Exploitation or eutrophication as threats for fisheries? Disentangling social and ecological drivers of ecosystem changes in Lake Victoria, Tanzania.

Lake Victoria is the second largest freshwater lake and supports the largest freshwater commercial fishery in the world. Eutrophication and fisheries drive Lake Victoria’s changing ecosystem with far-reaching consequences for exploitation patterns, livelihoods and trade. Yet, no attempt has been made to link the two processes to understand feedbacks in food webs, resource use patterns and trade.

Our main objective is to unravel the social and ecological drivers of ecosystem change and to develop long-term strategies to deal with the risks of these ecosystem changes. The working hypothesis is that continued eutrophication presents a much graver risk to the resource base and livelihoods of coastal populations than fishing pressure. Initially eutrophication resulted in a higher carrying capacity for Nile perch, because food availability increased through enhanced primary production.

This increased carrying capacity resulted in an apparent resilience of the exploited Nile perch stocks to increased fishing pressure. However, the compensation of increased fishing pressure by increased production could fail when eutrophication becomes too strong,because of increasing self-shading by algae and by an increasing anoxic hypoliminion, which can cause catastrophic fish kills. Optimal nutrient concentrations to support fisheries may have already been exceeded, which could negatively affect Nile perch biomass.

The responses of the fishery to changes caused by increased eutrophication as well as the responses of Nile perch stocks to the combined impacts of size selective fishing and eutrophication will be researched in four inter-related projects that:

1. analyse social factors that drive decisionmaking processes of individuals in the fishery;
2. analyse ecological factors, including sizeselection, that drive decisions about spatial effort allocation by fishermen;
3. analyse the impact of eutrophication and Nile perch predation on food web structure, and
4. model the interactions and feedbacks resulting from eutrophication and fishery, as most likely factors driving changes in Lake Victoria’s food web.

Scenarios to assess management under nonsteady state conditions will be developed in collaboration with international scientific experts and with regional and national government and research institutions. A networking NGO will be instrumental in communicating results to fishing communities and the general audience.