Hydrobiologia 279/280: 367-376, 1994. J. J. Kerekes (ed.), Aquatic Birds in the Trophic Web of Lakes. © 1994 Kluwer Academic Publishers. Printed in Belgium. 367 Effects of Nile perch (Lates niloticus) introduction into Lake Victoria, East Africa, on the diet of Pied Kingfishers (Ceryle rudis) Jan H. Wanink & Kees (P. C.) Goudswaard Haplochromis Ecology Survey Team (HEST)/Research Group on Ecological Morphology, Zoologisch Laboratorium, Rijksuniversiteit Leiden, P.O. Box 9516, 2300 RA Leiden, The Netherlands Key words: Ceryle rudis, Lates niloticus, feeding ecology, species introduction, Lake Victoria Abstract In recent years the ichthyofauna of Lake Victoria, the world's largest tropical lake, has gone through dramatic changes. The population of Nile perch, a large predator which has been introduced into the lake by man, increased explosively at the expense of many haplochromine cichlid species. At the same time, numbers of a small cyprinid (dagaa) rose sharply. Previously Pied Kingfishers on Lake Victoria fed mainly on haplochromines. Only the youngest nestlings depended on dagaa as primary food. The current diet of adult birds clearly reflects the changes which have occurred in the fish community. Pellet analysis reveals a shift towards a diet composed of almost 100% dagaa. The change in prey species composition has increased the number of fish a kingfisher needs to catch daily in order to meet its energetic demands, because: (1) the mean size of haplochromines is larger than that of dagaa; (2) the mean size of dagaa has decreased since the increase in Nile perch; (3) the weight of dagaa is lower than that of haplochromines of equal size; (4) mainly juvenile dagaa and adults in poor condition are accessible to kingfishers. Introduction Lake Victoria, the world's largest tropical lake with a surface area of about 69000 km 2 , is situ- ated across the equator (0°21' N-3° 0 ' S; 31 ° 39'-34 ° 53' E) at an altitude of 1135 m. It is shared by the East African countries Tanzania, Uganda and Kenya. Compared to the other Great African Lakes Tanganyika (max. depth 1470 m) and Malawi (= Nyasa; max. depth 704 m), Lake Victoria is shallow (max. depth 79 m; mean depth 40 m). The lake also differs from the other two in having a relatively low transparency. Secchi-disc readings from the southern part of the lake reached a maximum value of 2.75 m at a 30 m deep offshore sampling station (Wanink et al., in prep.). Inshore waters, used as feeding areas by the Pied Kingfisher (Ceryle rudis L.), are much less transparent with minimum values of 0.2- 0.6 m in shallow bays (De Beer, 1989; Wanink et al., op. cit.). Annual variations in water tem- perature range from 23-26 C near the bottom and from 23-28 C at the surface (Talling, 1966), with the largest fluctuations occurring in the in- shore waters. Values of pH vary between 7.0 and 8.6 (Talling, op. cit.). Calcium concentration is