use of compound feeds, pesticides and antibiotics. The spillage of these substances into natural aquatic systems may negatively affect these ecosystems.

African fish biodiversity is not well-known; only very few species are well-known, particularly the Cichlid family (Tilapia and Nile perch being the star fish groups). Large fish such as Distichodus sexfasciatus, Labeo sp, Mormyrops anguilloides, Bagrus sp., Synodontis sp., Schilbe mistus, etc. and various catfish species are not well-known. The western, southern and eastern African sub-regions aquatic ecosystems and biodiversity are quite well studied, while the Congo Basin is poorly known and scientific work is needed.

 Africa has a large potential for fish farming, with 37% of its surface area suitable for artisanal aquaculture and 43% suitable for commercial fish production (Aguilar- Manjarrez and Nath, 1998). Africa contributed about 4% to global aquaculture production and has been expanding production since 1984 at a rate equal to or greater than the global rate, albeit from a much smaller base (FAO, 1997). Traditional extensive African aquaculture systems are common throughout SSA region, especially in the West African sub-region (ICLARM-GTZ, 1991). Nigeria and Egypt provide 90% of output from African aquaculture, Madagascar and Zambia (together) contribute 4% and the rest of the continent provides 5-6% (FAO, 2000b).

Drain-in pond types (ouedos and ahlos) are mainly used to culture tilapias (Cichlids) in West and central Africa. In the D.R. Congo, especially the Imbongo region (Kikwit), drain-in ponds are traditionally used to culture tilapias, catfish (locally named ngolo) such as Clarias angolensis, C. lazera, C. gariepinus, etc., and a common edible aquatic herb (Hydrocharis chevalieri) primarily for local consumption. Where cultivated, each family in the village possesses its own pond for Hydrocharis cultivation (Brummett and Noble, 1995; Musibono and Mbale, 1995).

Over 90% of cultured fish in SSA come from earthen ponds of 200-500 m2 fed with locally available, low-cost agricultural by-products and with limited yields (1000-2000 kg ha-1 yr-1) (King, 1993). Periurban areas are promising zones for the development of aquaculture as an important source for nutrients and income generation and are playing an essential role in aquaculture, especially in Malawi, Cameroon, Nigeria, Ghana, Zambia and D.R. Congo.

In Zambia, the Kafue Fisheries Company, with 1870 ha, is the largest integrated fish farm in Africa. They produce indigenous tilapias (Oreochromis andersonni, O. mossambicus, O. niloticus), catfish (Clarias gariepinus) and carps (Cyprinus carpio). In the D.R. Congo, aquaculture was important during the colonial period (prior to the 1960s). Many aquacultural stations, such as Gandajika (Kasai), Nzilo (Katanga), Atwum/Imbongo (Bandundu), Kasangulu (Bas-Congo), were productive. After independence, the US Peace Corps tried to rehabilitate these ponds and to promote new aquaculture stations countrywide without success. Now aquaculture is getting more attention around large cities. In Kinshasa, for example, the Monastery “Notre-Dame de l’Assomption”, Kimpoko/Nsele, Maluku, Kasangulu, Bateke, Mungulu-Diaka aquaculture stations are promising. Flooded areas along river courses (e.g., Congo, Kasai, Kwilu, Lamon rivers) are also periodically used for fish culture (Musibono, 1992).

          Aquaculture in the SSA is still negligible compared to the potential offered by water resources and aquatic species (fish, crustaceans, snail, alligators, plants, etc.). AKST input into capture fishing and aquaculture is still very low, though increasing. Over 7,502 freshwater fish species are distributed in natural water bodies of 48 countries. Africa also boasts large natural and man-made lakes, which are important fish habitats and conservation areas (WRI, 1998).

Bio-invasion. Alien species are organisms that have been introduced intentionally or accidentally outside of their natural range. Alien invasive species are considered to be the most detrimental to pristine ecosystems and their dependent biodiversity (Williamson, 1996; McNeely, 2001). The Nile perch, Lates niloticus, intentionally introduced to Lake Victoria (Uganda) in the 1960s has tremendously reduced the indigenous tilapia population (WRI, 2002). In the Congo River, the invasive fish species Heterotis niloticus, accidentally introduced from Upper Ubangi River in the 1970s, is colonizing water bodies, especially rivers (Shumway et al., 2002).

Invasive species can be plants, algae, microorganisms, fish or other aquatic taxa. Water hyacinth, Eichornia crassipes, brought from South America by colonial administrations in the 1800s, is now widely spread in rivers and lakes, ponds, etc. In ponds, aquatic ferns, Salvinia nymphllula, Salvinia molesta and the Nile salad Pistia stratiotes, which are native, have become invasive and are reducing fish production. A native fish Citharinus gibbosus of the Congo River seasonally becomes invasive rendering fishing less productive. The water hyacinth case is a key example of the complexity of invasive species establishment and management (Rachmeler, 2003; Bartley and Martin, 2004; Howard, 2004).

Management of invasions. The best management strategy is based on prevention. Similar to other ecosystems, control of invasion in water-dependent ecosystems can be accomplished through:

1. Mechanical methods (removal, destruction, trapping or catching);
2. Chemical application (control by pesticides/herbicides and poisons);
3. Biological process (control of exotics being done by exotic biocontrol agents);
4. Ecosystem manipulation (such as the management of watersheds, water management, pollution control, competition with crops or local species); and
5. Integrated management (based on the association of some or all above strategies).

Biological pest control may be the best solution from the ecosystem health perspective, but the response may be slow. In SSA water bodies, for example, biological control of water hyacinth Echhornia crassipes using insects was not successful (Rachmeler, 2003).

The best invasive control, in many cases, is to give economic value to the invasions. When invasive fish species like Heterotis niloticus (also named Kongo sika or Zaiko in the D.R. Congo) is converted into capture and well marketed, fishing pressures on it will increase (reducing the negative impact on the ecosystem).