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Assessment of the Ecological Status and Threats of Welala and Shesher Wetlands, Lake Tana Sub-Basin (Ethiopia)







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    Shesher and Welala FloodplainWetlands (Lake Tana, Ethiopia):Are They Important Breeding Habitats for Clarias gariepinus and theMigratory Labeobarbus Fish Species? 2012
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    This study aims at investigating the spawning migration of the endemic Labeobarbus species and C. gariepinus from Lake Tana, through Ribb River, to Welala and Shesher wetlands. The study was conducted during peak spawning months (July to October, 2010). Fish were collected through overnight gillnet settings. A total of 1725 specimens of the genus Labeobarbus (13 species) and 506 specimens of C. gariepinus were collected. Six species of Labeobarbus formed prespawning aggregation at Ribb Riv er mouth. However, no Labeobarbus species was found to spawn in the two wetlands. More than 90% of the catch in Welala and Shesher wetlands was contributed by C. gariepinus. This implies that these wetlands are ideal spawning and nursery habitats for C. gariepinus but not for the endemic Labeobarbus species. Except L. intermedius, all the six Labeobarbus species (aggregated at Ribb River mouth) and C. gariepinus (spawning at Shesher andWelala wetlands) were temporally segregated.
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    Growth, biomass, and production of two small barbs (Barbus humilis and B. tanapelagius, Cyprinidae) and their role in the food web of Lake Tana (Ethiopia) 2012
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    Growth, biomass and production of two small barbs (Barbus humilis and Barbus tanapelagius) and their role in the food web of Lake Tana were investigated. From length–frequency distribution of trawl monitoring surveys growth coefficient, U0 values were estimated at 3.71–4.17 for B. humilis and 3.70–4.14 for B. tanapelagius, respectively. Values for B. humilis were confirmed in pond experiments. Mean biomass of the small barbs was 13.3 kg fresh wt ha-1, with B. humilis being most abundant in the littoral and sub-littoral zones, whereas B. tanapelagius was most abundant in the sub-littoral and pelagic zones. The two small barbs had a production of 53 kg fresh wt ha-1 year-1. Although their P/B ratios of about 4.0 were relatively high for small cyprinids, both their biomass and production were low in comparison with other small fish taxa in other tropical lakes. Of the zooplankton production only about 29% was consumed by the small barbs. However, they did not utilize calanoid copepods, which were responsible for approximately 57% of the zooplankton production and it is likely that small barb production was food limited during certain periods of the year. Piscivorous labeobarbs consumed about 56% of the small barbs production annually, but additionally, Clarias gariepinus, and many bird species were also preying on them. Therefore, limitation of Barbus production by predation during certain periods in the year cannot be excl uded.
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    The decrease in aquatic vegetation in Europe and its consequences for fish populations (1987) 1987
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    A diverse aquatic vegetation is essential to maintain a diverse fish fauna. The fish is an important part of a complex network of relations between nutrients, phytoplankton, epiphytes, herbivorous invertebrates, the aquatic vegetation and fish. In Northwest Europe and North America and probably in the rest of the industrialized world, the (submersed) aquatic vegetation (macrophytes) is rapidly disappearing from eutrophicated waters. The decrease is well documented. As a consequence of abun dant growth of epiphytes, which are better competitors for inorganic carbon and light in highly eutrophicated waters than submersed aquatic macrophytes are, the condition of the aquatic vegetation becomes worse. Shallow, eutrophic, relatively clear water that is rich in water plants, can change to phytoplankton dominated turbid water, within short time. This change may occur without a remarkable increase in the actual nutrient loading. Invertebrate grazers like snails, macrocrustaceans and cl adoceran zooplankters are able to protect aquatic macrophytes against the negative effects of this competition by removing epiphytes and phytoplanktonic algae. As a man predator on invertebrates, the fish indirectly influences the well-being of the aquatic vegetation. There is evidence that aquatic macrophytes are the source of biochemical compounds that negatively affect the growth of algae (allelopathy) and attract grazers. These processes are mainly found in model systems and under semi-na tural conditions. Their ecological significance still has to be tested in the field.A situation with turbid, phytoplankton dominated, water without aquatic vegetation can continue after removing nutrients from effluents because: (i) blue-green algae (phytoplankters) may excrete toxic substances, negatively affecting the growth of aquatic macrophytes; (ii) abundantly occurring young fish, but also invertebrate animals like mysids, prey on the bigger (phytoplankton grazing) cladocerans; (iii) acid rain, polluted bottom sediments and/or bird flocks contribute to the nutrient loading of a water body. Restoration techniques are: lowering the nutrient loading in combination with protection of the remaining stands of reed, replanting of aquatic plants, creation of artificial refugia for zooplankton and manipulation of young-of-the-year fish populations. Chemical and mechanical control of "nuisance" growth and heavy stocking with herbivorous fish including the common carp (Cyprinus carpi o) have to be omitted or executed very carefully to avoid phytoplankton-dominated turbid water. In small systems with "nuisance" growth, stocking (50-150 kg/ha, max. 250 kg/ha) with grass carp (Ctenopharyngodon idella) can improve the water quality.

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