-Nile tilapia (Oreochromis niloticus L.) is the dominant of the introduced tilapiines in many East African lakes and has flourished in the presence of introduced Nile perch (Lates niloticus L.). We explored the hypothesis that O. niloticus exhibits increased omnivory in response to a decline in abundance of haplochromine cichlids. First, we quantified variation in habitat use and diet of O. niloticus in Lake Nabugabo, Uganda. Second, we compared the diet of O. niloticus in lakes with (Nabugabo, Victoria) and without (Mburo, Wamala, Nyamusingiri, Kyasanduka) introduced Nile perch. In Lake Nabugabo, a higher level of phytoplanktivory was observed in small juveniles than in larger fish and in wetland ecotone areas where haplochromines were most abundant. An omnivorous diet dominated by detritus and invertebrates was recorded for O. niloticus in lakes Nabugabo and Victoria, while a predominantly herbivorous diet was characteristic of O. niloticus in lakes without Nile perch. Availability of a broad food base in lakes where inshore insectivores have been reduced may explain the increased omnivory recorded in lakes Nabugabo and Victoria.

In this study, we compare the diet, life history characters and abundance of Nile tilapia (Oreochromis niloticus) stocks between two crater lakes in western Uganda (Lake Nyamusingiri and Lake Kyasanduka) that differ in their history of fishing pressure. Both lakes support native fish communities, but also harbour populations of introduced O. niloticus. Lake Nyamusingiri was characterized by a higher relative and absolute abundance (CPUE) of haplochromine cichlids and a lower abundance of O. niloticus than in Lake Kyasanduka. In addition, the O. niloticus population in Lake Nyamusingiri exhibited a smaller mean size and a lower relative abundance of juveniles than in Lake Kyasanduka. These differences may reflect, at least in part, heavy exploitation of O. niloticus in this lake, as compared with Lake Kyasanduka where fishing was banned in 1988. In both lakes, the maximum size and size at maturity was small relative to other larger water bodies in the region. The Nile tilapia in both lakes were primarily herbivorous; with phytoplankton dominating the diet. In general, the condition of O. niloticus in the crater lakes was low relative to other larger lakes in the region. The low catch per unit effort of O. niloticus in Lake Nyamusingiri (7.8%) relative to Lake Kyasanduka (54.9%) suggests that overexploitation of the fishery is occurring, calling for careful monitoring and management of the system.

a b s t r a c tDue to the production of toxins, cyanobacteria may adversely affect economically important fish such as Nile tilapia Oreochromis niloticus in tropical lakes. We studied the diet composition and factors affecting the diet of Nile tilapia in two tropical lakes where cyanotoxins were present. Particle-bound microcystins were present in all analysed water samples, ranging in concentration from 0.00012 to 1.11 and from 0.006 to 0.254 mg L À 1 in Murchison Bay in Lake Victoria and Lake Mburo, respectively. Detritus and phytoplankton were the main dietary components of the Nile tilapia, with phytoplankton contributing to over 30% by volume of stomach contents. The cyanobacteria Microcystis spp., which are also the most likely source of microcystins in the lakes, accounted for more than 80% of ingested phytoplankton. Microcystis spp. were also the most abundant cyanobacteria in both lakes (460%). We found no significant relationship between the contribution of phytoplankton in Nile tilapia diet and the concentration of microcystins in the water but we found a close association between water transparency and the contribution of insects to Nile tilapia diets in Murchison Bay. Our results further show that none of the other measured environmental variables was a good predictor of diet items in Nile tilapia. Adult Nile tilapia in our study lakes, rely heavily on filter feeding, particularly under conditions of low water transparency, trapping detritus and phytoplankton cells especially colonies. They can ingest more mobile prey like insects and insect larvae when the water transparency and visibility increases.

The tilapiine fish Oreochromis esculentus, is endemic to only lakes Victoria and Kyoga and a few satellite lakes in the two lake basins. It was the most important commercial fish species during the first half of the 20th century in the two lake basins but because of over-exploitation and competition with introduced tilapiines, its stocks declined and, by 1980s, O. esculentus was virtually absent from the two main lakes. Lakes Mburo, Kachera, Kayanja and Kayugi in the Lake Victoria basin, and Nabisojjo, Kawi and Lemwa, in the Lake Kyoga basin, where remnants of the species survived were investigated using experimental fleets of gillnets from 1997 to end of 2002 with the objective of determining the species length frequency distribution, condition factor, feeding and breeding to guide its management, conservation and enhancement. Fish caught in Lake Kayugi, where diatoms dominated in the stomach contents attained the largest size, were most fecund and had a high value of condition factor K. These findings suggest that diatoms were vital in the survival of O. esculentus. However, with the recent shift of algal communities from diatoms to blue-green algae in most aquatic systems, assimilation of blue-green algae by O. esculentus should be investigated.

Quantitative zooplankton sampling was carried out biweekly during 16 months in Lake Nyahirya, a Western Uganda crater lake . Six out of 24 rotifer species recorded from the lake were quantitatively important (Horaella brehmi, Brachionus angularis, B. caudatus, Keratella tropica, Filinia longiseta and F opoliensis, and three others were common (Brachionus falcatus, Asplanchna sieboldi and Conochiloides natans) . Rotifer peaks were correlated with rainy periods and relatively clear water, which coincide with the European spring and autumn . All species were able to deal with hypoxic conditions . A clear niche separation can be observed between different genera and between species of the same genus . The population dynamics of the rotifers seem to be ruled by repeated irregular environmental fluctuations. Rainfall appears to be a primary steering factor.