Isotopic evidence of functional overlap amongst the resilient pelagic fishes of Lake Victoria, Kenya

Abstract

The Lake Victoria ecosystem once hosted a diverse fish community dominated by a large species flock of haplochromine cichlids. Today this fish assemblage is highly altered by anthropogenic activities, with at least half of the indigenous species either extinct or very rare. The fauna and flora of the tropic’s largest lake are still in flux, and little is known about even the most basic ecological questions, such as the source of carbon and nitrogen for its spectacular productivity. The actual food-web structure is difficult to determine with results based on traditional gut analysis. At the moment both scientists and conservationists are at odds as to whether there is any evidence of functional replacement or functional overlap amongst the resilient pelagic fishes of Lake Victoria. This study used a static stable isotope model to investigate the source of carbon and the extent of the trophic overlap amongst three pelagic fishes, Rastrineobola argentea, Yssichromis laparogramma and Y. fusiformis in the Kenyan waters of Lake Victoria. The d13C of plants ranged from )8.8 to )24.6&, while fishes ranged from )18.6 to )24.5&, suggesting assimilation of mostly C3 sources for the fish species. On the basis of the isotope model, it is evident that Yssichromis laparogramma, Yssichromis fusiformis, and Rastrineobola argentea obtained from 90 to 97.5%, with an average of 94.2%, of their food from the same trophic level. The very high degree of overlap in the effective trophic level of these three zooplanktivores has important ecological and conservation implications.