic blocking of the Blue Nile River (Figure 1). It is the source of the Blue Nile. It is 78 km long and 67 km wide, and its area of 3150-3500 km 2 constitutes about 50 per cent of the total inland water area of Ethiopia. The mean annual outflow is about 4.km 3 per year, which is about 7 per cent of the total flow of the Blue Nile. It is a shallow lake, with a maximum depth of 14 m and a mean depth 8.9 m.
The main tributaries to the lake are Little Abay,Gumara,Ribb,Magech,Infranz, and Gedla. There are lagoons and wetlands on all sides of the lake resulting from hydrological and land use changes. The Dembea Plain to the north, the Fogera Plain to the east and the Kunzila Plain to the south-west are low areas bordering the lake which are often flooded during the rainy season
The Lake Tana area has warm temperate climate. The average temperature is 21.7 o C , with a maximum yearly variation of 5 o C. Temperature has two peaks, one around May-June at the start of the rainy season and the other around October-November at the start of the dry winter season. The minimum temperature ranges from 6 C in December-January to 16 C in May. The climate is seasonal and is dominated by dry season from October-November until May-June. The rainy season (kremt)runs from July through August and September, with the maximum monthly rainfall occurring in July. The mean annual rainfall is about 1500 mm, of which 54 percent falls in the months of July and August, when the rainfall can be 250-300 mm/month.
The seasonal rains cause the lake level to fluctuate regularly with an average difference between the minimum, in May-June, and the maximum, in September-October of about 1.5 m. The largest recorded increase in the lake level was about 2.5 m in 1997. Most of the small rivers dry out shortly after the big rains. Significant inflow comes from three major rivers in the South, namely the GilgelAbai, Rib, and Gumara which carry a large amount of silt resulting from severe erosion, thereby increasing the turbidity of the water in the Bahar Dar Gulf. The transparency of the Secci disk depth is reduced during rainy season to 37 cm. Due to the shallow saucer-shape of the lake, and the fairly strong winds starting in the afternoons - becoming stronger after sunset - the transparency for the whole lake ranges from 130 cm during rainy season to 182 cm during dry season (Secchidepth V ). The transparency of water is also different during the early hours of the day and in the afternoon.
LakeTana feeds the Blue Nile, which in turn provides 85 per cent of the water supply to Sudan and Egypt through the Nile (Howell & Allan, 1994). It also provides some of the water supply for Bahar Dar, and is a significant source of water supply for the rural population around the lake
Tropical shallow lakes are, in general, highly productive of fish. The potential fish production of Lake Tana is estimated by the Ministry of Agriculture to be 13,000 tonnes per annum. Until the last couple of years, production was about 1000 tonnes per annum. However, this has dropped by about 50 per cent in recent years to 600 tonnes in1999 and 500 tonnes in 2000