Vloedvlaktes van de Wolga als proeftuin voor de lage landen

Rivieren zijn van nature dynamische systemen met sterk wisselende waterafvoeren. Doordat in het vroege voorjaar een extra hoeveelheid water door de rivieren stroomt, afkomstig van regenval of het smelten van sneeuw, komen periodiek grote oppervlaktes laaggelegen land onder water te staan. Leo Nagelkerke gaat in op de functie van deze vloedvlaktes voor visse

Wageningen UR, wetenschappelijk centrum voor vis

Nederland kent een rijke traditie in aquatische ecologie. Diverse universiteiten, HBO-opleidingen en instellingen houden zich bezig met de biologie van zowel het zoete als het zoute water. Opmerkelijk is dat er daarbij verhoudingsgewijs weinig aandacht wordt besteed aan vissen. Wageningen Universiteit en Research Centrum (WUR) vormt daarop een uitzondering. Binnen deze universiteit richten diverse onderzoeksgroepen zich op vissen. De bekendste zijn de leerstoelgroep Aquacultuur en Visserij en het voormalige rijksinstituut voor visserijonderzoek, het RIVO. In dit artikel legt Leo Nagelkerke, universitair docent bij de leerstoelgroep Aquacultuur en Visserij, uit waar zij zich zoal mee bezighoude

Linking Flow Regime, Floodplain Lake Connectivity and Fish Catch in a Large River-Floodplain System, the Volga-Akhtuba Floodplain (Russian Federation)

River-floodplain systems are amongst the most productive—but often severely impacted—aquatic systems worldwide. We explored the ecological response of fish to flow regime in a large river-floodplain system by studying the relationships between (1) discharge and inundated floodplain area, with a focus on spatial and temporal patterns in floodplain lake connectivity, and (2) flood volume and fisheries catch. Our results demonstrate a non-linear relationship between discharge and floodplain inundation with considerable hysteresis due to differences in inundation and drainage rate. Inundation extent was mostly determined by flood volume, not peak discharge. We found that the more isolated lakes (that is, lakes with a shorter connection duration to the river) are located at higher local elevation and at larger hydrological distance from the main rivers: geographical distance to the river appears a poor predictor of lake isolation. Although year-to-year fish catches in the floodplain were significantly larger with larger flood volumes in the floodplain, they were not in the main river, suggesting that mechanisms that increase catch, such as increased floodplain access or increased somatic growth, are stimulated by flooding in the floodplain, but not in the river. Fish species that profit from flooding belong to different feeding guilds, suggesting that all trophic levels may benefit from flooding. We found indications that the ecological functioning of floodplains is not limited to its temporary availability as habitat. Refugia can be present within the floodplain itself, which should be considered in the management of large rivers and their floodplain

Serial correlation and inter-annual variability in relation to the statistical power of monitoring schemes to detect trends in fish populations

We studied the effects of inter-annual variability and serial correlation on the statistical power of monitoring schemes to detect trends in biomass of bream (Abramis brama) in Lake Veluwemeer (The Netherlands). In order to distinguish between `true¿ system variability and sampling variability we simulated the development of the bream population, using estimates for population structure and growth, and compared the resulting inter-annual variabilities and serial correlations with those from field data. In all cases the inter-annual variability in the field data was larger than in simulated data (e.g. for total biomass of all assessed bream ¿¿=¿0.45 in field data, and ¿¿=¿0.03¿0.14 in simulated data) indicating that sampling variability decreased statistical power for detecting trends. Moreover, sampling variability obscured the inter-annual dependency (and thus the serial correlation) of biomass, which was expected because in this long-lived population biomass changes are buffered by the many year classes present. We did find the expected serial correlation in our simulation results and concluded that good survey data of long-lived fish populations should show low sampling variability and considerable inter-annual serial correlation. Since serial correlation decreases the power for detecting trends, this means that even when sampling variability would be greatly reduced, the number of sampling years to detect a change of 15%·year¿1 in bream populations (corresponding to a halving or doubling in a six-year period) would in most cases be more than six. This would imply that the six-year reporting periods that are required by the Water Framework Directive of the European Union are too short for the existing fish monitoring schemes

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

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

Effecten van Graskarper op de kwaliteit van watersystemen

In deze literatuurstudie is geëvalueerd wat de effectiviteit en ecologische effecten zijn van het uitzetten van graskarpers (Ctenopharyngodon idella) als beheermaatregel om plantengroei te beperken. Daarnaast zijn de effecten van het uitzetten van graskarpers vergeleken met die van maaibeheer. Belangrijkste conclusie is dat het uitzetten van graskarpers als beheermaatregelen voor het verwijderen van watervegetatie vraagt om een grondige afweging

Один из способов управления технологическим процессом дуговой сталеплавильной печи

В статье приводится описание комплексной математической модели, используемой для рационального управления работой дуговых электросталеплавильных печей.In article the description of the complex mathematical model used for rational management by work of arc electrosteel-smelting furnaces is resulted