Wageningen University and Research Centre
Wageningen Marine Research
East Africa’s Lake Victoria provides resources and services to millions of people on the lake’s shores and abroad. In particular, the lake’s fisheries are an important source of protein, employment, and international economic connections... more
East Africa’s Lake Victoria provides resources and services to millions of people on the lake’s shores and abroad. In particular, the lake’s fisheries are an important source of protein, employment, and international economic connections for the whole region. Nonetheless, stock dynamics are poorly understood and currently unpredictable. Furthermore, fishery dynamics are intricately connected to other supporting services of the lake as well as to lakeshore societies and economies. Much research has been carried out piecemeal on different aspects of Lake Victoria’s system; e.g., societies, biodiversity, fisheries, and eutrophication. However, to disentangle drivers and dynamics of change in this complex system, we need to put these pieces together and analyze the system as a whole. We did so by first building a qualitative model of the lake’s social-ecological system. We then investigated the model system through a qualitative loop analysis, and finally examined effects of changes on the system state and structure. The model and its contextual analysis allowed us to investigate system-wide chain reactions resulting from disturbances. Importantly, we built a tool that can be used to analyze the cascading effects of management options and establish the requirements for their success. We found that high connectedness of the system at the exploitation level, through fisheries having multiple target stocks, can increase the stocks’ vulnerability to exploitation but reduce society’s vulnerability to variability in individual stocks. We describe how there are multiple pathways to any change in the system, which makes it difficult to identify the root cause of changes but also broadens the management toolkit. Also, we illustrate how nutrient enrichment is not a self-regulating process, and that explicit management is necessary to halt or reverse eutrophication. This model is simple and usable to assess system-wide effects of management policies, and can serve as a paving stone for future quantitative analyses of system dynamics at local scales.
East Africa’s Lake Victoria provides resources and services to millions of people on the lake’s shores and abroad. In particular, the lake’s fisheries are an important source of protein, employment, and international economic connections... more
East Africa’s Lake Victoria provides resources and services to millions of people on the lake’s shores and abroad. In
particular, the lake’s fisheries are an important source of protein, employment, and international economic connections for the whole
region. Nonetheless, stock dynamics are poorly understood and currently unpredictable. Furthermore, fishery dynamics are intricately
connected to other supporting services of the lake as well as to lakeshore societies and economies. Much research has been carried out
piecemeal on different aspects of Lake Victoria’s system; e.g., societies, biodiversity, fisheries, and eutrophication. However, to
disentangle drivers and dynamics of change in this complex system, we need to put these pieces together and analyze the system as a
whole. We did so by first building a qualitative model of the lake’s social-ecological system. We then investigated the model system
through a qualitative loop analysis, and finally examined effects of changes on the system state and structure. The model and its
contextual analysis allowed us to investigate system-wide chain reactions resulting from disturbances. Importantly, we built a tool that
can be used to analyze the cascading effects of management options and establish the requirements for their success. We found that
high connectedness of the system at the exploitation level, through fisheries having multiple target stocks, can increase the stocks’
vulnerability to exploitation but reduce society’s vulnerability to variability in individual stocks. We describe how there are multiple
pathways to any change in the system, which makes it difficult to identify the root cause of changes but also broadens the management
toolkit. Also, we illustrate how nutrient enrichment is not a self-regulating process, and that explicit management is necessary to halt
or reverse eutrophication. This model is simple and usable to assess system-wide effects of management policies, and can serve as a
paving stone for future quantitative analyses of system dynamics at local scales.
particular, the lake’s fisheries are an important source of protein, employment, and international economic connections for the whole
region. Nonetheless, stock dynamics are poorly understood and currently unpredictable. Furthermore, fishery dynamics are intricately
connected to other supporting services of the lake as well as to lakeshore societies and economies. Much research has been carried out
piecemeal on different aspects of Lake Victoria’s system; e.g., societies, biodiversity, fisheries, and eutrophication. However, to
disentangle drivers and dynamics of change in this complex system, we need to put these pieces together and analyze the system as a
whole. We did so by first building a qualitative model of the lake’s social-ecological system. We then investigated the model system
through a qualitative loop analysis, and finally examined effects of changes on the system state and structure. The model and its
contextual analysis allowed us to investigate system-wide chain reactions resulting from disturbances. Importantly, we built a tool that
can be used to analyze the cascading effects of management options and establish the requirements for their success. We found that
high connectedness of the system at the exploitation level, through fisheries having multiple target stocks, can increase the stocks’
vulnerability to exploitation but reduce society’s vulnerability to variability in individual stocks. We describe how there are multiple
pathways to any change in the system, which makes it difficult to identify the root cause of changes but also broadens the management
toolkit. Also, we illustrate how nutrient enrichment is not a self-regulating process, and that explicit management is necessary to halt
or reverse eutrophication. This model is simple and usable to assess system-wide effects of management policies, and can serve as a
paving stone for future quantitative analyses of system dynamics at local scales.
- by Paul van Zwieten and +4
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Lion (Panthera leo) ecology and livestock conflicts in Waza National Park, Cameroon Jacco van Rijssel Supervisors: Drs. ... Lion (Panthera leo) ecology and livestock conflicts in Waza National Park, Cameroon Jacco van Rijssel Environment... more
Lion (Panthera leo) ecology and livestock conflicts in Waza National Park, Cameroon Jacco van Rijssel Supervisors: Drs. ... Lion (Panthera leo) ecology and livestock conflicts in Waza National Park, Cameroon Jacco van Rijssel Environment and Development Student Report no. ...
- by Jacco van Rijssel and +1
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- Environment
East Africa's Lake Victoria provides resources and services to millions of people on the lake's shores and abroad. In particular, the lake's fisheries are an important source of protein, employment, and international economic connections... more
East Africa's Lake Victoria provides resources and services to millions of people on the lake's shores and abroad. In particular, the lake's fisheries are an important source of protein, employment, and international economic connections for the whole region. Nonetheless, stock dynamics are poorly understood and currently unpredictable. Furthermore, fishery dynamics are intricately connected to other supporting services of the lake as well as to lakeshore societies and economies. Much research has been carried out piecemeal on different aspects of Lake Victoria's system; e.g., societies, biodiversity, fisheries, and eutrophication. However, to disentangle drivers and dynamics of change in this complex system, we need to put these pieces together and analyze the system as a whole. We did so by first building a qualitative model of the lake's social-ecological system. We then investigated the model system through a qualitative loop analysis, and finally examined effects of changes on the system state and structure. The model and its contextual analysis allowed us to investigate system-wide chain reactions resulting from disturbances. Importantly, we built a tool that can be used to analyze the cascading effects of management options and establish the requirements for their success. We found that high connectedness of the system at the exploitation level, through fisheries having multiple target stocks, can increase the stocks' vulnerability to exploitation but reduce society's vulnerability to variability in individual stocks. We describe how there are multiple pathways to any change in the system, which makes it difficult to identify the root cause of changes but also broadens the management toolkit. Also, we illustrate how nutrient enrichment is not a self-regulating process, and that explicit management is necessary to halt or reverse eutrophication. This model is simple and usable to assess system-wide effects of management policies, and can serve as a paving stone for future quantitative analyses of system dynamics at local scales.
Eutrophication is an increasing global threat to freshwater ecosystems. East Africa's Lake Victoria has suffered from severe eutrophication in the past decades which is partly responsible for the dramatic decline in haplochromine cichlid... more
Eutrophication is an increasing global threat to freshwater ecosystems. East Africa's Lake Victoria has suffered from severe eutrophication in the past decades which is partly responsible for the dramatic decline in haplochromine cichlid species diversity. However, some zooplanktivorous and detri-tivorous haplochromine species recovered and shifted their diet towards macro invertebrates and fish. We used four formalin preserved cichlid species caught over the past 35 years to investigate whether stable isotopes of these fish are reflecting the dietary changes, habitat differences and if these isotopes can be used as indicators of eutrophication. We found that d 15 N signatures mainly reflected dietary shifts to larger prey in all four haplochromine species. Shifts in d 13 C signatures likely represented habitat differences and dietary changes. In addition, a shift to remarkably heavy d 13 C signatures in 2011 was found for all four species which might infer increased primary production and thus eutrophication although more research is needed to confirm this hypothesis. The observed temporal changes confirm previous findings that preserved specimens can be used to trace historical changes in fish ecology and the aquatic environment. This highlights the need for continued sampling as this information could be of essence for reconstructing and predicting the effects of environmental changes.
Stable isotopes are increasingly being used to infer past and present trophic interactions in light of environmental changes. The Lake Victoria haplochromine cichlids have experienced severe environmental changes in the past decades that,... more
Stable isotopes are increasingly being used to infer past and present trophic interactions in light of environmental changes. The Lake Victoria haplochromine cichlids have experienced severe environmental changes in the past decades that, amongst others, resulted in a dietary shift towards larger prey. We investigated how the changed environment and diet of the haplochromines influenced stable isotope values of formalin-then-ethanol-preserved cichlid specimens, and then investigated how these values differed among species before (1977–1982) and after substantial environmental changes (2005–2007). We found a small preservation effect on both d 13 C and d 15 N values, and significant differences in isotope values among haplochromine species collected before the environmental changes. In contrast, there was a remarkable similarity in d 13 C and d 15 N values among species collected from the contemporary ecosystem and two out three species showed significantly different stable isotope values compared to species of the historic ecosystem. In addition, we found a putative isotopic gradient effect along our 5-km-long research transect indicating that the studied demersal species are more stenotopic than previously thought. The environmental changes have resulted in dietary change and overlap of the hap-lochromines which provides insight into the trophic plasticity of these species, which are often considered trophic specialists.
Textbook examples of adaptive radiation often show rapid morphological changes in response to environmental perturbations. East Africa’s Lake Victoria, famous for its stunning adaptive radiation of cichlids, has suffered from... more
Textbook examples of adaptive radiation often show rapid morphological changes in response to environmental perturbations. East Africa’s Lake Victoria, famous for its stunning adaptive radiation of cichlids, has suffered from human-induced eutrophication over the past decades. This cultural eutrophication is thought to be partly responsible for the dramatically reduced cichlid biodiversity, but climatic variability in itself might also have contributed to the eutrophication which resulted in low oxygen levels and decreased water transparency. To determine how recent environmental changes have influenced the lake and its cichlids over the past 50 years, we gathered environmental and meteorological variables and compared these with gill surface area of four cichlid species. We found that during the period of severe eutrophication and temperature increase (1980s), reduced wind speeds coincided with a reduction in oxygen levels and a decrease in both water temperature and transparency. The gill surface area in three out of the four cichlid species increased during this period which is consistent with adaptive change in response to increased hypoxia. During the 2000s, wind speeds, oxygen levels, water transparency and water temperature increased again, while cichlid gill surface area decreased. Our results imply that climatic changes and especially wind speed and direction might play a crucial role in tropical lake dynamics. The changes in Lake Victoria’s water quality coincide with fluctuations in cichlid gill surface area, suggesting that these fish can respond rapidly to environmental perturbations but also that climatic variability, together with continued eutrophication, might be detrimental to the lake’s cichlid biodiversity.
- by Jacco van Rijssel and +2
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- Climate Change, Hypoxia, Eutrophication, Cichlidae
- by Ian Donohue and +35
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- by Ian Donohue and +35
Theory suggests that speciation with gene flow is most likely when both sexual and ecological selection are divergent or disruptive. Divergent sexual and natural selection on the visual system have been demonstrated before in sympatric,... more
Theory suggests that speciation with gene flow is most likely when both sexual and ecological selection are divergent or disruptive. Divergent sexual and natural selection on the visual system have been demonstrated before in sympatric, morphologically similar sister species of Lake Victoria cichlids, but this does not explain the subtle morphological differences between them. To investigate the significance of natural selection on morphology during speciation, we here ask whether the prevalence of disruptive ecological selection differs between sympatric sister species that are at different stages of speciation. Some of our species pairs do (Pundamilia) and others do not (Neochromis) differ distinctively in sexually selected male nuptial coloration. We find that (i) evidence for disruptive selection, and for evolutionary response to it, is prevalent in traits that are differentiated between sister species; (ii) prevalence of both predicts the extent of genetic differentiation; and (iii) genetic differentiation is weaker in species pairs with conserved male nuptial coloration. Our results speak to the existence of two different mechanisms of speciation with gene flow: speciation mainly by sexual selection tightly followed by ecological character displacement in some cases and speciation mainly by divergent ecological selection in others.
- by Jacco van Rijssel and +3
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- Speciation, Gene Flow, Sexual Selection, Morphology