Patterns of resurgence and anthropogenic influence on trophic sources and interactions among fishes of Lake Victoria, Kenya

Lake Victoria, the world's largest freshwater lake in the tropics, along with its associated rivers, was once home to a spectacular assemblage of fish species dominated by two families: Cichlidae and Cyprinidae. However, in the last three decades the lake has undergone a precipitous loss of ecological complexity and ecosystem functions driven by habitat degradation, species invasions, overharvesting and pollution. Indigenous fish species dwindled with many feared extinct. The resulting trophic reorganization may be the most dramatic ever witnessed by ecologists. Past focus has been on the littoral fishes of Lake Victoria; this study examined the status of the major riverine and pelagic fish species. 13C and 15N isotopes were used to infer human influences on dietary sourcing to both indigenous and introduced fish species. The dual isotope approach was also used to determine trophic interactions amongst the dominant fish species in the watershed.; Survey of the rivers using electrofishing equipment revealed considerable numbers of the large cyprinids B. altianalis and L. victorianus, species that are now very rare in the lake. Their habitat preferences were assessed, revealing that reduced range and habitat due to human activities can dampen their resurgence. All fishes showed isotopic signatures depicting both C3 and C4 plants as sources of carbon. B. altianalis and L. victorianus from sites under direct industrial influence were enriched in heavy carbon year-round, indicating site fidelity for what should be migratory species. This strong imprint of land use patterns implies that surviving sub-populations of these two species are now nonmigratory.; On the basis of an isotope model and signatures, many fish species had unexpectedly high trophic overlap and hence functional redundancy in the rivers and pelagic realms. This could help to explain the lake's remarkable ecosystem resiliency in the wake of a mass extinction.; I additionally conclude that the reemergence patterns of fishes in the different aquatic compartments of the Lake Victoria basin are a sign of hope. If persistent elements of the indigenous fish community were nurtured and human activity regulated under a unifying systems approach, a future diverse and ecologically efficient Lake Victoria could well be achieved.