Ecosystem structure drivers and simulation scenarios of a shallow Lake Ziway, Ethiopia

A trophic model was constructed for Lake Ziway, Ethiopia, using EwE suite aiming to analyse and evaluate food-web structure, function and ecosystem properties. The input parameters were obtained from a survey of primary producers, zooplankton, macroinvertebrates and fish studies and published materials. Various levels of fishing effort were used to simulate and produce different scenarios. The biomass flow of the ecosystem was restricted between trophic levels I and II contributing 99.76%. The mean transfer efficiency was only 4.4%. The fishery catches consumed 2.5% of the primary production, and most of the production remains within the system unutilised (ecotrophic efficiency = 0.47). Fish groups were highly constrained by a combination of fishing and predation mortality as supported by high EE. Ecosim scenario analysis indicated that decreasing fishing effort of beach seine by half would increase the biomass of carp to six times in 10-year period, which might have increased turbidity and competition with tilapia. Furthermore, a high abundance of African catfish may escalate predation mortality on tilapia. Thus, contrary to what the managerial body expects, tilapia may not benefit from beach seine restriction alone. The models have captured the increasing fishing yield of carp, which was not the case when the model was created.