Observations préliminaires sur la répartition et les fluctuations de l'ichtyomasse exploitable dans les différentes zones écologiques du lac Ihema (bassin moyen de l'Akagera, Rwanda)

Preliminary considerations on distribution and changes in fishable ichtyomass in the ecological areas of lake Ihema (Akagera middle, Foodplain, Rwanda). Since 1981, commercial fishing is accomplished in lake Ihema with gillnets of 50 mm meshes and a mean of 175 tons of fish was caught per year during the first three years. Simultaneously, experimental fishing was introduced since 1982, for which ten gillnets of different meshes are used. Fishes of the two general Tilapia and Clarias are captured in commercial fishing, but in experimental one, about ten taxons of fish are caught. For this paper, we examined statistics obtained from experimental catches during 1982 and 1983. Apart from Tilapia and Clarias, small Cichlidae of Haplochromis group, two species of Mormyridae and Synodontis spp represent a potential which may be exploited with other kinds of gear. Concerning productiveness of fishing areas in lake Ihema, littoral zones (= inshore) are more rich than pelagic (= offshore) ones and the northern sector in which Akagera river enters the lake is the most productive. Species of fish are differently represented in ecological zones, following the kind of their food (planctonophagous, detritivorous or carnivorous) and the places they can reproduce. Catches fluctuate monthly and annually probably because of natural changes in fish stock according to fluctuations of climatic conditions, but also because of increasing intensity of commercial fishing. For Tilapia, reduction in the quantity captured may also be due to introgressive hybridization and competition between T. macrochir, T. nilotica and their hybrids