Status of Water Quality and its Implication on Fishery Production.
Within the past five decades, it has been realized that people significantly influence the water quality, which, in turn, affects the fisheries. Since the 1960s, Lake Victoria and its basin have come under increasing and substantial pressure from a variety of human activities. These include deforestation, over-fishing, intense cultivation, animal husbandry and introduction of exotic fish species. Human activities have disrupted biogeochemical cycles with have consequences for the water quality and fisheries of Lake Victoria. Measurements and theoretical analyses have clearly connected observed changes in the watershed and water quality and conclusions have been drawn that aquatic biota including fisheries are strongly influenced by disturbances around and within Lake Victoria The Lake Victoria basin has clearly shown signs of population pressure and associated land use changes to and within the lake. Nutrient enrichment in the water column, sediment and in rainfall over Lake Victoria accelerated after the 1960s. Phosphorus concentrations have risen by a factor of 2 to 3. Atmospheric loads of nutrients, especially of nitrogen (N) and phosphorus (P) have increased due to biomass burning and deforestation in the catchment. These high nutrient concentrations support elevated algal primary production and algal biomass that have risen by factors of 2, and 6 to 8 respectively. Algal and invertebrates species composition have responded to changes in water quality. Algae are now dominated by the blue-greens with types that are potentially toxic. Elevated algal production probably supports the 4 to 5-fold rise in fish production in the lake since the 1950s. Increased fish production is good as more local people have turned to the lake for their livelihood and fish exports to international markets earn foreign exchange for the riparian states. But problems associated with nutrient enrichment (eutrophication) impacts of elevated algal biomasses and proliferation of the obnoxious water hyacinth (Eichhornia crassipes) threatens continued beneficial use of the lake resources. High algal biomass and increased sediment have contributed to a 4-fold reduction in the lake transparency since the 1960s. Reduced lake transparency is believed to have accelerated hybridization of some cichlids that are picky in choosing mates and use visual cues of bright male coloration to identify suitors of their own species. Deoxygenation of deep waters is another undesirable change that has precluded a stable demersal fishery. Seasonal bottom oxygen deficiency created by decomposition of algal biomass and aggravated by stronger thermal stability directly affects the distribution of organisms including invertebrates and fish. Low oxygen has led to loss of approximately 50% of aerated fish habitat since the 1960s, and also lowers potential fish production. Among the invertebrates, Caridina nilotica has become a keystone species as it is resilient to low oxygen conditions. Low oxygen conditions (anoxia) and algal blooms are often associated with fish kills in Lake Victoria.
| Main Authors: | , , , , |
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| Format: | Book Section biblioteca |
| Language: | English |
| Published: |
Lake Victoria Fisheries Organization
2005
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| Subjects: | Water quality, Fishery development, Nutrients (mineral), Algae, Eutrophication, |
| Online Access: | http://hdl.handle.net/1834/7202 |
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