Abstract
The fish fauna of Gebel Aulia reservoir (White Nile) consists of 52 species with Alestes baremoze Joannis and Brycinus nurse Riippell dominating in numbers and biomass. The seasonality and condition of A. baremoze, A. dentex (L.) and B. nurse are described in relation to various limnological factors in the reservoir, close to the dam. The zooplankton assemblage is small-sized, consists mainly of small Cladocera and shows striking seasonality. The hydrological regime, temperature and the grazing effect of fish are the major factors influencing its development. The three species belong to a eurytopic but basically riverine fish guild. They show a seasonal cycle of abundance with high numbers at periods of water storage (October-April) and low numbers during riverine conditions. This seasonal pattern of occurrence, the size structure of the population and the maturity indices give strong evidence that no resident population is present at the study site, but upstream migrations of the immature fish occur at low water level (May-September). Return migrations of juvenile fish into the area at the dam proper take place from October until April.
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