{"refrec":{"BRefID":206261,"RR":"<b>Stevens, M.; Van den Neucker, T.; Gelaude, E.; Baeyens, R.; Jacobs, Y.; Mouton, A.; Buysse, D.; Coeck, J.</b> (2011). Onderzoek naar de trekvissoorten in het Schelde-estuarium. Voortplantings-en opgroeihabitat van rivierprik en fint. <i>Rapport van het Instituut voor Natuur- en Bosonderzoek</i>, R.2011.14. INBO: Brussel.  71 pp.","BEntID":198369,"PublicFlag":1,"CheckedFlag":0,"wosflag":null,"vabbflag":null,"RefStringPartII":". <i>Rapporten van het Instituut voor Natuur- en Bosonderzoek</i>, R.2011.14. INBO: Brussel.  71 pp.","DocTypID":13,"DocType":"Reports","MarineFlag":1,"FreshFlag":0,"BrackishFlag":1,"TerrestrialFlag":0,"Authorstring":"Stevens, M.; Van den Neucker, T.; Gelaude, E.; Baeyens, R.; Jacobs, Y.; Mouton, A.; Buysse, D.; Coeck, J.","OrigTitleTranslFlag":0,"Authorstringtrunc":"Stevens, M. <i>et al.</i>","Englishabstract":"Migratory fish such as river lamprey and twaite shad are important indicators of ecosystem functioning. Over the past century, most migratory fish have disappeared from the river Scheldt due to human impacts. The previous study on migratory fishes in the Scheldt showed however that most species show the first signs of recovery (Stevens et al., 2009). For both river lamprey and twaite shad there are strong indications that they reproduce in the Scheldt. However, the spawning and nursery habitats of both species are unknown and it is unclear whether the preconditions for a sustainable recovery are met. The spawning and nursery habitat of river lamprey can be located through targeted sampling of the larvae in the sediment. Sampling with fyke nets showed that adult river lamprey migrate mainly to the Bovenschelde and Zwalmbeek. In both rivers a number of locations were selected, which are, according to the literature, expected to be suitable habitats for the larvae of river lamprey. Wadable sites were sampled with a specially designed sediment pump and the deeper sites with a Van Veen grab. In neither of these rivers, however, river lamprey larvae could be found and no spawning sites could be identified. Possible reasons for the lack of larvae in the samples are (1) that no suitable larval habitat is present in the studied areas, (2) that the larval density in the investigated habitats is low and hence sampling frequency should be increased, (3) that the River Bovenschelde and the River Zwalm are not the main spawning grounds for river lamprey in the Scheldt. Telemetry of adult river lamprey could be a possible solution to locate the spawning grounds. In order to improve the reproduction and survival of river lamprey in the River Bovenschelde, the migration barriers in the Scheldt and its tributaries should be cleared and sufficient larval habitat should be availability. Larval habitat could be created in the River Zwalm and other tributaries through the restoration of natural banks. In addition, mud and sand banks in the Bovenschelde should be protected as much as possible as potential larval habitat.The population of twaite shad in the Scheldt is too small to identify the critical habitats by sampling in the field. Therefore, a habitat suitability model for spawning and larval shad was constructed based on literature data. Hereto, we first selected the environmental variables that determine habitat suitability. Next, for each variable the tolerance range was determined. Finallly, the variables were combined using fuzzy logic in order to determine the degree of suitability of a habitat. The model predicts the presence of suitable spawning habitat in the Upper Zeeschelde, upstream of the River Durme. Later in the season, when the water temperature rises, suitable spawning habitat is also present in the Rivers Kleine Nete and Grote Nete. Suitable habitat for larval shad is located mainly in the Upper Zeeschelde upstream Rupelmonde and in the River Rupel. Spawning of twaite shad takes place in the main channel and during their ontogeny the larvae migrate to the edges of the main channel and to side channels.Therefore, in areas with suitable spawning and larval habitat, both the main and side channels need protection. In particular mudflats, sand flats and subtidal low dynamic habitats should be safeguarded. Dredging of these habitats thus mortgages the recovery of the twaite shad population in the Scheldt. The oxygen concentration in the estuary has been greatly improved in recent years.However, in summer a low-oxygen zone in the freshwater area persists, comprising the upstream migration of adults and the survival of larvae. Periodic hypoxic conditions should therefore be avoided and a minimum oxygen content of 5 mg / l is essential for both adults and larvae. During the last century, hydrodynamics in the estuary has increased markedly. As a result, larvae have more difficulties in maintaining their position in suitable habitat. Actions that increase the river/tidal flow or eliminate local retention areas should therefore be avoided.","AbstractOtherLang":"Trekvissen zoals rivierprik en fint zijn belangrijke indicatoren voor het functioneren van het ecosysteem. Door menselijke invloeden verdwenen tijdens de voorbije eeuw de meeste trekvissen uit de Schelde. De voorgaande studie naar trekvissen in de Schelde toonde echter aan dat voor de meeste soorten een bescheiden herstel is ingezet (Stevens et al., 2009). Voor zowel rivierprik als fint zijn er belangrijke aanwijzingen dat ze zich voortplanten in het stroomgebied van de Schelde. Voor beide soorten zijn de paai- en opgroeihabitatten echter niet gekend en het is onduidelijk of de randvoorwaarden aanwezig zijn voor een duurzaam herstel. Het paai- en opgroeihabitat van rivierprik kan gelokaliseerd worden via een gerichte bemonstering van de larven in het sediment. Bemonstering met fuiken toonde aan dat volwassen rivierprikken vooral migreren naar de Bovenschelde en de Zwalmbeek. In beide waterlopen werden een aantal locaties geselecteerd waarvan op basis van literatuuronderzoek verwacht wordt dat het geschikte habitatten zijn voor de larven van rivierprik. De doorwaadbare locaties werden bemonsterd met een speciaal ontworpen sedimentpomp en de diepere locaties met een Van Veengrijper. In geen van beide waterlopen werden echter rivierpriklarven aangetroffen, waardoor ook geen paaiplaatsen konden aangeduid worden. Mogelijke redenen voor het ontbreken van larven in de stalen zijn (1) dat er geen geschikt larvaal habitat aanwezig is in de onderzochte zones, (2) dat de larvale densiteit in de onderzochte habitatten te laag is en de staalnamefrequentie verhoogd moet worden om ze te detecteren, (3) dat de Zwalm en de Bovenschelde toch niet de belangrijkste paaigebieden voor rivierprik in de Schelde zijn. Mogelijk kan telemetrie van adulte rivierprikken gebruikt worden om de paailocaties te vinden. Om de voortplanting en overlevingskansen van rivierprikken in de Bovenschelde te verbeteren moeten de migratieknelpunten in de Schelde en de zijrivieren passeerbaar gemaakt worden en moet voldoende larvaal habitat beschikbaar zijn. In de Zwalm en andere zijrivieren kan larvaal habitat gecreëerd worden door de aanleg van natuurlijke oevers.Daarnaast moeten slibbanken in de Bovenschelde zo veel mogelijk bewaard worden als potentieel larvaal habitat. De populatie van fint in de Schelde is nog te klein om de essentiële habitatten te identificeren via bemonsteringen in het veld. Daarom werd gekozen om op basis van literatuurgegevens een habitatgeschiktheidsmodel op te stellen voor paaiende finten en fintlarven. Hiervoor werden eerst de omgevingsvariabelen geselecteerd die de habitatgeschiktheid bepalen. Vervolgens werden voor elke variabele de tolerantiegrenzen bepaald. De variabelen werden tenslotte via vage logica (fuzzy logic) gecombineerd om een uitspraak te doen over de mate van geschiktheid van een habitat.Het model voorspelt dat er geschikt paaihabitat aanwezig is in de Zeeschelde stroomopwaarts van de Durme en later op het jaar, wanneer de watertemperatuur stijgt, ook in de Kleine en Grote Nete. Geschikt habitat voor larvale fint bevindt zich vooral in de Zeeschelde stroomopwaarts Rupelmonde en in de Rupel.De voortplanting van fint vindt in de hoofdgeul plaats en de larven verplaatsen zich tijdens hun ontwikkeling naar de randen van de hoofdgeul en naar nevengeulen en zijarmen. In zones met geschikt paai- en larvaal habitat moeten dus zowel de hoofd- als de nevengeul beschermd worden. Hierbij moeten vooral slikken, platen en laagdynamische subtidale habitatten gevrijwaard worden. Het wegbaggeren van deze habitatten hypothekeert dan ook het herstel van de fintpopulatie in de Schelde. Het zuurstofgehalte in het estuarium is de voorbije jaren sterk verbeterd, maar in de zomer blijft er een zuurstofarme zone in het zoetwater gebied aanwezig. Hierdoor komt destroomopwaartse migratie van adulten en de overleving van larven in het gedrang.Periodisch zuurstofarme condities moeten dan ook vermeden worden en een minimaal zuurstofgehalte van ","BibLvlCode":"MS","StandardTitle":"Onderzoek naar de trekvissoorten in het Schelde-estuarium. 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