Beginning in the mid-1980s Lake Victoria experienced severe eutrophication and it was suggested that deteriorating water quality might lead to a collapse of its fisheries. A series of lake-wide surveys carried out 1999-2001 and 2005-2009 revealed that the temperature of the lake had risen by > 1 °C since 1927, with more rapid warming of the deeper waters reducing the thermal gradient in the water column and thus weakening stratification and the extent and severity of deoxygenation. The chlorophyll a concentrations in open water decreased since the 1980s, while Secchi disc visibility increased, indicating a reduced severity of algal blooms. Chlorophyll a was higher and Secchi disc visibility lower in inshore waters but there has been no deterioration in these areas since the 1980s. The conductivity remained unchanged, although it was about 50% greater in the semi-enclosed Nyanza Gulf than in the open lake. The water quality of the lake has therefore improved considerably despite the fact than concentrations of plant nutrients have not decreased and the reasons why this may be the case are discussed. IntroductionLake Victoria is the world's second largest freshwater lake (area = 68,800 km 2 , mean depth = 35 m) and it supports one of the world's largest inland fisheries, yielding almost one million tons per annum (LVFO, 2009a). It has attracted much interest in recent years because of the extraordinary ecological changes that occurred in it over the last three decades, comparable in their magnitude to those in newly created man-made lakes. These changes were triggered in the mid-1980s by an introduced predatory fish, the Nile perch Lates niloticus (L.), and its subsequent destruction of the endemic haplochromine cichlids. These fishes, which formed a species flock with some 500 described forms (WITTE et al., 2007), accounted for over 80% of the fish biomass in the lake (KUDHONGANIA and CORDONE, 1974) but they were small fish thought to be of little commercial value. Nile perch were therefore introduced with the specific intention of converting haplochromines into a more * Corresponding author 210 L. SITOKI et al.

The Nile perch, Lates niloticus (L.), stock in Lake Victoria may be showing signs of overfishing as the average size of fish decreased rapidly in 2007 when the stock biomass fell by 50%. Fishers have increasingly abandoned large-meshed gillnets in favour of small hooks on long lines. This has allowed prey species (Rastrineobola argentea [Pellegrin] and haplochromines) to increase, shifting the fishery to one dominated by species at lower trophic levels. These changes do not reflect deterioration in the environment because evidence suggests that conditions in the lake have improved and prey species sensitive to deoxygenation have increased. Management authorities have been criticised for relying only on the control of fishing effort rather than taking a holistic approach. However, fishing effort appears to be the main cause of the fishery problems and is the only aspect that can be controlled as environmental problems are too intractable.K E Y W O R D S : biomass, demography, haplochromines, overfishing, Rastrineobola argentea.

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Lake Victoria, East Africa, supports a fishery that yields about one million tonnes per annum consisting predominantly of three species, Nile Perch (Lates niloticus), Nile Tilapia (Oreochromis niloticus) and a native sardine-like cyprinid called Dagaa (Rastrineobola argentea). The non-native Nile Perch is the most valuable of these species and supports an important commercial export industry; there are fears that overfishing, due to the growth of fishing capacity, is threatening the Nile Perch fishery. Based on its economic importance and the notion that overfishing is threatening the resource, the current fishery management system was developed to control fishing capacity and effort. This system, using the concepts of co-management, where fishing communities and stakeholders participate through community organizations called Beach Management Units (BMUs) to actively manage the fishery in partnership with the central government, has been criticized that it is “fishery-based,” focusing on a single species and taking no account of ecological conditions in the lake, nor other species. A more “holistic” approach, which places a greater emphasis on changing nutrient concentrations and primary productivity as drivers of fish populations, has been proposed. Though fishery biologists and managers on Lake Victoria recognize that ecological conditions affect fishery populations, there appears to be two major challenges hindering the implementation of such approaches: first, the lack of a coherent objective of the Lake Victoria fishery, and second, the challenges associated with incorporating and implementing concepts of nutrient information and multiple species into a practical fishery management program. This article describes the current fishery co-management program to determine the feasibility of implementing a holistic approach on Lake Victoria. It is concluded that whether a management system should be “holistic” or “fishery-based” is of little importance; what is needed on Lake Victoria are clear objectives and a management plan that will enable those objectives to be achieved, utilizing both ecological and fisheries data where appropriate.