Koeman en Bijkerk bv

AC, Netherlands

Koeman en Bijkerk bv

AC, Netherlands
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Witte F.,Leiden University | Witte F.,Netherlands Center for Biodiversity | Seehausen O.,University of Bern | Seehausen O.,Eawag - Swiss Federal Institute of Aquatic Science and Technology | And 6 more authors.
Aquatic Sciences | Year: 2013

During the past decades, major anthropogenic environmental changes occurred in Lake Victoria, including increased predation pressure due to Nile perch introduction, and decreases in water transparency and dissolved oxygen concentrations due to eutrophication. This resulted in a collapse of the haplochromine cichlids in the sub-littoral waters of the Mwanza Gulf in 1986-1990, followed by a recovery of some species in the 1990s and 2000s, when Nile perch densities declined. We studied two data sets: (1) haplochromines from sand and mud bottoms in the pre-collapse period; (2) haplochromines from sub-littoral areas during the pre-collapse, collapse and recovery periods. Water over mud is murkier and poorer in oxygen than water over sand, and differences in haplochromine communities in these natural habitats during the pre-collapse period may predict the effects of anthropogenic eutrophication during the collapse and recovery periods. In the pre-collapse period, haplochromine densities over sand and mud did not differ, but species richness over sand was 1. 6 times higher than over mud bottoms. Orange- and white-blotched colour morphs were most common at the shallowest sand station. More specifically, insectivores and mollusc-shellers had higher numbers of species over sand than over mud, whereas for mollusc-crushers no difference was found. Laboratory experiments revealed that mollusc shelling was more affected by decreased light intensities than mollusc crushing. During the pre-collapse period, spawning occurred year-round in shallow areas with hard substrates and relatively clear water. In deeper areas with mud bottoms, spawning mainly occurred during months in which water clarity was high. No effects of hypoxia on spawning periods were found. It follows that clearer water seems to support differentiation in feeding techniques as well as year-round spawning, and both may facilitate species coexistence. Water clarity is also known to be important for mate choice. These observations may explain why, since the decline of Nile perch, haplochromine densities have recovered, the numbers of hybrids increased and species diversity in the current eutrophic sub-littoral waters has remained 70 % lower than before the environmental changes. © 2012 The Author(s).

van Rijssel J.C.,Leiden University | van Rijssel J.C.,Naturalis Biodiversity Center | van Rijssel J.C.,Eawag - Swiss Federal Institute of Aquatic Science and Technology | van Rijssel J.C.,University of Bern | And 10 more authors.
Oecologia | Year: 2016

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. © 2016 Springer-Verlag Berlin Heidelberg

Kishe-Machumu M.A.,Tanzania Fisheries Research Institute | Kishe-Machumu M.A.,Leiden University | Voogd T.,Leiden University | Wanink J.H.,Leiden University | And 2 more authors.
Environmental Biology of Fishes | Year: 2015

The introduced Nile perch, Lates niloticus Linnaeus 1758 in Lake Victoria is considered to be a major contributor to the decline of haplochromine species. In the 1990s, the abundance of Nile perch declined and the recovery of some haplochromine species, mainly detritivores and zooplanktivores was observed. However, the resurgence of detritivores group was much slower than that of zooplanktivores. Differential Nile perch predation is thought to be the main cause for the differential recovery of these two groups. To test this, the frequency of occurrence (FOO) of the detritivores and zooplanktivores in the stomachs of the Nile perch and their FOO in the lake were compared. On their vulnerability to predation, body shape of the two groups was measured and compared with Nile perch mouth gape. The FOO for haplochromines in Nile perch stomach contents were 13.7 % detritivores and 86.3 % zooplanktivores, whereas in trawl catches they were 24.3 and 75.7 % respectively. The FOO of detritivores in the Nile perch diet was significantly lower than in the environment. The same holds for the FOO of juvenile haplochromines (13 % in the diet and 45 % in the environment). Body depth of detritivores (0.8–1.9 cm) in the diet of Nile perches was significantly larger than that of the zooplanktivores (0.6–1.7 cm). However, based on Nile perch mouth gape (2.0–5.3 cm), only Nile perches <13 cm TL (<1.9 m mouth gape) would not be able to swallow the largest detritivores. Thus, selective predation by Nile perch cannot explain the relatively slow resurgence of detritivores. © Springer Science+Business Media Dordrecht 2014.

Downing A.S.,University of Stockholm | Downing A.S.,Wageningen University | Downing A.S.,Netherlands Institute of Ecology | Van Nes E.H.,Wageningen University | And 47 more authors.
Ecology and Society | Year: 2014

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. © 2014 by the author(s).

Joordens J.C.A.,Leiden University | Kuipers R.S.,Onze Lieve Vrouwe Gasthuis | Kuipers R.S.,University of Groningen | Wanink J.H.,Leiden University | And 2 more authors.
Journal of Human Evolution | Year: 2014

From c. 2Ma (millions of years ago) onwards, hominin brain size and cognition increased in an unprecedented fashion. The exploitation of high-quality food resources, notably from aquatic ecosystems, may have been a facilitator or driver of this phenomenon. The aim of this study is to contribute to the ongoing debate on the possible role of aquatic resources in hominin evolution by providing a more detailed nutritional context. So far, the debate has focused on the relative importance of terrestrial versus aquatic resources while no distinction has been made between different types of aquatic resources. Here we show that Indian Ocean reef fish and eastern African lake fish yield on average similarly high amounts of eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA), and arachidonic acid (AA). Hence a shift from exploiting tropical marine to freshwater ecosystems (or vice versa) would entail no material difference in dietary long-chain polyunsaturated fatty acid (LC-PUFA) availability. However, a shift to marine ecosystems would likely mean a major increase in access to brain-selective micronutrients such as iodine. Fatty fish from marine temperate/cold waters yield twice as much DHA and four times as much EPA as tropical fish, demonstrating that a latitudinal shift in exploitation of African coastal ecosystems could constitute a significant difference in LC-PUFA availability with possible implications for brain development and functioning. We conclude that exploitation of aquatic food resources could have facilitated the initial moderate hominin brain increase as observed in fossils dated to c. 2Ma, but not the exceptional brain increase in later stages of hominin evolution. We propose that the significant expansion in hominin brain size and cognition later on may have been aided by strong directional selecting forces such as runaway sexual selection of intelligence, and nutritionally supported by exploitation of high-quality food resources in stable and productive aquatic ecosystems. © 2014 Elsevier Ltd.

Witte F.,Leiden University | Witte F.,Netherlands Center for Biodiversity | Silsbe G.M.,Netherlands Institute of Ecology | Hecky R.E.,University of Minnesota | And 7 more authors.
Hydrobiologia | Year: 2012

Possible causes of the increased algal blooms in Lake Victoria in the 1980s have been disputed by several authors; some suggested a top-down effect by the introduced Nile perch, whereas others suggested a bottom-up effect due to eutrophication. In this article the potential impact is established of grazing by fish on phytoplankton densities, before the Nile perch upsurge and the concomitant algal blooms in the Mwanza Gulf. The biomass and trophic composition of fish in the sublittoral area of the Mwanza Gulf were calculated based on catch data from bottom trawls, and from gill nets covering the whole water column. Estimates of phytoplankton production in the same area were made from Secchi values and chlorophyll concentrations. The total phytoplankton intake by fish was estimated at 230 mg DW m-2 day-1. The daily gross production ranged from 6,200 to 7,100 mg DW m-2 day-1 and the net production from 1,900 to 2,200 mg DW m-2 day-1. Thus, losses of phytoplankton through grazing by fish were about 3-4% of daily gross and 10-12% of daily net phytoplankton production. As a consequence it is unlikely that the phytoplankton blooms in the second half of the 1980s were due to a top-down effect caused by a strong decline in phytoplankton grazing by fish. © 2011 The Author(s).

McCartney K.,University of Maine at Presque Isle | Witkowski J.,University Of Szczecin | Jordan R.W.,Yamagata University | Daugbjerg N.,Universitetsparken 4 | And 11 more authors.
Marine Micropaleontology | Year: 2014

Silicoflagellate double skeletons are commonly considered to be pre-division stages, even though their life cycle is only partially resolved, especially with respect to reproduction. Double skeletons of the modern silicoflagellate genera Dictyocha Ehrenberg, Distephanus Stöhr, and Octactis Schiller are for the first time examined in detail by scanning electron microscopy in order to improve our understanding of how skeletal morphology relates to paired skeletons. A number of genus-specific mechanisms enable sibling skeletons to be held together at their abbasal surfaces, including a zig-zag design of the basal ring achieved via apical structure, strut attachment and pike rotation (in Distephanus and Dictyocha), and the presence of organic material binding the generally planar basal rings (in Octactis). Contrary to what is generally understood, the siblings are not mirror images of one another. Instead, the triple junctions formed by the skeletal elements of each apical structure are transposed across the middle of the dividing cell to produce a copy with the same rotation. Thus, two dome-shaped skeletons represent halves of a more spherical design, which suggests that the role of the silicoflagellate basal ring is to enable double skeleton formation, but the full implications of this have yet to be explored. Although the purpose of double skeleton formation in silicoflagellates remains unclear, observations from the fossil record indicate that differences in the relative alignment of doublet members can have a high significance for phylogeny. Differences in the doublet structure of living silicoflagellates call for a combined biological and geological perspective of the utility of maintaining Dictyocha, Distephanus and Octactis as separate genera. © 2014 Elsevier B.V.

Goudswaard P.C.,Leiden University | Goudswaard P.C.,Institute for Marine Resources and Ecosystem Studies | Katunzi E.F.B.,Tanzania Fisheries Research Institute | Wanink J.H.,Leiden University | And 3 more authors.
African Journal of Aquatic Science | Year: 2011

Although Nile perch Lates niloticus is assumed to be sensitive to low oxygen concentrations, it was found in deep water in Lake Victoria, where oxygen depletion is common during the rainy season. Since factors determining Nile perch distribution are not well understood its spatial distribution in the Mwanza Gulf of Lake Victoria was analysed in relation to depth, temperature and dissolved oxygen (DO) concentrations. Of these factors, DO concentration and the interaction of depth and DO concentration explained the distribution of Nile perch, whilst temperature had no significant effect. In periods of normoxia, Nile perch preferred to stay near the bottom at depths of 12-35 m, where densities of shrimps, their main prey, were high. However, Nile perch were apparently driven away from these areas by hypoxic conditions in the rainy season. They apparently escaped the seasonally hypoxic layers in deep water by horizontal inshore migration and by vertical movement in upward direction. Sudden upwelling of these deep layers is a threat to the Nile perch stock and the Nile perch fishery. © NISC (Pty) Ltd.

Van De Vijver B.,National Botanic Garden of Belgium | Verweij G.L.,Koeman en Bijkerk n.v | Van Der Wal J.,AQUON Boxtel | Mertens A.,Grontmij B.V
Phytotaxa | Year: 2012

A new cymbelloid diatom species, Encyonopsis neerlandica sp. nov., is described from several moorland pools from The Netherlands. The new species belongs to the complex of species around E. microcephala. Based on light and detailed scanning electron microscopy, the morphology of Encyonopsis neerlandica is discussed and compared to similar species from this complex. Encyonopsis neerlandica is characterized by its rather large valve dimensions, the narrowly to clearly lanceolate valve outline with convex to weakly convex margins and rostrate to subcapitate apices. The distal raphe fissures are ventrally deflected. There is a clear difference between the rounded areolae near the valve margin and the transapically elongated areolae near the axial area. Notes on the ecology of the species are included. © 2012 Magnolia Press.

Kishe-Machumu M.A.,Tanzania Fisheries Research Institute | Kishe-Machumu M.A.,Leiden University | Witte F.,Leiden University | Wanink J.H.,Leiden University | And 2 more authors.
Hydrobiologia | Year: 2012

Haplochromine cichlids used to be the main prey of the introduced Nile perch, Lates niloticus, in Lake Victoria. After depletion of the haplochromine stocks at the end of the 1980s, Nile perch shifted to the shrimp Caridina nilotica and to a lesser degree to its own young and the cyprinid Rastrineobola argentea. In the present study, we investigated the Nile perch diet in the northern Mwanza Gulf after resurgence of some of the haplochromine species and compared it with data collected in the same area in 1988/1989. It became clear that haplochromines are again the major prey of Nile perch. The dietary shift from invertebrate feeding (shrimps) to feeding on fish (haplochromine cichlids) occurs at a smaller size than it did when Nile perch were taking primarily dagaa and juvenile Nile perch as their fish prey. The apparent preference for haplochromines as prey has reduced the degree of cannibalism considerably, which may have a positive impact on Nile perch recruitment. © 2011 The Author(s).

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