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Downing A.S.,University of Stockholm | Downing A.S.,Wageningen University | Downing A.S.,Netherlands Institute of Ecology | Van Nes E.H.,Wageningen University | And 47 more authors.
Ecology and Society | Year: 2014

East Africa’s Lake Victoria provides resources and services to millions of people on the lake’s shores and abroad. In particular, the lake’s fisheries are an important source of protein, employment, and international economic connections for the whole region. Nonetheless, stock dynamics are poorly understood and currently unpredictable. Furthermore, fishery dynamics are intricately connected to other supporting services of the lake as well as to lakeshore societies and economies. Much research has been carried out piecemeal on different aspects of Lake Victoria’s system; e.g., societies, biodiversity, fisheries, and eutrophication. However, to disentangle drivers and dynamics of change in this complex system, we need to put these pieces together and analyze the system as a whole. We did so by first building a qualitative model of the lake’s social-ecological system. We then investigated the model system through a qualitative loop analysis, and finally examined effects of changes on the system state and structure. The model and its contextual analysis allowed us to investigate system-wide chain reactions resulting from disturbances. Importantly, we built a tool that can be used to analyze the cascading effects of management options and establish the requirements for their success. We found that high connectedness of the system at the exploitation level, through fisheries having multiple target stocks, can increase the stocks’ vulnerability to exploitation but reduce society’s vulnerability to variability in individual stocks. We describe how there are multiple pathways to any change in the system, which makes it difficult to identify the root cause of changes but also broadens the management toolkit. Also, we illustrate how nutrient enrichment is not a self-regulating process, and that explicit management is necessary to halt or reverse eutrophication. This model is simple and usable to assess system-wide effects of management policies, and can serve as a paving stone for future quantitative analyses of system dynamics at local scales. © 2014 by the author(s).

Marshall B.E.,Lake Victoria Fisheries Organisation | Ezekiel C.N.,Tanzania Fisheries Research Institute | Gichuki J.,Kenya Marine and Freshwater Fisheries Institute | Mkumbo O.C.,Lake Victoria Fisheries Organisation | And 2 more authors.
African Journal of Aquatic Science | Year: 2013

Climate change may threaten the fisheries of Lake Victoria by increasing density differentials in the water column, thereby strengthening stratification and increasing the intensity and duration of deoxygenation in the deeper waters. Between 1927 and 2008 the lake's temperature increased by 0.99 °C at the surface and by 1.34 °C at depths >50 m, with the rate of warming increasing most rapidly between 2000 and 2008. In February 2000 there were marked thermal discontinuities in the water column at a number of deep stations, with marked oxyclines at depths ranging from 30-50 m, and with all stations being anoxic from 50 m downwards. In contrast, in February 2007 the lake's temperature had risen, especially at the bottom, and both the thermal discontinuities and oxyclines were much reduced, only one station recording a dissolved oxygen concentration of <2.0 mg l-1 at 50 m. This may reflect the fact that deeper waters were warming faster, and the reasons for this are discussed. These data suggest that the impacts of warming on the thermal regime of African lakes may be highly variable and unpredictable and, in this case, may have reduced its threat to the fisheries. © 2013 Copyright © NISC (Pty) Ltd.

Mkumbo O.C.,Lake Victoria Fisheries Organisation | Marshall B.E.,Lake Victoria Fisheries Organisation
Fisheries Management and Ecology | Year: 2015

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. © 2014 John Wiley & Sons Ltd.

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