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Teal L.R.,University of Aberdeen | Teal L.R.,Institute for Marine Resource and Ecosystem Studies IMARES | Parker E.R.,CEFAS - Center for Environment, Fisheries and Aquaculture Science | SOlan M.,University of Aberdeen
Marine Ecology Progress Series | Year: 2010

Faunal mediated particle and porewater mixing (bioturbation) alters the structure ofthe surface sediment layer, forming a distinct mixed layer, where the majority of organicmatter degradation takes place. Current methods of assessing benthic habitat quality often reference this mixed layer as an indicator of benthic activity. Whilst a great deal of effort has been devoted to linking macro-invertebrate activity to the mixing depth, less attention has been given to defining what the mixing depth represents in terms of ecosystem process and function. Here, in situ sediment profile images are analysed using grey scale intensity analysis to distinguish the mixed zone and relate it to the physicochemicalenvironment in order to determine the biological, chemical and physical variables most influential in its formation. Significant differences were found between biogeochemical conditions within the mixed layer relative to the underlying historic sediment layer. These were attributed to a combination of environmental variables (Fe, Mn, Si, chlorophyll a and NO3 -) rather than a single dominant driver of change. Although these findings are consistent across multiple locations, the driver(s) that influence the depth of the mixed layer are site- and season-specific. The mixing depth thus provides a reasonable approximation of benthic ecosystem functioning, but when considering ecosystem process the link between the mixing depth and its driving factors (faunal mixing, food input, environmental conditions) is highly context-dependent. Conclusions on benthic community dynamics and ecosystem process, including assessments of habitat quality, cannot therefore be drawn from estimates of the mixing depth alone. © Inter-Research 2010.

Downing A.S.,Wageningen University | Downing A.S.,University of Stockholm | Galic N.,Wageningen University | Goudswaard K.P.C.,Institute for Marine Resource and Ecosystem Studies IMARES | And 5 more authors.
PLoS ONE | Year: 2013

Nile perch (Lates niloticus) suddenly invaded Lake Victoria between 1979 and 1987, 25 years after its introduction in the Ugandan side of the lake. Nile perch then replaced the native fish diversity and irreversibly altered the ecosystem and its role to lakeshore societies: it is now a prised export product that supports millions of livelihoods. The delay in the Nile perch boom led to a hunt for triggers of the sudden boom and generated several hypotheses regarding its growth at low abundances - all hypotheses having important implications for the management of Nile perch stocks. We use logistic growth as a parsimonious null model to predict when the Nile perch invasion should have been expected, given its growth rate, initial stock size and introduction year. We find the first exponential growth phase can explain the timing of the perch boom at the scale of Lake Victoria, suggesting that complex mechanisms are not necessary to explain the Nile perch invasion or its timing. However, the boom started in Kenya before Uganda, indicating perhaps that Allee effects act at smaller scales than that of the whole Lake. The Nile perch invasion of other lakes indicates that habitat differences may also have an effect on invasion success. Our results suggest there is probably no single management strategy applicable to the whole lake that would lead to both efficient and sustainable exploitation of its resources. © 2013 Downing et al.

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 4 more authors.
Freshwater Biology | Year: 2013

1. A collapse of Nile perch stocks of Lake Victoria could affect up to 30million people. Furthermore, changes in Nile perch population size-structure and stocks make the threat of collapse imminent. However, whether eutrophication or fishing will be the bane of Nile perch is still debated. 2. Here, we attempt to unravel how changes in food resources, a side effect of eutrophication, and fishing mortality determine fish population growth and size structures. We parameterised a physiologically structured model to Nile perch, analysed the influence of ontogenetic diet shifts and relative resource abundances on existence boundaries of Nile perch and described the populations on either side of these boundaries. 3. Our results showed that ignoring ontogenetic diet shifts can lead to over-estimating the maximum sustainable mortality of a fish population. Size distributions can be indicators of processes driving population dynamics. However, the vulnerability of stocks to fishing mortality is dependent on its environment and is not always reflected in size distributions. 4. We suggest that the ecosystem, instead of populations, should be used to monitor long-term effects of human impact. © 2013 Blackwell Publishing Ltd.

Mooij W.M.,Netherlands Institute of Ecology | Mooij W.M.,Wageningen University | Brederveld R.J.,WitteveenBos | de Klein J.J.M.,Wageningen University | And 26 more authors.
Environmental Modelling and Software | Year: 2014

Simulation modelling in ecology is a field that is becoming increasingly compartmentalized. Here we propose a Database Approach To Modelling (DATM) to create unity in dynamical ecosystem modelling with differential equations. In this approach the storage of ecological knowledge is independent of the language and platform in which the model will be run. To create an instance of the model, the information in the database is translated and augmented with the language and platform specifics. This process is automated so that a new instance can be created each time the database is updated. We describe the approach using the simple Lotka-Volterra model and the complex ecosystem model for shallow lakes PCLake, which we automatically implement in the frameworks OSIRIS, GRIND for MATLAB, ACSL, R, DUFLOW and DELWAQ. A clear advantage of working in a database is the overview it provides. The simplicity of the approach only adds to its elegance. © 2014 The Authors.

van de Wolfshaar K.E.,Institute for Marine Resource and Ecosystem Studies IMARES | Schellekens T.,Institute for Marine Resource and Ecosystem Studies IMARES | Poos J.-J.,Institute for Marine Resource and Ecosystem Studies IMARES | van Kooten T.,Institute for Marine Resource and Ecosystem Studies IMARES
PLoS ONE | Year: 2012

In many fisheries multiple species are simultaneously caught while stock assessments and fishing quota are defined at species level. Yet species caught together often share habitat and resources, resulting in interspecific resource competition. The consequences of resource competition on population dynamics and revenue of simultaneously harvested species has received little attention due to the historical single stock approach in fisheries management. Here we present the results of a modelling study on the interaction between resource competition of sole (Solea solea) and slaice (Pleuronectus platessa) and simultaneous harvesting of these species, using a stage-structured population model. Three resources were included of which one is shared with a varied competition intensity. We find that plaice is the better competitor of the two species and adult plaice are more abundant than adult sole. When competition is high sole population biomass increases with increasing fishing effort prior to plaice extinction. As a result of this increase in the sole population, the revenue of the stocks combined as function of effort becomes bimodal with increasing resource competition. When considering a single stock quota for sole, its recovery with increasing effort may result in even more fishing effort that would drive the plaice population to extinction. When sole and plaice compete for resources the highest revenue is obtained at effort levels at which plaice is extinct. Ignoring resource competition promotes overfishing due to increasing stock of one species prior to extinction of the other species. Consequently, efforts to mitigate the decline in one species will not be effective if increased stock in the other species leads to increased quota. If a species is to be protected against extinction, management should not only be directed at this one species, but all species that compete with it for resource as well. © 2012 van de Wolfshaar et al.

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