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.
Sitoki L.,Institute of Botany |
Gichuki J.,Institute of Botany |
Ezekiel C.,Tanzania Fisheries Research Institute |
Wanda F.,National Fisheries Resources Research Institute |
And 2 more authors.
International Review of Hydrobiology | Year: 2010
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. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Nunan F.,University of Birmingham |
Luomba J.,Tanzania Fisheries Research Institute |
Lwenya C.,Kenya Marine and Fisheries Research Institute |
Yongo E.,Kenya Marine and Fisheries Research Institute |
And 2 more authors.
Environmental Management | Year: 2012
The literature on fisheries co-management is almost silent on the issue of the movement of fisherfolk within fisheries, although such movement must have implications for the effectiveness of co-management. The introduction of co-management often involves the formation of new structures that should enable the participation of key stakeholder groups in decision-making and management, but such participation is challenging for migrating fishers. The article reports on a study on Lake Victoria, East Africa, which investigated the extent of movement around the lake and the implications of movement for how fishers participate and are represented in co-management, and the implications of the extent and nature of movement for co-management structures and processes. The analysis draws on the concept of space from the literature on participation in development and on a framework of representation in fisheries co-management in addressing these questions. The created space is on an 'invited' rather than open basis, reflecting the top-down nature of implementation and the desire to secure participation of different occupational groups, as well as women in a male-dominated sector. The more powerful boat owners dominate positions of power within the co-management system, particularly as the levels of co-management, from subdistrict to national, are traversed. The limited power and resources of boat crew are exacerbated by the degree and nature of movement around the lake, making effective participation in co-management decision-making a challenge. © Springer Science+Business Media, LLC 2012.
Rutaisire J.,National Fisheries Resources Research Institute |
Levavi-Sivan B.,Hebrew University of Jerusalem |
Aruho C.,National Fisheries Resources Research Institute |
Ondhoro C.C.,National Fisheries Resources Research Institute
Aquaculture Research | Year: 2015
Gonadal recrudescence in Barbus altianalis from River Nile, Jinja (33°05E; 0°45N) Uganda was studied prior to induced spawning. Oocytes in B. altianalis were found to develop in cohorts suggesting batch spawning throughout the year with pronounced spawning activities coinciding with rainfall peaks of April and September. There was a strong positive correlation between rainfall and gonadosomatic index (rS = 0.75, P = 0.008). B. altianalis had low fecundity (6.0 ± 2.0 per gram) and large-sized eggs (2.97 ± 0.1 mm). Induced spawning was successful with exogenous hormones and water current. Results from this study suggested that total striping may not be appropriate, but larger eggs would confer high survival of offsprings under optimal conditions. The success registered in induced spawning of B. altianalis provides a breakthrough in seed production for multiplication and culture of this economically important fish species. © 2013 John Wiley & Sons Ltd.
Nkalubo W.,National Fisheries Resources Research Institute |
Chapman L.,McGill University |
Muyodi F.,Makerere University
Aquatic Ecosystem Health and Management | Year: 2014
The diet of Nile Perch (8.0-121.0 cm total length [TL]) from the Ugandan waters of Lake Victoria was quantified through stomach content analysis of specimens collected from experimental catches and fish factory samples. A total of 7824 stomachs (5602 from experimental fishing and 2222 from factory samples) were examined, of which 34.8% contained food. Fish from the experimental catches were smaller (8.0-41.6 cm TL) and had a higher diversity of prey dominated by unidentifiable fish prey, haplochromine cichlids, Rastrineobola argentea, Odonata and Caridina nilotica, while larger fish (30.0-121.0 cm TL) from the factory samples had a predominance of fish remains and haplochromine cichlids. Nile Perch that had a high proportion of fish prey (versus invertebrates) in their stomachs showed a larger size for a given age, and were in a better condition (K = 1.24) than those that had primarily invertebrates (K = 1.10) in their stomachs. Nile Perch exhibited a much smaller size (15 cm versus 30 cm TL) at shift to piscivory in comparison to Nile Perch examined in earlier studies, when haplochromines were rare in Lake Victoria. The recovery of haplochromine cichlids coincident with declining Nile Perch densities illustrates the importance of developing sustainable management options that can define a proper balance between fishing mortality and Nile Perch predation. © 2014 Copyright Taylor & Francis Group, LLC.
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.
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.
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.
Kayanda R.,Tanzania Fisheries Research Institute |
Everson I.,Anglia Ruskin University |
Munyaho T.,National Fisheries Resources Research Institute |
Mgaya Y.,University of Dar es Salaam
Fisheries Research | Year: 2012
Results of estimation of target strength (TS) of physoclistous Nile perch using in situ TS measurements and deformed cylinder model (DCM) at 70 and 120. kHz are presented. The results of the two methods are compared with those from cage experiments conducted between 1999 and 2000. Cage experiments that assumed the 20. Log(L) relationship (TS = 20. Log(TL). -. 66.55) provided a good fit only over the length range of 18-38. cm total length, but overestimated the length of large fish (>60. cm TL). The DCM which lead towards a 30. Log(L) provides more accurate conversion over the full length range of single targets. An in situ study that made no assumptions about the shape, structure or orientation of the fish provided a close fit of acoustically estimated length frequency distributions to net hauls with the equations:. TS70=32.11 Log(TL)-86.97 at 70 kHz andTS120=30.15 Log(TL)-84.14 at 120 kHzIt is suggested that using this approach can provide valid TS to length relationships for acoustic surveys on species for which little information exists and there is little opportunity for direct observation of behavioural aspects that are known to affect TS such as tilt angle and vertical migration. © 2011 Elsevier B.V.