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.

Lake Victoria is the single most important fishery in Africa and has one of the highest concentrations of fish species in the world. Historically, the lake has been important in small, traditional, sustenance fishing, with a very diverse assemblage of cichlid fish, some of which are not found outside of the lake. However, the introduction of a voracious predator, the Nile perch, has drastically changed the fishery in Lake Victoria.

Nile perch. Photo: William Taylor

Nile perch is native to several waters in Africa, but it did not exist in Lake Victoria. In an effort to increase fish yield, Nile perch and Nile tilapia were introduced to the lake in the 1950s.³⁰ Nile perch eats a wide variety of fish, including many cichlids, the cyprinid Rastrineobola, and the prawn Caridina. It also eats its own juveniles to an appreciable extent. Many of the cichlids, plus the other fish, were important in the traditional fisheries in Lake Victoria.  Nile perch did establish in the lake but did not become dominant until the 1980s. At the same time, the populations of nearly all the other fishes declined in Lake Victoria. More than 200 species are thought to have become extinct; this has been described as the largest vertebrate extinction caused by a single human action,³¹ and Nile perch has been listed among the top 100 invasive species in the Global Invasive Species Database (2005). The lake food web is now dominated by Nile perch, and to a lesser extent, Nile tilapia.

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  Small traditional fisheries that were based on the native fish have been replaced by the large commercial fisheries.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