Hydrobiologia 513: 141–152, 2004. © 2004 Kluwer Academic Publishers. Printed in the Netherlands. 141 Diel vertical migration of major fish-species in Lake Victoria, East Africa Kees (P.C.) Goudswaard 1 , Jan H. Wanink 1 , Frans Witte 1,∗ , Egid F.B. Katunzi 2 , Michiel R. Berger 3 & David J. Postma 4 1 Institute of Biology Leiden, University of Leiden, P.O. Box 9516, 2300 RA Leiden, The Netherlands E-mail: witte@rulsfb.leidenuniv.nl 2 Tanzania Fisheries Research Institute, P.O. Box 475, Mwanza, Tanzania 3 Zoological Laboratory, University of Groningen, P.O. Box 14, 9750 AA Haren, The Netherlands 4 Department of Fish Culture and Fisheries, Agricultural University of Wageningen, P.O. Box 338, 6700 AH Wageningen, The Netherlands ( ∗ Author for correspondence) Received 27 January 2003; in revised form 23 July 2003; accepted 28 July 2003 Key words: haplochromine cichlids, Lates, Oreochromis, prey-detection, Rastrineobola, Schilbe Abstract Understanding of migration patterns is essential in the interpretation of hydro-acoustic stock assessment data of partly demersal partly pelagic fish stocks. In this paper we provide this kind of information for some species that were common in the Mwanza Gulf of Lake Victoria in the 1980s, before and after the upsurge of introduced Nile perch (Lates niloticus). Detritivorous haplochromines and Nile tilapia (Oreochromis niloticus), both stay near the bottom during day and night. Feeding seems to occur predominantly during the day. The zooplanktivorous haplochromines and dagaa (Rastrineobola argentea) dwell near the bottom by day and migrate towards the surface during the night. They seem to follow their prey, zooplankton and lake-fly larvae. Piscivorous nembe (Schilbe intermedius) show similar migration patterns to zooplanktivorous fishes, but their behaviour cannot be unambigu- ously explained by pursuit of prey. Nile perch to some extend migrate into the column at night, though the majority remains near the bottom. Feeding takes place during day and night. Introduction Stock estimates of Lake Victoria fishes were until re- cently done by bottom trawling (Kudhongania & Cor- done, 1974; Kukowski, 1978; Benda, 1981; Okemwa, 1981; Okaronon, 1994). In such estimates the vertical distribution of the species over the water column is not taken into account. Currently hydro-acoustic tech- niques, which do account for the vertical distribution of fish biomass, are used for stock assessments (Tum- webaze, 1997; Getabu et al., 2003). However, for a sound interpretation of the hydro-acoustic data, it is essential to have information about vertical move- ments of individual fish species. In this paper we provide such information for some species that were common in the Mwanza Gulf of Lake Victoria, before and after the upsurge of introduced Nile perch (Lates niloticus L.) in the 1980s. The species investigated comprise: haplochromine cichlids, nembe (Schilbe intermedius L.), Nile perch, Nile tilapia (Oreochromis niloticus L.) and dagaa (Rastrineobola argentea Pellegrin). Phytoplanktivor- ous Nile tilapia and piscivorous Nile perch were both introduced into Lake Victoria during the 1950s. Land- ings of Nile tilapia increased gradually from 1970s onwards (Goudswaard et al., 2002), while the popu- lation of Nile perch increased explosively during the 1980s (Ogutu-Ohwayo, 1990; Barel et al., 1991). Sub- sequently, the majority of the original species, domin- ated by haplochromine cichlids, both in species num- ber and biomass (Greenwood, 1974; Kudhongania & Cordone, 1974), disappeared from the lake and a new fish community developed (Ogutu-Ohwayo,