Browsing by Author "Okello, William"

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    Assessment of exploited fish species in the Lake Edward System, East Africa
    (Journal of Applied Ichthyology, 2021) Musinguzi, Laban; Bassa, Samuel; Natugonza, Vianny; Van Steenberge, Maarten; Okello, William; Snoeks, Jos; Froese, Rainer
    The unknown status of inland fish stocks hinders their sustainable management. Therefore, increasing stock status information is important for sustainable inland fisheries. Fisheries reference points were estimated for five exploited fish species (11 stocks) in the Lake Edward system, East Africa, which is one of the most productive inland water systems. The aim was to ascertain the status of the fisheries and establish reference points for effective management. The reference points were based on four linked stock assessment approaches for data-limited fisheries. Estimates showed poor stock status with the stocks defined as either collapsed, recruitment impaired or overfished. However, higher catches could be obtained under sustainable management. Estimates of maximum sustainable yield (MSY) and supporting biomass (Bmsy) are provided for 10 of the stocks as targets for rebuilding plans. The immediate target of management should be rebuilding biomass to Bmsy. Applicable measures include shifting length at first capture to the length that maximizes catch without endangering size structure and biomass, and livelihood diversification out of fisheries.
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    Characterization of Potential Threats from Cyanobacterial Toxins in Lake Victoria Embayments and during Water Treatment
    (Toxins, 2022) Olokotum, Mark; Humbert, Jean-François; Quiblier, Catherine; Okello, William; Semyalo, Ronald; Troussellier, Marc; Marie, Benjamin; Baumann, Kathrin; Kurmayer, Rainer; Bernard, Cécile
    Africa’s water needs are often supported by eutrophic water bodies dominated by cyanobacteria posing health threats to riparian populations from cyanotoxins, and Lake Victoria is no exception. In two embayments of the lake (Murchison Bay and Napoleon Gulf), cyanobacterial surveys were conducted to characterize the dynamics of cyanotoxins in lake water and water treatment plants. Forty-six cyanobacterial taxa were recorded, and out of these, fourteen were considered potentially toxigenic (i.e., from the genera Dolichospermum, Microcystis, Oscillatoria, Pseudanabaena and Raphidiopsis). A higher concentration (ranging from 5 to 10 g MC-LR equiv. L􀀀1) of microcystins (MC) was detected in Murchison Bay compared to Napoleon Gulf, with a declining gradient from the inshore (max. 15 g MC-LR equiv. L􀀀1) to the open lake. In Murchison Bay, an increase in Microcystis sp. biovolume and MC was observed over the last two decades. Despite high cell densities of toxigenic Microcystis and high MC concentrations, the water treatment plant in Murchison Bay efficiently removed the cyanobacterial biomass, intracellular and dissolved MC to below the lifetime guideline value for exposure via drinking water (<1.0 g MC-LR equiv. L􀀀1). Thus, the potential health threats stem from the consumption of untreated water and recreational activities along the shores of the lake embayments. MC concentrations were predicted from Microcystis cell numbers regulated by environmental factors, such as solar radiation, wind speed in the N–S direction and turbidity. Thus, an early warning through microscopical counting of Microcystis cell numbers is proposed to better manage health risks from toxigenic cyanobacteria in Lake Victoria.
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    High nutrient loading and climatic parameters influence the dominance and dissimilarity of toxigenic cyanobacteria in northern bays of Lake Victoria
    (Journal of Great Lakes Research, 2021) Olokotum, Mark; Troussellier, Marc; Escalas, Arthur; Hamlaoui, Sahima; Okello, William; Semyalo, Ronald; Humbert, Jean-François; Bernard, Cécile
    Eutrophication of Lake Victoria led to changes in its phytoplankton communities. However, different levels of eutrophication exist in the open lake and the bays, and between embayments. This study utilized spatial and temporal sampling of Napoleon Gulf and Murchison Bay, exhibiting different trophic conditions. Over one year, we investigated phytoplankton biomass, richness, diversity and dissimilarity, and related the dynamics of the dominant species to the limnological and climatic conditions. The results confirmed that Napoleon Gulf and Murchison Bay showed large differences in eutrophication status, with lower nutrient concentrations in Napoleon Gulf than in Murchison Bay, where a strong gradient was observed from inshore to offshore areas. These nutrient dynamics resulted in a 4 to 10 fold higher chlorophyll-a in Murchison Bay than in Napoleon Gulf. From the embayments, 135 phytoplankton taxa were recorded with no significant differences in alpha diversity. However, high dissimilarity in community structure was observed in beta diversity, mostly due to a turnover among the dominant toxigenic species. Thus, from a similar species pool, there was a shift in the dominant toxigenic cyanobacteria from Microcystis flos-aquae and M. aeruginosa in Murchison Bay, Dolichospermum circinale and Planktolyngbya circumcreta in Napoleon Gulf to D. circinale in the offshore stations. These cyanobacteria are toxigenic taxa with known health hazards. Using partial least square models, we showed that both climatic variables (e.g. wind, solar radiation) and levels of inorganic dissolved nutrients (e.g. SRP, NO3 – , and NH4 + ) are the main drivers of differences and dominance in cyanobacteria communities in northern Lake Victoria.
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    Need for harmonized long-term multi-lake monitoring of African Great Lakes
    (Journal of Great Lakes Research, 2022) Plisnier, Pierre-Denis; Kayanda, Robert; MacIntyre, Sally; Obiero, Kevin; Okello, William
    To ensure the long-term sustainable use of African Great Lakes (AGL), and to better understand the functioning of these ecosystems, authorities, managers and scientists need regularly collected scientific data and information of key environmental indicators over multi-years to make informed decisions. Monitoring is regularly conducted at some sites across AGL; while at others sites, it is rare or conducted irregularly in response to sporadic funding or short-term projects/studies. Managers and scientists working on the AGL thus often lack critical long-term data to evaluate and gauge ongoing changes. Hence, we propose a multi-lake approach to harmonize data collection modalities for better understanding of regional and global environmental impacts on AGL. Climate variability has had strong impacts on all AGL in the recent past. Although these lakes have specific characteristics, their limnological cycles show many similarities. Because different anthropogenic pressures take place at the different AGL, harmonized multilake monitoring will provide comparable data to address the main drivers of concern (climate versus regional anthropogenic impact). To realize harmonized long-term multi-lake monitoring, the approach will need: (1) support of a wide community of researchers and managers; (2) political goodwill towards a common goal for such monitoring; and (3) sufficient capacity (e.g., institutional, financial, human and logistic resources) for its implementation. This paper presents an assessment of the state of monitoring the AGL and possible approaches to realize a long-term, multi-lake harmonized monitoring strategy. Key parameters are proposed. The support of national and regional authorities is necessary as each AGL crosses international boundaries.
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    A review of the socioecological causes and consequences of cyanobacterial blooms in Lake Victoria
    (Harmful Algae, 2020) Olokotum, Mark; Mitroi, Veronica; Troussellier, Marc; Semyalo, Ronald; Bernard, Cécile; Montuelle, Bernard; Okello, William; Quiblier, Catherine; Humbert, Jean-François
    Africa is experiencing high annual population growth in its major river basins. This growth has resulted in significant land use change and pollution pressure on the freshwater ecosystems. Among them, the Lake Victoria basin, with more than 42 million people, is a unique and vital resource that provides food and drinking water in East Africa. However, Lake Victoria (LV) experienced a progressive eutrophication and substantial changes in the fish community leading to recurrent proliferation of water hyacinth and cyanobacteria. Based on an extensive literature review, we show that cyanobacterial biomasses and microcystin concentrations are higher in the bays and gulfs (B&Gs) than in the open lake (OL), with Microcystis and Dolichospermum as the dominant genera. These differences between the B&Gs and the OL are due to differences in their hydrological conditions and in the origins, type and quantities of nutrients. Using data from the literature, in this paper we describe the multiple ways in which the human population growth in the LV watershed is connected to the increasing occurrence of cyanobacterial blooms in the OL and B&Gs. We also described the already documented consequences of cyanobacterial blooms on food resources and fishing and on direct water use and water supply of local populations, with their potential consequences on the human health. Finally, we discuss the actions that have been taken for the protection of LV. Although many projects have been implemented in the 15 past years in order to improve the management of waste waters or to reduce deforestation and erosion, the huge challenge of the reduction of cyanobacterial blooms in LV by the control of eutrophication seems far from being achieved.
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    Status of Kigezi minor Lakes: A limnological survey in the Lakes of Kisoro, Kabale and Rukungiri Districts
    (International Journal of Water Resources and Environmental Engineering, 2016) Dias M. Tibihika, Papius; Okello, William; Barekye, Alex; Mbabazi, Dismas; Omony, Jimmy; Kiggundu, Vincent
    A limnological survey was conducted in six Lakes: Mulehe, Mutanda, Kayumbu, Chahafi, Bunyonyi and Nakasanda, of South-Western Uganda (SWU) in September 2014. The baseline survey aimed at characterizing the high altitude water bodies for enhanced future fisheries productivity. The results indicated that Lake Nakasanda had the highest temperature (24°C). Although other physic-chemical conditions of the water bodies varied, they were within acceptable ranges for fish productivity with no extreme cases reported. Chlorophyll-a was recorded highest in Lakes Nakasanda (16.8±3.4 μgL-1) and Bunyonyi (14.2±9.5 μgL-1). Rotifers were the most dominant zooplankton taxa with a total of 16 species and Lake Mulehe had the highest abundance (2,089,423±4,511 individuals m-2) while the least abundance were recorded in Lakes Nakasanda (471,608±35,505 individuals m-2) and Chahafi (473,354±15,279 individuals m-2). Among the macro-invertebrates, Diptera was the most abundant with Lake Mutanda recording the highest species richness of 8. The recorded absence or fewer individuals of some zooplankton and macro-invertebrate species/groups in certain aquatic ecosystems were attributed to the water quality. The variation in water quality, results from human activities and this may impart negative implications on fish productivity if not remedied.