Studies of the food of introduced Nile tilapia, Oreochromis niloticus (L.) with respect to size, habitat and season were conducted between November 1998 and October 2000 in Kenyan waters of Lake Victoria. Stomach contents of 1980 specimens collected by demersal trawl and seining were analysed. Nile tilapia originally known to be herbivorous, feeding mostly on algae has diversified its diet to include insects, fish, algae and plant materials. The major diet of fish <5 cm total length was zooplankton whereas bigger fish included a wider range of food items in their diet. There was spatial variation in diet with insects and algae dominating in the gulf and open water habitats respectively. There was no seasonal variation in the food items ingested and diel feeding regime indicated that O. niloticus is a diurnal feeder. The shift in diet could be due to ecological and environmental changes in Lake Victoria, which have been associated with changes in composition and diversity of fish and invertebrate fauna, emergence and dominance of different flora including water hyacinth Eichhornia crassipes (Mart.) Solms-Laub., and algae communities. The feeding habit of O. niloticus is discussed in the context of changes occurring in the lake. et invertébrée, à l'émergence et à la dominance d'une certaine flore, y compris la jacinthe d'eau, Eichhornia crassipes (Mart.) Solms-Laub., et de communautés d'algues. Les habitudes alimentaires d' O. niloticus sont discutées dans le contexte des changements survenus dans le lac.

The life history characteristics of introduced Nile tilapia (Oreochromis niloticus L.) in Lake Victoria, including, sex ratio, fecundity, reproduction, weight‐length relationship and body condition were studied and compared with those of other populations. Samples were collected by trawling and seining in the Kenyan sector of Lake Victoria between June 1998 and December 2000. Males predominated over females (sex ratio 1.42 : 1 : 00). O. niloticus spawned throughout the year but with a peak between December and June. Length at first maturity was (mean ± SD) 30.81 ± 0.09 for females and 34.5 ± 6 0.48 for males. There was little seasonal variation in relative condition, which ranged from 0.92 to 1.05 in males and 0.94 to 1.07 in females. Gonadosomatic index (GSI) was low during the postspawning period (July to October) and high during the protracted breeding period (December and June). Fecundity ranged from 905 to 7619 oocytes for fish of 28 to 51 cm total length (TL) respectively. The relationships between fecundity (F) and total length (L), weight (W) and ovary weight (OW) were: F = 8.159L1.53, F = 96.269W0.4504, F = 1806 + 39.4OW. The slope b of the weight‐length relationship was 3.08–3.32 for males and 3.07–3.22 for females. Growth was allometric in both cases and was significantly different from the expected value of 3. The life history strategy of O. niloticus is discussed in context of environmental changes occurring in the lake.

Lake Victoria is African's most important source of inland fishery production, exhibiting an annual catch of ≈500 000 mt. The lake moderates local climate, serves as a means of transport, and a source of human food and raw material for the fishmeal industry. The introduction of Nile perch transformed the fisheries from a locally based artisanal fishery to a national and international capital investment industry. The fisheries produce an annual income of $US 600 million, providing employment opportunities for over 3 million people. Although the benefits of transforming the fishery of Lake Victoria appear to be quite remarkable, the negative impacts of this development are even more dramatic.Several challenges face the lake fishery, including environmental degradation, the introduction of exotic species, overfishing, and a fish export ban. Nutrients inputs into the lake have resulted in eutrophication and increased algal blooms. Deteriorated water quality and increased anoxia have caused the displacement and decline of some fish species. Exotic tilapiines have hybridized, displacing the indigenous fish species. Nile perch predation has caused the disappearance of once abundant native fish species. The once complex food web is now dominated by Nile perch. Unhygienic fishing methods have led to several fish export bans, while water hyacinth invasion in the lake has affected water quality, fishing and transportation possibilities. The current top-down management and use of different regulations by the riparian states have been a major drawback to sustainable management of the lake. There is a need for a paradigm shift, in which the lake is considered to be one ecosystem and relevant stakeholders are involved in the decision-making process.

Lake Victoria is one of the African Great Lakes, and an important source of affordable protein food in the form of fish. It provides employment, income, and export earnings to the riparian communities. Despite this importance there are major concerns about the status and exploitation of the fisheries. This study assesses if current extraction rates/yield of Nile perch, Lates niloticus (L.), from Lake Victoria are sustainable for the foreseeable future. The paper reviews trends in catch and effort in the Nile perch fishery and models the expected scenarios using ECOPATH. The fishery exhibits, classic indicators of intensive fishing, erring towards overexploitation, including: (i) decline in total estimated catch of Nile perch in recent years from the peaks in the 1990s. This is coupled with a shift in contribution of catches from higher trophic level species (Nile perch) to lower trophic level (dagaa) species. (ii) Three major trends in the fishing effort are evident: (a) almost doubling of the number of fishers and fishing boats between 1990 and 2000, and the even greater expansion between 2000 and 2002; (b) a large scale increase in the number of gears operational in the lake; and (c) the propensity for use of ÔillegalÕ gears. (iii) Catchper-unit-effort has declined from about 80 to 45 kg per boat day. (iv) Predictive modelling (ECOSIM) of the future of the Nile perch fisheries under a scenario of increased fishing effort suggests that the fisheries are unsustainable and will decline in the long term. It appears that the Nile perch stocks in the three riparian countries are under intense fishing pressure, and unless concerted action is taken, the potential for degradation of the resources is prevalent. In view of the importance of these fisheries, it is recommended that a precautionary approach to further intens-ification of the fisheries is adopted until such time empirical evidence shows that the fisheries are capable of further expansion and intensification. The main options for management are devolvement of responsibilities for the fisheries to the communities, enforcement of existing regulations, improved monitoring and data collection processes, reducing post-harvest losses and increasing the value of the products to the export market.

Lake Victoria is Africa's most important source of inland fishery production, exhibiting annual catches of ≈ 400 000 mt. The predatory Nile perch, Lates niloticus, and the herbivorous tilapiines, Oreochromis niloticus, Oreochromis leucostictus, Tilapia zillii and Tilapia rendalii, were introduced in Lake Victoria in the 1950s and 1960s. Nile perch were introduced to convert the abundant, but bony, haplochromines to fish flesh, while the tilapiines were introduced to boost the declining fishery. Since that time, the fisheries of Lake Victoria have undergone dramatic social and ecological changes. The catches increased tremendously, changing the fishery from artisanal to commercial, in turn increasing fisher income and employment opportunities. However, there was a decline and, in some cases, the disappearance of many indigenous fish species, especially the haplochromine cichlids. This reduction was attributed to overexploitation, predation, and competition and hybridization with the introduced species. The decline of the native fish species has had impacts on the trophic and ecological status of the lake. Nile perch now dominate the formerly complex food web. The loss of phytoplanktivorous haplochromines has contributed to an increase in algal blooms, reduction in water quality and occasional fish kills. Water hyacinth, Eichhornia crassipes, invaded Lake Victoria in 1988, with high rates of infestation in shallow waters and bays, which are breeding and nursery grounds for most fish species. Catches of Nile perch decreased following the infestation, while those of Nile tilapia, lung fish (Protopterus aethiopicus) and mud fish (Clarias gariepinus) increased. Haplochromines species also showed some signs of recovery. In view of all these changes, the future of the Lake Victoria fishery is uncertain.