[Show abstract][Hide abstract] ABSTRACT: External body colour is an important trait contributing to phenotypic diversity and individual fitness in fish species. In this study, we use a combination of experimental techniques and field observations to examine patterns of colour divergence in the introduced Nile perch population of Lake Nabugabo, Uganda. We aim to determine whether the proportion of black–brown body colouration of Nile perch differs over a number of size classes, across ecologically distinct wetland edge and forest edge habitats, and whether these differences are the result of rapid (physiological) or ontogenetic (morphological) colour change. We found substantial colour differences in Nile perch between habitats, but trends were not consistent across size classes. Small Nile perch (<15 cm SL) from wetland edge habitats had darker skin pigmentation than those from forest edge; however, no significant colour differences existed between medium (15–40 cm SL) and large (>40 cm SL) Nile perch. Inter-habitat differences in colour in the small size class, and shifts in colour from juvenile to adult appear to be the result of morphological colour change associated with distinct ontogenetic shifts in resource use.
[Show abstract][Hide abstract] ABSTRACT: Introduced species can have profound direct ecological impacts on native species, yet their potential indirect effects remain relatively unexplored. For instance, introduced predators may directly affect some native species via predation, which may in turn have indirect consequences for other species that are released from competition.We explore this possibility in East Africa's Lake Victoria basin, where the introduction of the predatory Nile perch, Lates niloticus, in the 1950s and 60s contributed to the overall or local extinction of hundreds of native fish by the 1980s. We ask whether this dramatic change in assemblage composition has led to competitive release and niche expansion in Rastrineobola argentea, a resilient native cyprinid that has thrived in this highly perturbed ecosystem.To address this question, we compare the trophic ecology of R. argentea before (1966) and after (2011) the introduction of the Nile perch in Lake Victoria; and across eight satellite lakes that differ in their history of Nile perch invasion. Using this combination of spatial and temporal comparisons, we test for increases in dietary niche breadth (niche expansion) and changes in the level of individual specialisation of R. argentea in invaded versus uninvaded contexts.In our historical comparison, we find good evidence for dietary niche expansion and an increase in interindividual diet variation in R. argentea over time. Across lakes, however, strong bottom-up effects (i.e. variation in prey availability) appear to obscure any potential top-down effects of the Nile perch introduction on the trophic ecology of R. argentea.Overall, we find substantial temporal and spatial variation in the diet and niche breadth of R. argentea, but the underlying drivers remain uncertain, given the complexity of both anthropogenic and natural ecological changes in Lake Victoria over the past century. Understanding both the direct and indirect impacts of introduced species is challenging, but important for successful long-term management of human-altered ecosystems.
[Show abstract][Hide abstract] ABSTRACT: Inland fisheries represent an important source of protein and income for many communities, particularly in the tropics. It has been shown that ectotherms living in climatically stable tropical environments tend to be thermal specialists, and that some of them achieve their optimal metabolic performance at temperatures near their upper tolerance limits. Consequently, tropical freshwater fishes may be exceptionally sensitive to even small changes in temperature anticipated from global climate change. Despite that threat, the thermal biology of tropical freshwater fishes has received little attention. This project aims to assess whether the thermal sensitivity of fish metabolism and energetics can be used to forecast the vulnerability of key tropical freshwater fishes to climate change. Experiments are conducted at three locations (Brazil, Uganda, and Cambodia), where we quantify the metabolic capacities of two species acclimated for 3 weeks to three water temperature treatments (ambient, ambient + 2°C, and ambient + 4°C). The project addresses a time sensitive conservation problem, with a goal to contribute to the protection of culturally and socio-economically important fish species in the tropics.
American Fisheries Society 144th Annual Meeting; 08/2014
[Show abstract][Hide abstract] ABSTRACT: The use of non-lethal experiments to elucidate behavioural and physiological thresholds to environmental stressors can provide valuable data for identifying threats to, and critical habitat of, imperilled species. Increased turbidity contributes to population declines and loss of fish diversity globally, but the complex direct and indirect effects of this stressor on imperilled species are not well understood.Response to turbidity was examined in a group of closely related North American fishes (Notropis spp.), including four imperilled shiners: pugnose shiner (N. anogenus), bridle shiner (N. bifrenatus), blacknose shiner (N. heterolepis), blackchin shiner (N. heterodon), and the more common mimic shiner (N. volucellus). Two experiments quantified (i) the behavioural response of each species to progressively increasing turbidity, and (ii) critical swimming speed of fishes acclimated in turbid- and clear-water treatments to provide insights into effects of low turbidity on aerobic performance.Very low levels of turbidity (
[Show abstract][Hide abstract] ABSTRACT: Gene flow among populations in different selective environments should favor the evolution of phenotypic plasticity over local adaptation. Plasticity in development is a common response to long-term hypoxia in some widespread African fishes, including Pseudocrenilabrus multicolor, a cichlid that exploits both normoxic (high oxygen) rivers/lakes and hypoxic (low oxygen) swamps. Previous studies have shown that fish from normoxic and hypoxic sites differ in many traits, including gill size, brain size and body shape, and that much of this variation reflects developmental plasticity. However, these earlier studies focused on areas in Uganda where gene flow between swamp and river or lake populations is high. In this study we tested the hypothesis that P. multicolor from a relatively isolated lake population (Lake Saka, Uganda) exhibit low levels of plasticity in traits related to oxygen uptake. Multiple broods of P. multicolor from Lake Saka were reared under low and high dissolved oxygen, and traits related to gill size, brain mass and body shape were quantified. Surprisingly, both gill size and brain mass showed high levels of developmental plasticity. We suggest that high levels of plasticity, particularly in the gill size of P. multicolor, reflects low costs of maintaining the plastic response, even in relatively isolated populations.
[Show abstract][Hide abstract] ABSTRACT: African tropical lakes provide vital ecosystem services including food and water to some of the fastest growing human populations, yet they are among the most understudied ecosystems in the world. The consequences of climate change and other stressors on the tropical lakes of Africa have been informed by long-term analyses, but these studies have largely focused on the massive Great Rift Valley lakes. Our objective was to evaluate how recent climate change has altered the functioning and services of smaller tropical lakes, which are far more abundant on the landscape. Based on a paired analysis of 20 years of high-resolution water column data and a paleolimnological record from a small crater lake in western Uganda, we present evidence that even a modest warming of the air (∼0.9°C increase over 20 years) and changes in the timing and intensity of rainfall can have significant consequences on the dynamics of this common tropical lake type. For example, we observed a significant nonlinear increase (R(2) adj = 0.23, e.d.f. = 7, p<0.0001) in thermal stability over the past 20 years. This resulted in the expansion of anoxic waters and consequent deterioration of fish habitat and appears to have abated primary production; processes that may impair ecosystem services for a vulnerable human population. This study on a system representative of small tropical crater lakes highlights the far-reaching effects of global climatic change on tropical waters. Increased research efforts into tropical aquatic ecosystem health and the development of sound management practices are necessary in order to strengthen adaptive capabilities in tropical regions.
PLoS ONE 01/2014; 9(1):e86561. · 3.53 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The African cichlid, Pseudocrenilabrus multicolor victoriae is a eurytopic fish that exhibits high levels of developmental plasticity in response to dissolved oxygen availability. In this study, F1 offspring from three sites in the Mpanga River drainage of Western Uganda characterized by different dissolved oxygen (D.O.) regimes were reared under normoxic or hypoxic conditions. After 1year, enzymes were measured to determine the tissue metabolic capacity of four different tissues: muscle, heart, brain and liver. The enzymes measured were pyruvate kinase [PK], lactate dehydrogenase [LDH], citrate synthase [CS], and cytochrome C oxidase [CCO], and an additional two, malate dehydrogenase (MDH) and fructose 1,6-bisphosphatase (FBPase), were examined in the liver only. Individuals reared under hypoxia exhibited elevated levels of LDH and CCO in the heart; and depressed activity levels of brain CS and liver CCO and MDH relative to normoxia-reared sibs. Results from this study demonstrate that long-term exposure to hypoxia during development can induce changes in the metabolic capacities of P. multicolor. This flexibility may be important in facilitating persistence in variable and/or novel environments, and in the face of increasing global hypoxia.
Comparative biochemistry and physiology. Part B, Biochemistry & molecular biology 08/2013; · 1.61 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Population level response to hypoxia has become an issue of global significance because of increased frequency and intensity of hypoxic events worldwide, and the potential for global warming to exacerbate hypoxic stress. In this study, we sequenced two nuclear intronic regions and a single mitochondrial region across seven populations of the African cyprinid, Barbus neumayeri from two river drainages in Uganda: the Rwembaita Swamp-Njuguta River System and the Dura River. We then examined two indices of population structure, G ST and Jost's D, to detect links between oxygen availability and genetic variation and to determine if population genetic structure was associated with (i) dissolved oxygen regime (hypoxia or normoxia), (ii) geographical distance, or (iii) a combination of dissolved oxygen regime and geographical distance. Our results indicate that over a large scale (between drainages), geographical distance significantly affects the genetic structure of populations. However, within a single drainage, dissolved oxygen regime plays a key role in determining the genetic structure of populations. Within the Rwembaita-Njuguta system, gene flow was high between locations of similar oxygen regimes, but low between areas characterized by divergent oxygen regimes. Interestingly, G ST analyses appear to yield less realistic measures of population structure than Jost's D, suggesting that caution must be taken when interpreting and comparing the results from different studies. These results support the idea that aquatic dissolved oxygen can act as a selective force limiting gene flow among populations of aquatic species and therefore should be considered when implementing conservation plans and assessing environmental impact of human activities.
Ecology and Evolution 06/2013; 3(6):1495-1506. · 1.66 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Aquatic hypoxia is generally viewed as stressful for aerobic organisms. However, hypoxia may also benefit organisms by decreasing cellular stress, particularly that related to free radicals. Thus, an ideal habitat may have the minimum O2 necessary to both sustain aerobic metabolism and reduce the need to scavenge free radicals and repair free radical damage. The ability of aquatic organisms to sustain aerobic metabolism relates in part to the ability to maximize gas diffusion, which can be facilitated by small body size when O2 uptake occurs across the body surface, by a large gill surface area, or by the ability to use atmospheric air. We use water-breathing organisms in chronically hypoxic papyrus (Cyperus papyrus) swamps of East Africa to test the hypothesis that cellular-level benefits of hypoxia may translate into increased fitness, especially for small organisms. A review of recent studies of fingernail clams (Sphaerium sp.) shows that clams living in sustained hypoxia have minimized oxidative stress and that these cellular-level benefits may lead to increased fitness. We suggest that organisms in the extreme conditions in the papyrus swamps provide a unique opportunity to challenge the conventional classification of hypoxic habitats as 'stressful' and normoxic habitats as 'optimal.'
Comparative biochemistry and physiology. Part A, Molecular & integrative physiology 04/2013; · 2.20 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Parallel adaptive radiation events provide a powerful framework for investigations of ecology's contribution to phenotypic diversification. Ecologically driven divergence has been invoked to explain the repeated evolution of sympatric dwarf and normal lake whitefish (Coregonus clupeaformis) species in multiple lakes in eastern North America. Nevertheless, links between most putatively adaptive traits and ecological variation remain poorly defined within and among whitefish species pairs. Here, we examine four species pairs for variation in gill, heart, and brain size; three traits predicted to show strong phenotypic responses to ecological divergence. In each of the species pairs, normals exhibited larger body size standardized gills compared to dwarfs - a pattern that is suggestive of a common ecological driver of gill size divergence. Within lakes, the seasonal hypoxia experienced in the benthic environment is a likely factor leading to the requirement for larger gills in normals. Interestingly, the morphological pathways used to achieve larger gills varied between species pairs from Québec and Maine, which may imply subtle non-parallelism in gill size divergence related to differences in genetic background. There was also a non-significant trend toward larger hearts in dwarfs, the more active species of the two, whereas brain size varied exclusively among the lake populations. Taken together, our results suggest that the diversification of whitefish has been driven by parallel and non-parallel ecological conditions across lakes. Furthermore, the phenotypic response to ecological variation may depend on genetic background of each population.
Ecology and Evolution 03/2013; 3(3):546-57. · 1.66 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Aquatic hypoxia can affect predator-prey interactions by altering the success rate of the predator and/or
the vulnerability of prey. For example, in the Lake Victoria basin of East Africa, native prey exploit hypoxic
wetlands as refugia from predation by introduced Nile perch (Lates niloticus). Here, it is predicted that species
exploitation of wetlands depends on their hypoxia tolerance relative to the heterogeneity of wetland hypoxia. In this
study, we compared the hypoxia tolerance of four fish taxa that differ in their use of hypoxic wetlands in Lake
Nabugabo, Uganda: the cichlid Pseudocrenilabrus multicolor victoriae that inhabits the dense swamp interior; and
three taxa that inhabit wetland ecotones including Nile tilapia (Oreochromis niloticus), L. niloticus and juvenile
endemic haplochromine cichlids. We characterised hypoxia tolerance by exposing fish to progressive hypoxia and
quantifying variation in aquatic surface respiration (ASR). The effect of body size on tolerance was explored in
L. niloticus by quantifying ASR behaviour across a range of size classes. ASR behaviour was also compared
between O. niloticus groups from wetland versus open-water habitats to detect habitat-associated intraspecific
variation. The most tolerant taxon was the swamp specialist P. multicolor, indicated by its low ASR thresholds and
small percentage of fish using ASR during the final sample interval. The other three taxa did not differ in ASR
behaviour, and no differences were detected between O. niloticus groups. Body size effects were present for
L. niloticus suggesting a lower tolerance to hypoxia in larger-bodied individuals, thus limiting their ability to
Ecology of Freshwater Fish 01/2013; 22:421-429. · 1.94 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Nile perch (Lates niloticus) was introduced to the Lake Victoria basin in the 1950s and 1960s and eventually became one of the most valuable commercial species of East Africa's inland fisheries. Intense fishing-induced mortality may be contributing to dramatic ecological change in this species (reductions in body size and catch rate) and reinforcing patterns of ecological divergence in some lakes in the region. We used radio telemetry to characterize Nile perch movement and home range size in Lake Nabugabo, Uganda and quantified patterns of habitat selection by Nile perch in the heavily-fished near-shore areas of this system. Nile perch exhibited high site tenacity with daily movement averaging ∼400 m/day and home range size averaging 0.83 km2. Body size and water temperature were both important predictors of Nile perch movement patterns. In the near-shore area of the lake, Nile perch selected regions characterized by low temperature and high oxygen conditions, and tended to prefer forest edge over wetland edge. The level of site tenacity exhibited by Nile perch suggests that ecological isolation between fish using forest and wetland ecotones may be higher than expected. These results also have important implications for development of territorial jurisdiction by Beach Management Units, the decentralized structure of fisheries management in the region.
Fisheries Research 01/2013; 137:18–29. · 1.70 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: 1. Wetlands in the Lake Victoria basin serve as structural and hypoxic refugia for some native fishes against predation by introduced Nile perch (Lates niloticus); however, little is known about the fine-scale patterns of distribution and abundance of these refuge inhabitants. 2. This study sought to quantify wetland ecological gradients and determine where peaks in native fish abundance and richness ('peak refugia') occurred using Lake Nabugabo, Uganda as a model system. 3. Extensive wetland transects were sampled between June and August, 2011 to measure ecological variation over distance from the lake–wetland edge. 4. Wetlands were characterized by strong clines in water temperature, pH, dissolved oxygen (DO), depth and vegetation density, and narrow peak refugia were found precisely at the lake–wetland edge. Community richness and diversity tended to be greater in areas with higher DO and lower temperature, pH, and vegetation density. It is interesting that areas encroached upon by a native emergent macrophyte (hippo grass, Vossia cuspidata) had more extreme physico-chemical conditions and supported fewer native fish species. 5. These results demonstrate the importance of wetland edges in the maintenance of native fish fauna in the Lake Victoria basin, and suggest that the continued expansion of hippo grass may reduce the accessibility of wetlands as refugia. 6. We recommend that the use of spatially explicit management approaches such as the development of secure buffer zones surrounding wetland edges to protect peak refugia, and the need for hippo grass control to minimize its effects on this important refuge.
[Show abstract][Hide abstract] ABSTRACT: Damselfly larvae, important predators and prey in many freshwater communities, may be particularly sensitive to hypoxia because their caudal lamellae (external gills) are frequently lost. In this study, we address how lost lamellae interact with low oxygen to affect respiration and behavior of the widespread North American damselfly Ischnura posita. Results showed no effect of lost lamellae on resting metabolic rate or critical oxygen tension. Ventilation behaviors increased only when dissolved oxygen (DO) was at or below 25% saturation, and these behaviors were not affected by the number of lamellae. Use of the oxygen-rich surface layer occurred almost exclusively at the lowest dissolved oxygen level tested (10% saturation, 2.0 kPa). Damselflies that were missing lamellae spent more time at the surface than individuals with intact lamellae. The negative relationship between body size and time at the surface, and the negative relationship between body mass and critical oxygen tension suggest that larger I. posita may be more hypoxia tolerant than smaller individuals. Overall, I. posita was minimally affected by missing lamellae and seems well adapted to low DO habitats. Average critical oxygen tension was very low (0.48 kPa, 2.4% saturation), suggesting that individuals can maintain their metabolic rate across a broad range of DO, and behaviors changed only at DO levels below the hypoxia tolerance thresholds of many other aquatic organisms.
Journal of insect physiology 11/2012; · 2.24 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: This study describes the metabolic capacities of the African cichlid Pseudocrenilabrus multicolor victoriae from four sites in Uganda, East Africa. Fish were captured during the dry season, from two aquatic systems in different regions (Lake Nabugabo and Mpanga River). Within the Lake Nabugabo region, individuals were sampled from Lake Kayanja (normoxic) and Lwamunda Swamp (hypoxic); within the Mpanga River system, individuals were sampled from Bunoga and Kahunge (characterized by seasonal variation in dissolved oxygen (D.O.)). Enzyme activity levels of pyruvate kinase, lactate dehydrogenase, citrate synthase, and cytochrome C oxidase were measured in four tissues: white skeletal muscle, heart, brain, and liver. Two additional enzymes were measured in the liver, malate dehydrogenase and fructose 1,6-bisphosphatase. Regional differences between enzyme activities in most tissues were evident; however, little variation was observed between two sites within a region despite differences in D.O. In general, P. multicolor from the Mpanga River system displayed greater anaerobic enzyme activity in white skeletal muscle, lower gluconeogenic enzyme activity in the liver, and an overall higher enzyme activity in the heart and brain tissues than fish from the Nabugabo region. The latter may reflect a long-term adaptation to low-oxygen conditions at the metapopulation level in the Nabugabo region.
Comparative biochemistry and physiology. Part B, Biochemistry & molecular biology 11/2012; · 1.61 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: 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.