The changing ecosystem of East Africa’s Mare Nostrum: Using ichthyology collections to identify the changes in the Lake Victoria region

Abstract

The East African lakes of the Lake Victoria region are vital in providing food, water and transportation to one of the continent’s most densely populated regions. These lakes, which besides Lake Victoria also contain lakes Edward, George, Albert and Kivu all support productive inland fisheries. Additionally, they are biodiversity hotspots that harbour over 700 species of endemic fishes. The majority of these belong to the genus Haplochromis, which radiated in each of these lakes in situ through explosive speciation. Because of these radiations, these lakes have served as natural laboratories in speciation research, resulting in extensive collections that are currently housed in natural history museums. Human interference has led to severe ecosystem degradation in many of these lakes. An example hereof is provided by the fishery of lakes Edward and George, which used to be almost entirely focussed on Nile Tilapia, Oreochromis niloticus. Catches of this species have dwindled to the extent that it is currently only the third-most fished species in these lakes. Using collections, we can investigate whether this change in community structure coincided with a change in trophic preferences in the fisheries target species. However, nowhere were ecosystem changes as profound as in Lake Victoria. Here, a combination of invasive species and eutrophication led to an ecosystem turnover in the 1980’s in which a complex ecosystem with many trophic interactions shifted into a simple one with a strong top-down control exerted by the invasive Nile Perch. However, there is still debate whether this change was caused by the introduction of the Nile Perch, Lates niloticus, or whether it was eutrophication that changed the ecosystem. To investigate this, we used museum collections of fishes that were compiled before and after the changes in the ecosystem occurred. We used parasites as proxies of ecosystem complexity using both collections of Lake Victoria proper as well of those of the neighbouring lakes where these stressors are naturally present. This approach allowed us to use a time-for-space approach with Nile perch being native to Lake Albert, Lake George being naturally eutrophic, while Lake Edward being most similar to historic Lake Victoria. Finally, we investigated the origin of the invasive Nile Perch by comparing specimens from Lake Victoria with those of the (potential) source populations in Lake Albert and Lake Turkana.