As discussed in Chapter 4: Freshwater, these systems have a central role in local livelihoods, providing food and water, water for agriculture, tourism, recreation and hydrological power. The introduction of IAS has placed these opportunities under threat. Non-native species of fish and plants have been introduced into freshwater systems to enhance food production, control pests such as mosquitoes, and to promote water purification. However, in many cases they have had adverse effects.

Table 1. Effects of some characteristic aquaculture-related introductions in Africa
Aquaculture Introduction Environmental Impact

Oreochromis niloticus to Kenya Displaced endemic Oreochromis esculentus in Lake Victoria
Tilapia zillii to Uganda Displaced Oreochromis variabilis in Lake Victoria
Osphronemus goramy to Mauritius Naturalized, minimal
Oreochromis macrochir and Tilapia rendalli to Cameroon Naturalized, unknown
Cyprinus carpio to Kenya Displacement of local species
Cyprinus carpio to Zambia Not established
Cyprinus carpio to Malawi Not established
Cyprinus carpio to Zimbabwe Naturalized
Oreochromis niloticus to Zimbabwe Introgression and reduced catches of indigenous tilapias
Clarias gariepinus to Cameroon Naturalized
Carassius auratus to Madagascar May have introduced parasites
Chinese carps to Ethiopia Reportedly naturalized
Ctenopharyngodon idella to South Africa Introduced fish tapeworm
Cyprinus carpio to Madagascar Naturalized
Cyprinus carpio to South Africa Reduced catches of local species; introduced 7 exotic parasites
Heterotis niloticus to Côte d’Ivoire, Cameroon, CAR, Gambia, Congo Naturalized, unknown
Oncorhynchus mykiss to Morocco Unknown
Salmo trutta to South Africa Eradication of local species
Oreochromis niloticus to Madagascar Genetic introgression and replacement of local species

Source: Brummet 2002

Although the use of alien species in aquaculture has had many positive effects, when these species escape or become invasive they can cause significant ecosystem damage. The Mozambique tilapia is an invasive in Northern Africa and has spread worldwide through introductions for aquaculture, as well as into the coastal waters of several WIO countries (IUCN/SSC/ISSG). Established populations in the wild are the result of intentional releases as well as escapes from fish farms. The same is true of the Nile tilapia, from Northern Africa, which has become an IAS in Southern Africa. This species can hybridize with the Mozambique tilapia, threatening its very existence in its native habitat (van der Vaal 2002).

Procambarus clarkii (Louisiana crayfish) has been introduced for aquaculture as well as a biological control agent for snail hosts of bilharzia. It escaped from aquaculture sites and is responsible for the disappearance of water lilies and submerged vegetation, as well as many species of snails, in the wetlands of Eastern and Southern Africa (Howard and Matindi 2003). It threatens the existence of smaller fish, and its habit of burrowing can result in damage to dams and reservoirs. It tolerates a wide range of salinities, oxygen-poor conditions, high pollution and fluctuating water levels. These factors, along with the ability of adult crayfish to travel long distances across land, have made it an effective invasive species (GISP 2004). In Kenya, it was introduced into Lake Naivasha where it has supported a lucrative export industry to Europe. The crayfish has become a keystone species in the lake, resulting in a cyclical boom-or-bust scenario with plants and other fish disappearing and then recovering (GISP 2004).

Box 5: Water hyacinth wreaks havoc

The water hyacinth (Eichhornia crassipes), imported into Africa from the Amazon River Basin in South America, is exploding into large infestations and is causing serious disruption to environments, economies and societies. Rivers, dams, lakes and other water bodies have become infested.

Lake Victoria is one of the most affected lakes. Although it is not known when or how the plant first entered the lake, reports suggest that it has been present in the lake since the 1980s. By 1998 it covered about 20 000 ha and huge mats paralysed activities in ports, villages and bays. At times it has even prevented large and small boats from leaving their harbours. Through biological control methods the water hyacinth has been effectively brought under control, and now only covers 2 000 ha of the lake surface.

In West Africa, the water hyacinth is a major problem, affecting more than 550 km along the Niger River. Countries such as Niger, Mali, Côte d’Ivoire, Nigeria and Senegal use various approaches to control its spread. The measures include chemical, biological and mechanical control.

The economic impacts of the water hyacinth, in seven African countries, have been estimated at between US$20- 50 million every year. Across Africa costs may be as much as US$100 million annually.

Sources: UNEP 2003a, McNeely and others 2001, GISP 2004

Extent of water hyacinth coverage in Murchison Bay, Lake Victoria, Uganda (1995, 1999).

Source: UNEP/GRID and USGS EROS Data Centre in UNEP 2004