The Netherlands has two natural bodies of water grading from salt to fresh: the Ems-Dollard in the north and the Western Scheldt in the south of the country. The Western Scheldt is the only estuary in the southwestern delta area that is not closed off from the North Sea by a flood defence. However, the Western Scheldt is not completely natural. To allow large ship access to the ports of Vlissingen, Terneuzen and Antwerp, the estuary has been deepened and widened for decades.
Brackish water transition zones where fresh river water flows to the saline sea are a scarce type of environment throughout Europe. Their special natural values are under pressure. Reason enough to protect the Western Scheldt (and the adjacent upstream Sea Scheldt) via the Birds/Habitats Directive and designate it as a Natura 2000 area. The Western Scheldt also falls under the scope of the Water Framework Directive, which attempts to restore and guarantee a good environmental status of fresh and brackish waters. The legal obligations under these directives include conducting research, including on fish.
The Western Scheldt is an important migration route for migratory fish that swim from freshwater to saltwater or vice versa, and is a spawning ground and nursery for various fish species. Some, for example the sea and river lamprey, twaite shad and anchovy, have a special protection status. In order to be able to assess how these fish species are faring, Wageningen Marine Research (WMR) has set up a monitoring program on behalf of Rijkswaterstaat that focuses on the pelagic fish stock, i.e. the fish species that live in the water column, usually in schools. By fishing for pelagic fish in several places in the Western Scheldt every year in the spring and autumn, a good idea of the trends in the fish stocks present is derived. Moreover, Belgium and the Netherlands are acting together on the basis of the Scheldt Treaties: the same monitoring as in the Western Scheldt is carried out in the Sea Scheldt, by the same skipper.
Fishing is done in a traditional way with a so-called stow net. A ship with two large frame nets on either side anchors in the water stream - and while the tidal current flows through the deployed nets - all fish that go with the flow end up in the net. The catch is sorted on board and later analysed at WMR. The results are reported annually.