What affects fisheries in African lakes: Climate change or fishing effort? A case study from Lake Kariba

Abstract

Lake Kariba, created in 1958, experienced changes, notably the decline of the Limnothrissa miodon fishery, which have been attributed to climatic change. Air temperatures rose abruptly by 1.1°C between 1980 and 1981, but the temperature of the lake did not follow this pattern. Temperatures at 10 m depth increased by 8% between 1961 and 1971, remained stable until 1984, then declined and by 1992 the temperature was about 7% lower than in 1961. The causes of this are unclear but it followed the hot and dry El Niño droughts of 1982-83 and 1991-92. The lake warmed again by 2007-2011 with temperatures at 10 m being about 10% higher than in 1961, while at 40 m it was 16% higher indicating a faster warming rate in deeper waters. The thermocline fell from 15-20 m in 1968 and 1986 to 20-25 m in 2011 and the temperature gradient decreased by > 50%. The epilimnion became more homogenous, with no evidence of anoxia in the upper 20 m in 2007-08, and the normal monomictic thermal regime may change, thus affecting nutrient circulation and the seasonal abundance of plankton. Both zooplankton and phytoplankton communities have evolved since the lake was created, but these changes occurred before any evidence of warming. The fishery has been declining since 1996 but there is no evidence that climate change is responsible; the number of fishing vessels is presently about three times the recommended level and fishing effort is almost certainly the main cause of the problems. All African lakes support fisheries and it is essential to consider fishing, which changes fish species composition, demographics and abundance, characters that could also be affected by climate change.