Chapter 5 - Plankton and fish nutrition in African lakes

Abstract

Africa, the second-largest continent, is featured by a unique lake system with diverse nature and productivity. African lakes play a critical role in preserving and balancing the global ecosystem. To stabilize the ecosystem of these lakes, a balance must be achieved between their primary productivity (plankton), the main food source for the early stages, and their consumers (fishes). Although several studies on primary productivity and fisheries have been conducted on the Great Lakes and North African lakes, many of Africa’s inland lakes have received less attention. Lake Victoria, for example, is the world’s second-largest freshwater lake; its total annual catch was estimated to be around one million tons, with more than 500 species until the early 1950s. This figure has been reduced by more than half as a result of eutrophication problems. Cichlid fish are the most abundant in African lakes; their omnivorous feeding habits make their early life stages more reliant on plankton for feeding, and this causes the abundance and diversity of plankton communities to have a significant impact on fish production. So, ecosystem-based fisheries management is critical for examining the biology and ecological interactions of the fish. In African lakes, recent global warming has affected plankton biomass and productivity, and the decline in zooplankton production and consequent drop in fish stocks has resulted from a decrease in the production of palatable chlorophytes and an increase in Cyanobacteria as water temperatures rise. The production of plankton varies greatly among African lakes, reflecting their trophic status. Lake Kainji, Nigeria, is an example of oligotrophic lakes, with only 0.3gCm2/h phytoplankton, while the nutrient-rich (eutrophic) Lake Nasser, Egypt, is at the top of the production scale, with an average output of 4.48gcm2/h.