Ontogenic Changes in Prey Ingested by Nile perch (Lates niloticus) Caught in Nyanza Gulf of Lake Victoria, Kenya Nicholas O. Outa, 1 * Edwine Yongo 2 and Jameslast A. Keyombe 3 1 Centre for Research and Technology Development, Maseno University, Kisumu, 2 Department of Fisheries and aquatic Sciences, University of Eldoret, Eldoret, and 3 Kenya Marine and Fisheries Research Institute (KMFRI), Naivasha, Kenya Abstract A total of 289 fish were analysed in this study, with Caradina nilotica (44%) being the dominant food item in their diet, while tilapia (8%) contributed the least. Haplochromines, unidentified fish prey and juvenile Nile perch also were observed. An ontogenic shift was observed, with C. nilotica contributing highest (61.1%) of the diet of fingerlings, but decreasing to 21% in the diet of adults. Nile perch contributed 35.7% of the diet in the adults and 8.1% in the fingerlings. This study indicates Nile perch is a predatory fish, feeding mainly on C. nilotica, haplochromines, tilapia, Nile perch and other fish materials. Key words cannibalism, Caradina nilotica, feeding, haplochromines, ontogenic shift. INTRODUCTION Studies on the feeding habits of fishes have been con- ducted over the years to better understand fish dynamics and reproduction patterns. Nile perch (Lates niloticus) has supported the fisheries of Lake Victoria since its introduction in the lake in the 1950s (Cowx et al. 2003; Yeko et al. 2009). The main aim of introducing Nile per- son was to convert the bony haplochromines in the lake into more fleshy fish that would be more economically beneficial to the fisher folk around the lake (Goudswaard et al. 2008). The subsequent increased population of Nile perch in Lake Victoria has been cited as the cause for disappearance of most of the haplochromines and other fish species within the lake (Njiru et al. 2005). Haplochro- mine cichlids were previously the main prey of the intro- duced Nile perch in Lake Victoria (Njiru et al. 2005). After the decline of the haplochromine stocks at the end of the 1980s, however, Nile perch shifted to the shrimp C. nilotica, and to a lesser degree to its own young and the cyprinid Rastrineobola argentea, as well as juvenile tilapiines as primary food sources (Kishe-Machumu et al. 2012). The introduction of the Nile perch was observed to also affect the feeding habit of the Pied Kingfisher, which shifted from a predominantly haplochromine diet to a diet dominated by R. argentea (omena) because of the decline of the former within the lake (Wanink et al. 1994), illustrating the long-term effects of introducing Nile perch into the lake. The type and size of food items consumed by fish change with the age and size of the fish. This is based on the observation that fish can only feed on items that can fit in their mouth, as well as what food sources their guts can digest. As fish continue to mature, their diges- tive systems become more developed because of more developed digestive enzymes, and a widening and elonga- tion of the gut, thereby making it possible for the fish to digest more complex food. This phenomenon, known as an “ontogenic shift” has been observed for several fish species, including Nile perch (Goudswaard et al. 2006) and tilapia (Njiru et al. 2004; Outa et al. 2014) in lakes Naivasha and Victoria. The present study provides information on the feed- ing habits of Nile perch within the Nyanza Gulf of Lake Victoria, focusing mainly on the main diet, as well as the changes that occur in the type of prey items ingested at different stages in the development of the fish within the gulf. *Corresponding author. Email: nichouta@gmail.com Accepted for publication 2 February 2017. Lakes and Reservoirs: Research and Management 2017 20: 1–5 © 2017 John Wiley & Sons Australia, Ltd Doi: 10.1111/lre.12163