Increase in Anoxia in Lake Victoria and Its Effects on the Fishery

Njiru, M.; Nyamweya, Chrisphine S.; Gichuki, John; Mugidde, R.; Mkumbo, O.C.; Witte, Frans

doi:10.5772/27461

Cited by 20 publications

(25 citation statements)

References 86 publications

(159 reference statements)

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“…5), confirms the importance of shallow habitats for juvenile Nile perch (Mkumbo, 2002). Surprisingly, on both local and regional scales, we did not find strong relationships between Nile perch densities and oxygen concentrations as was found in other studies in Lake Victoria (Wanink et al, 2001;Mkumbo, 2002;Goudswaard et al, 2011;Njiru et al, 2012;Nyboer & Chapman, 2013;Taabu-Munyaho et al, 2013). On a local scale we did not find any relationships between oxygen concentration and Nile perch density (Fig.…”

Section: Factors Determining Nile Perch Distributionscontrasting

confidence: 41%

“…Experiments showed that Nile perch was relatively intolerant to hypoxia (Schofield & Chapman, 2000). Therefore, hypoxic waters can act as refugia for hypoxia-tolerant haplochromine species in swamps, wetlands, and deep water (Hecky, 1994;Chapman et al, 2002;Paterson & Chapman, 2009;Njiru et al, 2012) and for the freshwater shrimp Caridina nilotica (Roux) (Kaufman, 1992;Hecky, 1994;Njiru et al, 2012), both important prey for Nile perch. Hypoxia also affects the size class distribution of fish, because it can limit physical growth and reproduction, resulting in a reduction of maximum fish size and length at maturity (Pauly, 1981(Pauly, , 1984.…”

Section: Introductionmentioning

confidence: 99%

“…Several studies showed the relationship of dissolved oxygen and Nile perch densities (Wanink et al, 2001;Chapman et al, 2002;Mkumbo, 2002;Balirwa et al, 2003;Paterson & Chapman, 2009;Goudswaard et al, 2011;Njiru et al, 2012;Nyboer & Chapman, 2013;Taabu-Munyaho et al, 2013). In Lake Victoria seasonal stratification of the water column results in decreased oxygen concentrations near the bottom, especially in deeper water.…”

Section: Introductionmentioning

confidence: 99%

“…Hypoxia also affects the size class distribution of fish, because it can limit physical growth and reproduction, resulting in a reduction of maximum fish size and length at maturity (Pauly, 1981(Pauly, , 1984. Furthermore, large-sized Nile perch are affected more by low oxygen than small-sized Nile perch, because of their smaller gill surface area to biomass ratio (Njiru et al, 2012). Sudden upwelling of anoxic waters caused incidental massive kills of large-sized Nile perch in Kenyan waters and in the Mwanza Gulf in the 1980s (Ochumba, 1990;Kaufman, 1992;Wanink et al, 2001) and fish kills of Nile perch at smaller scales (i.e., individual fish floating dead or gasping for air rather than groups of fish) have been observed in 2009-2011 on at least three occasions during the breakdown of stratification (I. J. M. Cornelissen and H. K. Peter, personal observations).…”

Section: Introductionmentioning

confidence: 99%

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Nile perch distribution in south-east Lake Victoria is more strongly driven by abiotic factors, than by prey densities

et al. 2015

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We studied the effects of environmental driving factors (maximum depth, visibility, oxygen, temperature, and prey densities) on the distribution and diet composition of Nile perch (Lates niloticus) in south-east Lake Victoria from 2009 to 2011. We tested the hypotheses that (i) Nile perch distribution is regulated by the same environmental factors on a local scale (Mwanza Gulf) and on a regional scale (Mwanza Gulf, Speke Gulf and the open lake in Sengerema district), and (ii) driving factors act differently on different Nile perch size classes. Fish were sampled with gillnets. Nile perch densities were highest in the shallow part of the Mwanza Gulf and during the wet seasons, mainly caused by high densities of juveniles. The environmental driving factors explained Nile perch distributions on both regional and local scales in a similar way, often showing non-linear relationships. Maximum depth and temperature were the best predictors of Nile perch densities. Prey densities of shrimp and haplochromines did not strongly affect Nile perch distributions, but did explain Nile perch diet on a local and regional scale. We conclude that abiotic variables drive Nile perch distributions more strongly than prey densities and that feeding takes place opportunistically.

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Section: Factors Determining Nile Perch Distributionscontrasting

confidence: 41%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Nile perch distribution in south-east Lake Victoria is more strongly driven by abiotic factors, than by prey densities

et al. 2015

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“…Peripheries for most of these lakes are populous and user conflicts have amplified the demand for fisheries resources culminating into overfishing (Ogutu-Ohwayo et al 1997). Previous limnological studies regarding the herein mentioned EA water bodies have indicated increased eutrophication, with oxygen levels and transparency (Secchi depths) decreasing, Chlorophyll a and siltation increasing, among others, all attributed to environment/catchment destruction particularly in Lake Victoria basin (Njiru et al 2012). Additionally, translocations of fish species to nonnative homes, for instance a voracious predator, Nile perch (Lates niloticus) from Lake Albert to Victoria, Kyoga and Nabugabo, also have left majority of the cichlids depleted through predation pressure, a concern to the future sustainability of the resources (OgutuOhwayo et al 1997).…”

Section: Study Areas/locationsmentioning

confidence: 99%

Anthropogenic impacts on the contextual morphological diversification and adaptation of Nile tilapia (Oreochromis niloticus, L. 1758) in East Africa

et al. 2018

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Nile tilapia occurs naturally in East Africa where it's an economically important species. Many of the natural populations of Nile tilapia have been affected by anthropogenic activities including translocations, associated with programmes aimed at enhancing capture fisheries and aquaculture productivity. Using geometric morphometric analyses, we tested the hypothesis that such anthropogenic activities have augmented admixture among natural populations of Nile tilapia and influenced the geographical distribution of morphological variation within the species. Our expectation was that Nile tilapia shape divergent might be consistent with reportedly anthropogenic activities in nonnative environments. To test this hypothesis, we analyzed the shapes of 490 individuals from thirteen populations; three farms, six natives and four nonnative natural populations. Our analysis revealed that the most pronounced shape diversification was observed in seven populations; three nonnatives (Victoria, Kyoga and Sindi farm) and four natives (Albert, River Nile, George and Turkana). The features responsible for the observed morphotypes were mainly related to the orientation of the anterior region of the fish and may be due to diversifying selection in response to new environmental pressures (for nonnative populations), admixture or drift. Shape change in the nonnative high-altitude populations was unexpectedly conserved, suggesting recent introductions which may have not resulted in admixture or there was strong selection against change in the traits measured. On the other hand, the recorded morphotypic clusters explained the possible genetic link to their putative ancestral home. Our results were partially consistent with our prediction that the nonnative populations exhibited divergent morphotypes. We recommend further investigations with molecular genetics for followup of these findings.

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Lake Victoria

Renaut,

Owen

2023

Syntheses in Limnogeology

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Increase in Anoxia in Lake Victoria and Its Effects on the Fishery

Cited by 20 publications

References 86 publications

Nile perch distribution in south-east Lake Victoria is more strongly driven by abiotic factors, than by prey densities

Nile perch distribution in south-east Lake Victoria is more strongly driven by abiotic factors, than by prey densities

Anthropogenic impacts on the contextual morphological diversification and adaptation of Nile tilapia (Oreochromis niloticus, L. 1758) in East Africa

Lake Victoria

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