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Sharpe D.M.T.,McGill University | Wandera S.B.,National Fisheries Resources Research Institute NaFIRRI | Chapman L.J.,McGill University
Evolutionary Applications | Year: 2012

Fishing and introduced species are among the most important stressors affecting freshwaters and can also be strong selective agents. We examined the combined effects of commercial fishing and an introduced predator (Nile perch, Lates niloticus) on life history traits in an African cyprinid fish (Rastrineobola argentea) native to the Lake Victoria basin in East Africa. To understand whether these two stressors have driven shifts in life history traits of R. argentea, we tested for associations between life history phenotypes and the presence/absence of stressors both spatially (across 10 Ugandan lakes) and temporally (over four decades in Lake Victoria). Overall, introduced Nile perch and fishing tended to be associated with a suite of life history responses in R. argentea, including: decreased body size, maturation at smaller sizes, and increased reproductive effort (larger eggs; and higher relative fecundity, clutch volume, and ovary weight). This is one of the first well-documented examples of fisheries-induced phenotypic change in a tropical, freshwater stock; the magnitude of which raises some concerns for the long-term sustainability of this fishery, now the most important (by mass) in Lake Victoria. © 2012 Blackwell Publishing Ltd. Source


Okello W.,National Fisheries Resources Research Institute NaFIRRI | Kurmayer R.,Austrian Academy of Sciences
Lakes and Reservoirs: Research and Management | Year: 2011

This study investigated the seasonal development of phytoplankton and potential microcystin (MC)-producing cyanobacteria and MC concentrations in freshwater lakes in Uganda. During 1year (May 2007-April 2008), monthly measurements were made of chemical and physical characteristics, phytoplankton composition and MC concentrations in a hypertrophic crater lake (Lake Saka), in shallow eutrophic lakes (Lakes Mburo, George and Edward) and in Lake Victoria (Murchison Bay, Napoleon Gulf). Throughout the study period, cyanobacteria (composed of the genera Anabaena, Aphanocapsa, Chroococcus, Cylindrospermopsis, Microcystis, Planktolyngbya and Planktothrix) dominated, always contributing >50% to the total phytoplankton biovolume. All samples from all sampling sites were found to contain MC. Samples from Lake Saka had the maximum MC concentration (10μgL-1) in July 2007. The minimum concentration (0.02μgL-1) was recorded in Lake George for the months of May and June 2007 and January and April, 2008. Intermediate MC concentrations (0.1-2.5μg MC-LR eq.L-1) were observed at all the sampling sites in the other three lakes. Highly significant positive linear relationships between the total MC concentration and Microcystis cell numbers were observed for all sampling sites. Relating the total MC concentrations to Microcystis cells revealed a >100-fold variation in the average MC contents per cell between lakes. While Microcystis from Lake George exhibited the lowest MC cell quotas (0.03-1.24fgcell-1), Microcystis from Lake Saka consistently exhibited maximum MC cell contents (14-144fgcell-1). It was concluded that MC production was because of the occurrence of Microcystis at all sampling sites. The populations differed consistently between sites and independent of the season in their average MC content per cell. © 2011 The Authors. Journal compilation © 2011 Blackwell Publishing Asia Pty Ltd. Source


Okello W.,Austrian Academy of Sciences | Okello W.,National Fisheries Resources Research Institute NaFIRRI | Portmann C.,Ecole Polytechnique Federale de Lausanne | Erhard M.,AnagnosTec GmbH | And 2 more authors.
Environmental Toxicology | Year: 2010

Microcystins (MCs) are cyclic heptapeptides, which are the most abundant toxins produced by cyanobacteria in freshwater. The phytoplankton of many freshwater lakes in Eastern Africa is dominated by cyanobacteria. Less is known, however, on the occurrence of MC producers and the production of MCs. Twelve Ugandan freshwater habitats ranging from mesotrophic to hypertrophic conditions were sampled in May and June of 2004 and April of 2008 and were analyzed for their physicochemical parameters, phytoplankton composition, and MC concentrations. Among the group of the potential MC-producing cyanobacteria, Anabaena (0-107 cells ml-1) and Microcystis (10 3-107 cells ml-1) occurred most frequently and dominated in eutrophic systems. A significant linear relationship (n = 31, r2 = 0.38, P < 0.001) between the Microcystis cell numbers and MC concentration (1.3-93 fg of MC cell-1) was observed. Besides [MeAsp3, Mdha7]-MC-RR, two new MCs, [Asp 3]-MC-RY and [MeAsp3]-MC-RY, were isolated and their constitution was assigned by LC-MS2. To identify the MC-producing organism in the water samples, (i) the conserved aminotransferase domain part of the mcyE gene that is indicative of MC production was amplified by general primers and cloned and sequenced, and (ii) genus-specific primers were used to amplify the mcyE gene of the genera Microcystis, Anabaena, and Planktothrix. Only mcyE genotypes that are indicative of Microcystis sp. were obtained via the environmental cloning approach (337 bp, 96.1-96.7% similarity to the Microcystis aeruginosa strain PCC7806). Accordingly, only the mcyE primers, which are specific for Microcystis, revealed PCR products. We concluded that Microcystis is the major MC-producer in Ugandan freshwater. © 2009 Wiley Periodicals, Inc. Source


Okello W.,Austrian Academy of Sciences | Okello W.,National Fisheries Resources Research Institute NaFIRRI | Ostermaier V.,Austrian Academy of Sciences | Portmann C.,Ecole Polytechnique Federale de Lausanne | And 2 more authors.
Water Research | Year: 2010

It is generally agreed that the hepatotoxic microcystins (MCs) are the most abundant toxins produced by cyanobacteria in freshwater. In various freshwater lakes in East Africa MC-producing Microcystis has been reported to dominate the phytoplankton, however the regulation of MC production is poorly understood. From May 2007 to April 2008 the Microcystis abundance, the absolute and relative abundance of the mcyB genotype indicative of MC production and the MC concentrations were recorded monthly in five freshwater lakes in Uganda: (1) in a crater lake (Lake Saka), (2) in three shallow lakes (Lake Mburo, George, Edward), (3) in Lake Victoria (Murchison Bay, Napoleon Gulf). During the whole study period Microcystis was abundant or dominated the phytoplankton. In all samples mcyB-containing cells of Microcystis were found and on average comprised 20 ± 2% (SE) of the total population. The proportion of the mcyB genotype differed significantly between the sampling sites, and while the highest mcyB proportions were recorded in Lake Saka (37 ± 3%), the lowest proportion was recorded in Lake George (1.4 ± 0.2%). Consequently Microcystis from Lake George had the lowest MC cell quotas (0.03-1.24 fg MC cell-1) and resulted in the lowest MC concentrations (0-0.5 μg L-1) while Microcystis from Lake Saka consistently showed maximum MC cell quotas (14-144 fg cell-1) and the highest MC concentrations (0.5-10.2 μg L-1). Over the whole study period the average MC content per Microcystis cell depended linearly on the proportion of the mcyB genotype of Microcystis. It is concluded that Microcystis populations differ consistently and independently of the season in mcyB genotype proportion between lakes resulting in population-specific differences in the average MC content per cell. © 2010 Elsevier Ltd. All rights reserved. Source


Everson I.,Anglia Ruskin University | Kayanda R.,Tanzania Fisheries Research Institute | Taabu-Munyaho A.,National Fisheries Resources Research Institute NaFIRRI
Lakes and Reservoirs: Research and Management | Year: 2013

Time series, using different echosounders or large-scale multiship acoustic surveys, can be criticized because equipment changes might affect the final results. This criticism was addressed previously by comparing the results from different vessels using echo integration on the target species. The acoustically estimated standing stock of Nile perch (Lates niloticus) in Lake Victoria, East Africa, declined to 50% between successive surveys six months apart in 2007, prompting the criticism that a change in echosounder was responsible for this observation. This concern has been addressed, using data from the same four small localities around the lake, Emin Pasha Gulf, Nyanza Gulf, Speke Gulf and the vicinity of the Sesse Islands, from six surveys, spanning the time when the change in echosounder occurred. For three of the locations, echo integration and single target detections within the first bottom echo indicated no significant differences in echosounder performance. Results from the fourth location, Sesse Islands, showed very low backscatter, possibly due to a layer of detritus on the lake bed. It is concluded that all data are equally comparable, providing echosounders are correctly calibrated with the vessel being stationary, although there may still be differences under operational conditions. Characteristics of intercalibration sites are discussed in this study. The results also show changes in substrate, likely attributable to local environmental changes between surveys. © 2013 Wiley Publishing Asia Pty Ltd. Source

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