Kenya Marine and Fisheries Research Institute, P.O.Box 1881, Kisumu, Kenya. E-mail: Kmfkisu@net 2000ke.com or Owili@yahoo.co.uk

Diets, selective feeding and the structure of the zooplankton community assemblage at the river-lake interface: Predatory impact by the zooplanktivorous Rastrineobola argentea and the juvenile Lates niloticus.

Trophodynamics of the zooplankton community were studied over a period of four months, focusing on zooplanktivorous Rastrineobola argentea and larval/juvenile Lates niloticus and their zooplankton prey. The major objective was to describe the distribution, abundance and the predatory impact by the mentioned fish species. The study was carried out along three transects set from the Sondu miriu, Awach and Oluch river mouths to approximately 2 km into Lake Victoria. The predator and prey abundances were estimated from beach seine catches and vertical hauls, respectively.

Copepoda dominated the zooplankton community. Zooplankton was the main prey item in the diet of R. argentea and the larval L. niloticus, however, there were negligible incidences of insectivory even in the smaller fish. Larger L. niloticus included the fresh water shrimp Caridina nilotica and fish in their diet. Spatial and temporal variations and ontogenic shifts were observed in the diets of both species. There was evidence of food resource partitioning at inter- and intra-species levels. Calanoida, Cyclopoida and Cladocera prey were utilised in different proportions depending on the site of sampling and the size of the predator. There was evidence of flexibility and preference of prey type. The relationships between predator and prey size were similar for both species, with size of prey consumed increasing with larval growth. Comparisons of fish diets reveal a strong selection for certain prey types as well as prey size. The study of predator and prey enables one to establish predictive models for fisheries management.