Assistant Professor Gettysburg College Environmental Studies Department, United States
Background/Question/Methods
Lake Turkana, located in northern Kenya on the Ethiopian border, is the world’s largest permanent desert lake. Over 90% of its flow originates from the Omo River in Ethiopia, which causes the lake to have seasonal rises and falls in water level. This river has been subject to the construction of five dams (the Gilgel Gibe dams) and several irrigation projects. These developments will impact the amount of water entering the lake, with implications for lake chemistry and productivity, littoral habitat availability, and the breeding biology of the lake’s fishes. The lake’s fisheries provide an important source of protein and income for the surrounding populations, particularly during periods of drought. Though ongoing upstream changes are likely to have serious impacts on the fishes and fisheries of Lake Turkana, this system remains drastically understudied. The purpose of this project is to develop an understanding of the food web and ecology of seven key fisheries species in Lake Turkana before the installation of the Gibe III dam. We used Bayesian mixing models in the statistical software R and stable isotope data (𝛿15N and 𝛿13C) collected between 2011 and 2013 at three sites in Lake Turkana to estimate key species’ diet.
Results/Conclusions
We found that site or site type (littoral or pelagic) was an important driver of diet for the majority of the species studied. There were also clear ontogenetic dietary shifts in four species. Aquatic insects, which are likely to be highly impacted by changes in water level and fluctuations, were an important diet item for many species, including key fisheries species Lates niloticus and Orecromous niloticus. Phytoplankton and zooplankton were an important diet item for low-trophic level fishes, including tilapias and pelagic schooling species. Previous research suggests declines in primary production due to dam-associated reductions in water fluctuations and, as a result, resultant nutrient pulses to the lake. Comparison of earlier diet studies to findings in this study show a decline in fish as a diet item in some larger predatory fish, which may be because of declines in the lake’s small pelagic planktivores. This study highlights vulnerabilities in the diet of key fishery species in a large lake experiencing ongoing ecosystem change. Findings of this study are the first step in understanding how upstream projects will impact the fisheries of Lake Turkana by providing a baseline assessment of the food web structure.
