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
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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.