Ecosystem Structure Trophic Link And Functioning Of A Shallow Rift Valley Lake The Case Of Lake Ziway (ethiopia

A trophic model was constructed for the Lake Ziway ecosystem using Ecopath withrnEcosim application software. This model was used to evaluate and analyze the food webrnstructure and other properties of this ecosystem. The mean Chlorophyll a biomass ofrn70.46 μg L−1was found which is higher than those of earlier reports. Wet weight value ofrnphytoplankton was estimated to be 15.99 tons km-2. A total of 4 cladocerans and 2rncopepods were identified. The biomass of zooplankton is dominated by Thermocyclopsrn(127.73 mg WW m-3). A total of fourteen families of macro-zoobenthos were identifiedrnwhich were dominated by chironomids (15924 individuals m-2) that were attached withrnthe macrophyte Typha anguistifola. The above ground biomass of macrophytesrnvegetation of Lake Ziway was estimated as 3011.68 tons WW km-2. The total mortalityrnvalue for Carp and Catfish was 0.74 and 1.34 year-1 respectively. The total annual catchrnfrom Lake Ziway was estimated at 1127 tons based on the catch and effort data (2008–rn2010). The estimated annual catch in tons per gear type was 683.11, 62.97 and 295.98rnfor beach seine, gill net and long line, respectively. The results of Ecopath with Ecosimrnmodeling showed the presence of six aggregated trophic levels (TLs). The fractionalrntrophic level assigned TLs of Lake Ziway ecosystem between 1.0 and 3.29. The biomassrnflow was highly restricted to I and II trophic levels which contributed 99.76 %. The meanrntransfer efficiency was only 4.4%. The fishery catch consumed 2.5% of the primaryrnproduction in the lake. Low ecotrophic efficiency (EE) of 0.47 was calculated for thernphytoplankton indicating that most of the production remains within the systemrnunutilized. A high value of EE was noted for carp (0.99), indicating that it is subjected tornhigh fishing pressure despite its low biomass. In Lake Ziway, EE values were high for allrnfish groups, indicating that the fish groups were highly constrained by a combination ofrnfishing and predation mortality. Management scenario using Ecosim analysis indicatedrnthat increasing catfish mortality by 1.50 times resulted in increment of tilapia to 1.6 timesrnthe current biomass in four years period and leveling of its biomass to 1.16 times therncurrent value at the end of the tenth year. Doubling in mortality of catfish resulted inrnincrement of tilapia biomass to 1.12 times the current biomass. Decreasing the effort ofrnbeach seine did not benefit the tilapia group as their biomass declined to 0.98 and 0.66rnwhich is related to biomass increment of catfish. The final scenario of increasing therneffort long line by 1.50 and 2.00 times benefited tilapia and significantly decreased therncatfish community from the ecosystem. The total analysis indicated that lake Ziwayrnecosystem has an open niche which can be filled with obligate herbivores to increase thernenergy transfer efficiency of the lake.rnKey words: Ecopath with Ecosim, Biomass, Transfer efficiency, Trophic modeling,rnSpecies introduction

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