Abstract:

This study examined the trophic structure of the fishes present in lakes Victoria and Kyoga, and the less impacted satellite lakes in the two basins using a combination of stable carbon (513C) and nitrogen (b15N) isotopes, and stomach content analyses between 2001 and 2004. Both stomach content and stable isotope analyses revealed that fish had a varied diet, which in part was influenced by their species and size, but in general displayed a lack of specialization. Invertebrates constituted the most important dietary component of most fish species (>50%) with the exception of the tilapiine species in the less disturbed lakes without Nile perch. Fish formed another important dietary item, especially for the predatory Nile perch the basis of the current fishery in lakes where it is found. Phytoplankton and detritus formed the basal carbon supporting the fish communities among the examined lakes. Ontogenetic dietary shift was only evident from stomach content analysis for Nile perch (Lates niloticus), consuming mainly invertebrates at a total length of (< 20 cm) and fish (between 20 and < 60 cm) but neither of the size groups were purely invertebrate eaters or piscivores. The high b15N signatures (7.35 — 8.28 %o) and trophic positions (2.5 - 2.6) of the Rastrineobola argentea found in stomachs of Nile perch (< 20%), in comparison to those of Nile perch (5.35 - 9.30 %o and 2.1 - 3.3 respectively) rules them out as major diets contributing to its somatic growth. It was apparent that the trophic structure of the examined lakes/areas was not structured in a simple linear fashion, but indicated omnivory, hence the 515N did not exhibit a typical assumed shift of 3-4 %o per trophic level. Based on b15N signatures the longest food chain lengths were recorded in Thruston Bay Lake Victoria (1.7) followed by lakes Bisina, Kayugi and Napoleon Gulf in Lake Victoria (1.5), Nawampasa (1.4) and the shortest were recorded in lakes Nabugabo and Nakuwa (< 0.7). Noteworthy, lakes Nabugabo and Nakuwa, with the shortest food chain lengths, like the main lakes, contain the predatory Nile perch. The b13C of the fishes in the main lakes ranged from -21.7 to -14.8 %o and -25.4 to —13.8 %o in lakes Victoria and Kyoga respectively and from —28.1 to —6.7 %o in the satellite lakes. Among the satellite lakes in the Lake Victoria basin, Nabugabo recorded the least range for 613C (-21.6 to -19.7 %o) and Lake Kachera the largest range (-20.5 to -9.4 %o). In the Kyoga lake basin, Lake Bisina recorded the largest range in 513C (-25.9 to 11.3 %o) and Lake Nawampasa the least (24.4 to 21.7 %o). The littoral areas of Lake Victoria and the satellite lakes without Nile perch were characterized by substantially longer food chains than those lakes with the Nile perch in the same drainage systems. The broad ranges in 513C are an indication of a wide range of food sources in the aquatic systems including external inputs. This study has indicated the importance of non-commercial fish species and other biota in trophic interactions as well as the potential factors that may affect ultimate energy sources supporting commercially important stocks.