FAO published a CIFA Technical Paper on the Status of African Reservoir Fisheries in 1984 (Kapetsky and Petr, 1984). Syntheses of information on the fisheries in reservoirs in seven countries were included; they present summaries of fishery research experience and development activities. As a companion to the update of the 1984 publication FAO considered it useful to prepare such summaries also for other large man-made lakes in Africa. It was decided to review existing literature and synthesize the information for artificial lakes with surfaces exceeding 300 km2.
A list of 13 reservoirs with their respective surface areas is presented in Table 1.1. It should be noted that potential fish yields (tonnes/year) are included, estimated by Crul (1992) using a yield-predicting model based on catch versus area relationships. Valuable background information on the reservoirs was obtained from the Source Book for the Inland Fishery Resources of Africa - SIFRA (Vanden Bossche and Bernacsek, 1990a, 1990b, 1991). The Source Book provides morphometric, chemical, physical as well as fisheries production data for the majority of African inland waters. Of the 13 selected reservoirs four are situated in Cameroon, two in Côte d'lvoire, two in Mali, one in Sudan, one in Tanzania, one in Zaire and two in Zambia.
Kapetsky (1986) stated that the annual fish production from reservoirs provide about 10% of the 1.4 million tonnes from Africa's inland waters. He concluded that development of reservoir fisheries had included a wide variety of activities- such as brush and tree clearing, boat-building, training in fishing methods, improvement of processing techniques, establishment of transport systems, market facilities and organization of fishermen's cooperatives and credit schemes. These activities were the result of numerous fishery development projects.
Entz (1984) summarized the information on reservoirs obtained during a number of FAO/UNDP projects in Africa. The objectives of those projects were, among others:
For fish stock estimates the morpho-edaphic index was applied to provide preliminary indications of catches to be expected. At later stages catch statistics and results of biological research provided more accurate assessments.
The present study presents an overview of information available on:
(The following paragraphs are based on Entz' general findings.)
Forecasting of the fisheries potential of tropical man-made lakes in their filling stage or during the transitional period is very difficult, even when based on morphoedaphic index (MEI) and stock assessments. Adjustments usually have to be made at a later date. The maximum sustainable yield can be estimated prior to regulation of fishing activities, before the stabilization phase has been reached, but the maximum sustainable yield (MSY) based on MEI and other parameters collected later on should be adjusted from time to time.
The riverine organisms do not disappear permanently from the reservoirs. When the initially low dissolved oxygen concentration rises, some elements of the riverine fauna may invade the reservoir from upstream and occupy some lacustrine niches in the lake. Some typically riverine fish may establish themselves in well-oxygenated, wave-exposed rocky habitats. Phytoplankton frequently causes blooms during the formative stage of a reservoir. Zooplankton, initially represented mainly by rotifers, but later on with an increasing proportion of copepods and cladocerans, may become abundant. There may still be some vacant niches, like those of plankton-feeding fish. The establishment of indigenous pelagic fish in new lakes is usually slow and the stocks may remain poor for a long time.
Initially, the unstable water level of the new reservoir does not allow a rooted emergent vegetation to form, but there may be an explosive growth of floating vegetation. When the submersed vegetation becomes more widespread, it provides an excellent habitat for chironomids, shrimps, molluscs and for periphyton feeders such as tilapias.
The spread of molluscs is frequently associated with the spread of the disease schistosomiasis, as molluscs such as Bulinus and Biomphalaria function as host organisms for Schistosoma parasites. Other invertebrates may become excellent food organisms for fish.
The biological succession is both temporarily and spatially differentiated along the axis and major limbs of the reservoir. Longitudinal differentiation is more obvious in reservoirs where the filling process takes several years. In some reservoirs, areas near the dam have lacustrine features, while the central sections of these lakes may still have a semi-riverine character - at least seasonally during floods; the upper reaches generally have riverine characteristics (Entz, 1984).
