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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.

Ojwang W.O.,Kenya Marine and Fisheries Research Institute | Ojuok J.E.,Kenya Marine and Fisheries Research Institute | Mbabazi D.,National Fisheries Resources Research Institute | Kaufman L.,Boston University
Aquatic Ecosystem Health and Management | Year: 2010

The fish community of Lake Victoria, East Africa, has continued to exhibit an intriguing degree of resiliency despite stress from anthropogenic activities. Frequent environmental perturbations include wild fluctuations in fishing pressure, limnological conditions, and lake levels. Surprisingly, many of the endemic and other indigenous fishes have survived, and some have even increased in numbers in recent years. While a positive development, this resiliency is a red flag to scientists because we do not understand it, or know whether it can be expected to continue. Furthermore, besides issues of immediate human well-being, there remain grave concerns about long term resource sustainability. This paper explores possible reasons for ecological resilience in the Lake Victoria fish community, with a focus on omnivory and functional redundancy as possible explanations. The analysis used published data based on both traditional gut content analysis (GCA) and stable isotope analysis (SIA). Trophic plasticity is ubiquitous among the surviving fishes of Lake Victoria. This, combined with an overall simplification of the food web, contributed to the lake community's current resilience. © 2010 AEHMS.

Taabu-Munyaho A.,National Fisheries Resources Research Institute | Taabu-Munyaho A.,University of Iceland | Kayanda R.J.,Tanzania Fisheries Research Institute | Everson I.,Anglia Ruskin University | And 2 more authors.
Journal of Great Lakes Research | Year: 2013

Stratification restricts habitable areas forcing fish to balance between favourable temperature and minimum dissolved oxygen requirements. Acoustic surveys conducted during the stratified and isothermal periods on tropical Lake Victoria indicated that stratification of temperature and dissolved oxygen (DO) affected vertical distribution of Nile perch. There was higher mean temperature (25.6±0.5°C) and lower DO (6.4±1.8mg/l) during stratified period compared to the isothermal period (mean temperature 24.9±0.3°C; mean DO 7.3±0.6mg/l). Higher mean densities of Nile perch were recorded in the coastal (0.44±0.03) and deep (0.27±0.02g/m3) strata during the stratified compared to the isothermal season (coastal: 0.24±0.01; deep: 0.12±0.02g/m3). In addition, Nile perch density in the upper 0-40m depth layers in the coastal and deep strata increased by over 50% from the isothermal to the stratified season. Daily landings from 65 motorised fishing boats between October 2008 and September 2010 show higher mean catch (26.29±0.17kg/boat/day) during stratified compared to the isothermal (23.59±0.15) season. Thermal stratification apparently compresses the habitat available to Nile perch and can potentially result in higher exploitation. Managers should evaluate the potential benefits of instituting closed seasons during the stratified period, and stock assessment models should take into account the seasonal niche compression. © 2013 Elsevier B.V.

Poste A.E.,University of Waterloo | Muir D.C.G.,Environment Canada | Mbabazi D.,National Fisheries Resources Research Institute | Hecky R.E.,University of Waterloo | Hecky R.E.,University of Minnesota
Journal of Great Lakes Research | Year: 2012

Nearshore regions of lakes are important sources of fish, and can be strongly influenced by anthropogenic inputs of nutrients as well as contaminants. This study characterizes food web structure, mercury concentrations, and biomagnification of mercury in two embayments in northern Lake Victoria that differ in their connectivity to the open lake, trophic status, and the influence of local anthropogenic pollution. Murchison Bay is a semi-confined hypereutrophic bay in a densely populated region, while Napoleon Gulf is mesotrophic and is well flushed with water from the open lake. Based on stable carbon and nitrogen isotope analysis, food web structure was similar at both sites, with short food chains and conspecific fish occupying similar trophic positions. However, there were strong differences in net phytoplankton δ15N and δ13C between sites; net phytoplankton δ13C was largely related to trophic status, while δ15N values appeared to be influenced by inputs of human waste and the prevalence of biological nitrogen fixation. Total mercury (THg) concentrations in fish were consistently below 200ng/g wet weight, and despite elevated THg concentrations in water in Murchison Bay, THg concentrations in net phytoplankton and fish from both embayments did not differ, highlighting that THg in water is not always a good predictor of concentrations in fish. We also observed that biomagnification of mercury was occurring at a lower rate in Murchison Bay than in Napoleon Gulf, and we propose that the hypereutrophic state of Murchison Bay may be acting to reduce potential Hg exposure for higher trophic level fish. © 2012 Elsevier B.V.

Fugere V.,McGill University | Kasangaki A.,National Fisheries Resources Research Institute | Chapman L.J.,McGill University
Ecosphere | Year: 2016

Land use changes such as deforestation and agricultural expansion strongly affect stream biodiversity, with several studies demonstrating negative impacts on stream alpha diversity. Effects of forest conversion on stream beta diversity are much harder to predict, both because empirical studies are few and because competing theories suggest opposite responses. Moreover, almost no data exist for tropical Africa, a region that is paradoxically a hotspot of both current deforestation and freshwater biodiversity. Here, we compared environmental variables, invertebrate community composition, and alpha and beta diversity of forested and deforested (agricultural) streams in and around Kibale National Park, Uganda. We found that forest conversion strongly influenced stream environmental variables and invertebrate community composition, and that agricultural land use reduced stream alpha diversity. However, among-stream beta diversity was greater across the agricultural landscape than inside the forest. Decomposing beta diversity into taxa replacement and richness differences demonstrated that replacement contributed a similar proportion to total beta diversity in both land use classes. Because of this greater beta diversity, the agricultural landscape had similar gamma diversity as the forested landscape despite its lower alpha diversity. We discuss conservation implications of these land use-associated biodiversity changes in a highly diverse yet little-studied deforestation hotspot. © 2016 Fugère et al.

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