EcoJogical-economic modelling and implications of land use change and wetlands extent on freshwater fisheries: the case of Lake Victoria (East Africa)

Watershed degradation and the conversion of wetlands to alternative uses affect
water quality, having profound implications for the freshwater ecosystem and fisheries of
Lake Victoria. Through dynamic simulations using the Ecopath approach we show that
eutrophication may explain the sudden upsurge of the introduced Nile perch during the
1980s. During these years an important trigger of change was an abrupt shift in primary
productivity due to an external shock, probably related to an El Nino-ENSO event. The
Nile perch population explosion during the 1980s caused a profound transformation ofthe
fishing industry leading to a dramatic increase in fishing effort. Our equilibrium analysis
from the base values of an Ecopath mo~l for year 1989 depicts an overfishing situation.
To quantify the combined effect of eutrophication and fishing effort on the-aggregate fish
stock and fishery we specify and estimate a biomass dynamics model, which embeds a
measure of phytoplankton biomass as an environmental variable. We found that in a
eutrophic state, which for the Kenyan waters of Lake Victoria was above an estimated
chlorophyll-a concentration threshold of 17.11 mg m-3
, a further increase in phytoplankton
biomass would cause a decrease in aggregate fish stock biomass and landings respectively.
We then model the relation between land use change and water quality, considering the
nutrient retention function of wetlands. First, we identify catchment-based indicators, in
particular price indicators and wetland extent, which may anticipate changes in
,.
environmental variables driving stock assessment models. We then study both nutrient
loading and wetland reclamation externalities to the fishery sector, focusing on the Yala
swamp, one of the largest wetlands in the Kenyan basin of Lake Victoria. Finally we
propose a spatially distributed land use tax as policy instrument for reaching the desired
objective of changing farmers' behaviour and maximising the total economic benefit to
society.