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LAKE TANA
Geophysical characteristics
Lake Tana is the largest lake in in Ethiopia located 12o0’N 37o15’E.
It is the source and the main reservoir of the Blue Nile.
The lake is of cultural historical importance and has many islands with churches and monasteries where remains of ancient Ethiopian emperors and treasures of the Ethiopian Orthodox church dating as far back as the 14th century is kept.
Lake Tana was formed by volcanic activity blocking the Blue Nile about 5 million years ago.
It is relatively shallow with an average depth of 9.7m and maximum depth of 14.8 m.
It is 84 km long and 68 km wide.
Its surface area varies between 3,000-3,500 km2 and its water level fluctuates by 2-2.5. annually due to variations in rainfall and evaporation.
It lies at the highest elevation among the large lakes of Africa of 1,788 m amsl.
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It has a catchment area of 12,000km2.
It has a volume of 29.6 km3.
The water residence time is 5.4 years.
The lake is surrounded by flood plains with many lagoons in north, east and southwest.
The lake has five major rivers flowing into it including Gilgel Abay, Gelda, Gumara, Rib, and Megech and >40 seasonal rivers.
The primary outflows is Blue Nile which is obstructed by the Blue Nile Falls.
The five main rivers contribute 95% of the inflow (Amare and Kameswara, 2011).
The annual inflow to the lake is 3843 million m3 yr-1 from direct rainfall, 3970 from gauged rivers, and 2729 from ungauged rivers. The water loss if through outflow of the Blue Nile (5183 MCM) and evaporation (4714 MCM) (Minychl, Aron, and Seifu et al., 2019).
The lake level is regulated by weirs constructed at the outlet to the Blue Nile.
The potential sources of conflicts are disputes between Egypt, Sudan, and Ethiopia over use of the waters of the Blue Nile due to lack of an agreement on sharing the waters of the Nile. This has been exacerbated by construction of the Great Ethiopian Renaissance Dam along the Blue Nile by Ethiopia which, is expected to regulate the quantity of water flowing downstream to Sudan and Egypt.
Demographic characteristics
The population density around Lake Tana is high, >150 persons/km2 as the basin is a major economic zone and this is increasing pressure on the lake and its resources (Amare and Kameswara, 2011).
The main economic activities are agriculture, fishing, transportation, tourism (religious and recreational) and sand mining.
The lake and the adjacent wetlands provide directly or indirectly a livelihood for more than 74 500,000 people (Adugnaw et al, 2019).
The government of Ethiopia has a comprehensive rural and urban water supply, sanitation and hygiene program.
The government of Ethiopia has a comprehensive rural and urban water supply, sanitation and hygiene program.
Fishes and fisheries
It is the most productive lake in Ethiopia with potential annual production of 10,000 and in 2011 landed 1,454 tons which contributes 50.2% of capture fishery production in Ethiopia (Lemma, 2017).
Lake Tana has 27 fish species 20 of which are endemic including one of the only two known cyprinid species flocks the other one was in Lake Lanao in the Philippines where it has been decimated by introduced species.
The fishery is dominated by Labeobarbus species some of which can grow to a size of up to 1 m long, Nile tilapia and catfishes.
The sizes of tilapia and the catfish have decreased, and the stocks of Labeobarbus spp that breed in tributaries have declined due to exploitation and habitat destruction.
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Nile perch, Lates niloticus, a species of the Nile system is absent in Lake Tana although caught in large numbers in Ethiopian Rift valley lakes such as Chamo. It spread to Lake Tana seems to have been prevented by Tisisat Falls just like the Marchison Fall prevented the species from Lake Albert to enter the Lake Victoria region.
Ethiopia, including the Lake Tana basin has enormous potential for aquaculture production due to large expanses of water and suitable climate but aquaculture was originally not much practiced in the country.
Since the 2000s government has developed a policy to promote of aquaculture in the country to facilitate increase in fish production to contribute to nutrition and income.
Two commercial farms (MIDGE 2000 PLC Cage Culture and Ashraf Industrial Group Cage Culture) plan to set up cage aquaculture farms on Lake Tana (Lemma, 2017).
The lake habitat
The waters in Lake Tana are relatively cold, 20-27 °C compared to other tropical lakes.
The lake is alkaline with a pH of 8.0 -8.6.
Secchi depth in shallow nearshore waters was 0.18-0.85 suggesting high algal productivity and eutrophication in the area.
TP is highest is the dry season in March 0.21 mg L−1 and lowest in the rainy season in August 0.14 mg L−1. The higher values of P in the dry season has been attributed to low flushing rate/high residence time, high retention capacity, and wind-induced re-suspension of phosphorus when the temperature increases in the dry months.
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TP is highest is the dry season in March 0.21 mg L−1 and lowest in the rainy season in August 0.14 mg L−1. The higher values of P in the dry season has been attributed to low flushing rate/high residence time, high retention capacity, and wind-induced re-suspension of phosphorus when the temperature increases in the dry months.
The highest values of TN of 2.6 mg L-1 were recorded in the rainy season in August and the lowest of 1.9 mg L-1 in dry season in March. The high level during the dry season are associated with transportation of nitrogen fertilizers used on the farms to the lake (Minychl, Aron, and Seifu et al., 2019).
Use of phosphate and nitrogen fertilizers in the lake basin is increasing the trophic status of the lake.
Chl_a was in 2020 estimated 4.36 - 8.82 µg/L.
The dominant algae in the lake are dominated by blue green algae such as Microcystis aeruginosa sp., Nostoc sp., and Anabaena sp. Which are characteristic of eutrophic lakes. They are also known to produce phyto-toxins which can be dangerous to aquatic like and consumers of water and aquatic organisms (Adugnaw et al, 2019).
The lake supports few invertebrates, fifteen species of molluscs, including one endemic freshwater sponge.
Water hyacinth invaded Lake Tana in 2011. By 2014, it was covering one-third of the lake's shoreline, around 128km, and within two years, it doubled from 20 000 to 40 000 hectares and by 2017 was covering 50 000 hectares of the lake (Minychl et al 2019).
Water hyacinths biomass in the lake was observed to increase rapidly as the lake became eutrophic with P increasing from 0.06 mg L-1 to 1.06 mg L-1 and is most common in shallow areas of <6 m deep with high P concentration.
Between 1973 and 2008, temperature increased, and rainfall and lake level declined apparently due to human induced climate change (Amare and Kameswara, 2011).
Basin factors
There are intense agricultural activities in the lake basin with crop agriculture covering 51%, pastoralism, 29% and the lake water 20% of the catchment.
The main agricultural activities in the lake basin are crops (grains, oilseeds, and coffee), pastoralism and fishing.
Conversion of land to agriculture, deforestation, and wetland degradation expose the lakes to siltation and nutrient enrichment from the catchment areas.
The catchment of the lake has, extensive wetlands, lagoons and swamps and much of the nearshore areas of the lake are <6m deep are wetlands according to the definition of wetlands in the Ramsar Convection.
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The wetlands including the lake are affected by silt loads from the intensive agricultural activities in the basin.
Clearing of wetlands affects one to the commercially important fish species, Labeobarbus which breeds in wetlands.
The lake is threatened by pollution from agro-chemicals used in agriculture in the catchment area and by urban wastes from rapidly growing cities in its basin. An estimated 2.15 million t yr-1 of organic matter rich in nutrients are deposited in the lake bed every year.
Sedimentation is a major threat to the lake and the wetlands in the basin. It is estimated that 12–37 million t yr-1 of sediments enter the lake and only 1.09 million t yr-1 of this leaves the lake every year. The rate of sedimentation is 11.7 _ 0.1 kg m-1 yr-1
There is a 460 MW hydropower station at Tana Beles at the outlet of the Blue Nile.
There are two major cities Bahir Dar in the south and Gonder in the north in sub-basins of the rivers flowing into the lake and are potential sources of pollution of the lake.
Lake Tana region has a thriving tourist industry centered on the islands which are home to Ethiopian Orthodox Church monasteries and on the lake.
The lake has about 230 species of birds, including more than 80 wetland species and is an important resting and feeding ground for migrant water birds.
It was designated a UNESCO world biosphere reserve in 2015.
Governance
The lake has great economic potential but is also threatened by degradation of catchment areas, drainage of land for agriculture, overgrazing, water hyacinth, siltation, pollution, agrochemicals, urban sewerages, fecal pollution, agricultural fertilizers, insecticides, herbicides, overfishing, industrial waste, and nutrient enrichment (N and P) (Adugnaw et al, 2019).
A number of programs and interventions are required to address the values and threats to enable the lake sustain ecosystem and livelihood services.
Programs
An evaluation of the extent of agriculture development activities in the Lake Tana basin was undertaken in 2017 to guide management.
Fisheries is increasingly becoming recognized under the agricultural sector policy, as a means of improving nutrition, food security and reducing poverty in the country.
There is a program to manage the UNESCO heritage site.
There is increasing awareness about the impact of water hyacinth to guide its management.
Policies
Ethiopia did not original have fisheries laws which exposed the fishery to overexploitation.
There is a fisheries regulation to guide management of the fisheries.
Institutions
The federal government is responsible for policy and the regional states for the application of the management system.
The Ministry of Agriculture (MoA) is responsible for the overall regulation and supervisory of fisheries management.
Fisheries research is under the Ethiopian Agricultural Research Organization (EARO).
Both the MoA and EARO are coordinated by the Ministry of Rural Development.
Fisheries and community based institutions are being promoted to promote sustainable management of fisheries resources.
Funding
Limited funding is a major constraint to fisheries development and management.
Fishery research centers have limited capacity to respond to problems in the fishery sector due to limited technical and financial capacities.
Management information systems
The federal and regional institutions lack effective structural linkage for information exchange making reporting indirect and not very effective.
Interventions
A mechanism should be developed to enable Ethiopia agree with downstream countries on regulation of the flow of the Blue Nile.
Protect and conserve the endemic cyprinid species.
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Explore opportunities for increasing fish production through aquaculture.
Determine the distribution of aquatic weeds especially water hyacinth and control its spread.
Put in place measures to promote sustainable land management practices.
Establish a buffer zone to protect the lake from agricultural, urban, industrial wastes, nutrients and pollutant from the catchment area.
Improve information exchange between federal, regional, and grassroot institutions
