Potential Infjuence of Fish Casge Farming on water quality, phytoplankton Biomass and plankton species composition in Fish ponds: a case study in the rift valley and North Shoa Reservoirs,Ethiopia.
2008
Fasil Degefu Abera(Researcher)
The potential impact of Tilapia (O. niloticus) cage culture on water quality parameters and subsequent primary production and/or biotic composition of small water bodies (Alagea and Yemlo reservoirs) in Ethiopia were investigated between October and February 2008. Core objective of this study was to assess the difference between the cages and open water in relation to water quality parameters with may be attributed to intensive imputs of fish waste and left over fish feed. It was observed that all physico-chemical water qulaity parameters including inorganic nutrients varied temporally over the study period coupled with dry and wet periods.The reservoir's trophic state raged from eutrophic to hypereutrophic with strong correlation between chlorophyll-a and total phosphorus.High solar radiation and elevated nutrient levels from tererrestrial runoffs probbly have bigger impacts on the eutrophic state than fish caging in the reservoirs. Dissolved inorganic nutrient concerntrations and other physical parameters showed no significant differences between the cages and open water except for dissolved oxygen and ammonium nitrogen, which were lower and higher in the cages respectively.The mean values oberved were 10µgL-1 and 13µgL-1 for SRP; 105µgL-1and 98µgL-1 for total phosphorus; 3mgL-1 and 3.2mgL-1 for nitrate-N concentration was 17µgL-1 and 6µgL-1 while nitrite-N concentration was 36.2µgL-1 and 38.7µgL-1 in the cages and open water respectively. The phytoplankton community was dominated by cyanobacteria (84% of the total phytoplnakton abundance) in particular by Anabaenopsis sp. in Alagea reservoir whilst Chlorophyta (70%) with pediastrum sp. as dominant genus prevailed in Yemlo reservoir.The zooplankton species composition of both reservoirs was simple and is highly dominated by rotifers which contributed 43 and 70% to the total zooplankton in Alagea and Yemlo reservirs respectively.
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