Modeling trophic interactions and the impact of an introduced exotic carp species in the Rift Valley Lake Koka, Ethiopia

The study aims to describe the energy flow and trophic structure, to assess the ecological stage of maturity, the ecosystem impact of the introduced exotic carp species and the current fishery on the Lake Koka ecosystem using a mass-balance trophic model. The Ecopath approach and software were used to represent the lake ecosystem, and input parameters were obtained from in-situ surveys and from published lake specific sources. The model was balanced as per the recommended ecological and thermodynamic constraints. The low ecotrophic efficiency found for adult catfish, primary producers and detritus suggest that these resources are not fully utilized, while other groups have moderate to high values. The annual fishery catch represents only about one sixth of the total fish biomass, and the exploitation rates of target species are well below 0.5 suggesting under-exploitation. The dietary intake of fish-eating birds is 5.8 t km-2 year-1 almost twice the catch, showing the lake's important role for sustaining the bird populations and hence, caution needs to be taken before implementing any further fisheries development intervention. The mean trophic level of the catch (2.55) is intermediate when compared to other tropical lake systems and reflects the mixed fishery of the lake. Mixed Trophic Impact and Niche Overlap analysis reveals that, contrary to what may be expected, the impact of the introduced exotic carp species on the lake system is less significant than that of the native species. Besides its socio-economic significance, the detritivorous carp seems to fill an unutilized niche and helps the system in recycling organic matter. The P/R value (3.2) and other system maturity indices indicate that Lake Koka is as yet an immature, developing system. The model can be used as a reference for exploring ecosystem-based fisheries management scenarios aimed at sustaining fish stocks, ecosystem services as well as ecological integrity.