Visbestandopnames in Vlaamse beken en rivieren afgevist in het kader van het 'Meetnet Zoetwatervis' 2007
Van Thuyne, G.; Breine, J. (2008). Visbestandopnames in Vlaamse beken en rivieren afgevist in het kader van het 'Meetnet Zoetwatervis' 2007. Rapport van het Instituut voor Natuur- en Bosonderzoek, R.2008.21. Instituut voor Natuur- en Bosonderzoek: Groenendaal. 154 pp.
Part of: Rapporten van het Instituut voor Natuur- en Bosonderzoek. Instituut voor Natuur- en Bosonderzoek: Brussel. ISSN 1782-9054
In this document we report about the surveys performed within the framework of the freshwater monitoring network. We assessed the fish assemblages in 149 sites in nine different river basins. Among the rivers we fished in the Dijle, the Leie and the Zenne. Fish assemblage data were obtained by electrofishing using a 5 kW generator with an adjustable output voltage of 300 to 500 V and a pulse frequency of 480 Hz. The number of electric fishing devices and the number of hand-held anodes used was 2 except when the river was smaller than 1 m. We sampled also fish using double fyke nets (type 120/80). Fish data recorded included speciesspecific fish densities, individual total lengths (TL) and wet weights (g). On each site we recorded different abiotic parameters. We give for each river basin the results of the fishing campaigns. In the River Dijle basin we surveyed the Dijle and some of its tributaries. The ecological quality of the River Dijle downstream Leuven improved compared to previous campaigns. The upstream locations have a better quality but in general we observe a positive evolution in the fish population. For the first time fish was caught in the River Zenne which is rather a remarkable result since this river had a reputation as an open sewer for years and is only recently purified. In the other tributaries we could only observe an improvement in the Voer. In average the sites in the River Dijle basin have a poor quality. In the River Demer basin we surveyed two tributaries each of them showing a decrease in water quality which is reflected in a poor score for the index of biotic integrity (IBI). In the River Nete Basin and River Maas basin we could not see a change of quality compared to previous surveys. Most of the locations score a moderate quality. 25% of the assessed sites in the River Maas basin have a good quality. In the River IJzer basin a positive trend was observed but the IBI still scores poor in most sites. A similar trend is observed for the water courses within the Gentse Kanalen which in average score moderate. In the basin of the Schelde (upstream Gent) we recorded a status quo camparing to previous surveys. The IBI of these water courses score bad or poor. In the River Leie basin we fished in the River Leie and some tributaries e.g. Heulebeek and Mandel. For the River Leie the positive trend observed in 2003 was confirmed. Species diversity and density increased spectacularly. And in average the river obtains a moderate IBI score. A quality improvement in the section between Deinze and Kortrijk is remarkable but most fish was caught in the so called “tourist Leie”. For the first time in the River Leie basin Stone loach was caught. In the River Schelde basin downstream Gent we surveyed some tributaries. In half of the sites no fish were caught and the other sites score mostly poor. A moderate quality was recorded in the Zwanebeek and the Zuidelijke Watergang. In the Barbierbeek a slight improvement was observed.
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