|Table of contents
CHALLENGES FOR REALIZING
FRESHWATER AND LIVELIHOODS
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
|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
|Oreochromis macrochir and Tilapia rendalli to Cameroon
|Cyprinus carpio to Kenya
||Displacement of local species
|Cyprinus carpio to Zambia
|Cyprinus carpio to Malawi
|Cyprinus carpio to Zimbabwe
|Oreochromis niloticus to Zimbabwe
||Introgression and reduced catches of indigenous tilapias
|Clarias gariepinus to Cameroon
|Carassius auratus to Madagascar
||May have introduced parasites
|Chinese carps to Ethiopia
|Ctenopharyngodon idella to South Africa
||Introduced fish tapeworm
|Cyprinus carpio to Madagascar
|Cyprinus carpio to South Africa
||Reduced catches of local species; introduced 7 exotic parasites
|Heterotis niloticus to Côte d’Ivoire, Cameroon, CAR, Gambia, Congo
|Oncorhynchus mykiss to Morocco
|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