Species Introductions in Lake Victoria: Nile perch
In addition to pollution and habitat fragmentation and destruction, the introduction of alien plant and animal species is a major contributor to biodiversity loss in the Nile River basin. The issues resulting from the introduction of the Nile perch into Lake Victoria are briefly described below to highlight this issue.
The introduction of Nile perch has improved fish yields from the lake.32 The market for Nile perch is strong, and large commercial fishery has developed. However, because of the high demand for Nile perch, most of the fish are exported. Small traditional fisheries that were based on the native fish have been replaced by the large commercial fisheries. Because Nile perch is oilier that other fish in the lake, it is more difficult to dry. This has caused an increased demand for wood from the surrounding forest for fires to dry the fish, and the loss of forest has caused an increase in nutrient and pollution runoff into the lake.33
It is not currently known why Nile perch only became dominant in the 1980s. One hypothesis is that eutrophication from increased nutrient inputs has caused the deeper areas of the lake to become depleted in oxygen;34 the low oxygen in the lakes has caused numerous fish die-offs. Eutrophication has also caused a shift in the phytoplankton species composition, and it is thought that these phytoplankton may be inedible to many of the lake’s cichlids. Therefore, eutrophication, coupled with the high predation pressure from Nile perch, may have tipped the balance in favour of Nile perch, increasing the numbers of this species and leading to loss of fish diversity in the lake.35
Nile perch is being overfished and its population size is now decreasing. This has caused a resurgence in some of the species that had decreased after Nile perch was introduced. Some species that were thought to be extinct are now being found in the lake again. Overfishing, in this case, may have led to an increase in fish diversity in Lake Victoria.36