The restoration of nature in large rivers loses its efficacy after approximately fifteen years. This is revealed by research conducted by Wageningen University & Research in collaboration with Rijkswaterstaat (the Dutch Public Works and Water Management Agency) on the diversity and number of typical river fish. To conserve these natural treasures, side channels should be dredged regularly.
'Side channels provide river fish with a safe place to reproduce and raise their offspring', says Twan Stoffers, the PhD candidate researching river fish. To this end, he inventoried and re-analysed most of the available data on river fish over the last three decades. Stoffers: 'flowing water is the most important thing for young fish to develop. If that is available, the other circumstances are often also adequate.'
The main reason for the decline in the quality of side channels is that they slowly clog and become overgrown, which causes a decrease in water flow. This reduces the quality of the environment for river fish. Spontaneous restoration through flooding no longer occurs, and neither are new channels spontaneously created, as they would in a natural river system. It has long been suspected that this is the reason the river fish populations in the Netherlands remain small.
The study, which was published in Science of the Total Environment (STOTEN), shows that human-made side channels have an expiration date. 'This is actually very hopeful', says Stoffers. 'Creating subsidiaries does indeed bring back typical river fish such as the common nase and barbel. But we now know that these nursery grounds require maintenance.'
Breeding grounds for river fish
Large rivers create side channels under natural circumstances, for example, as a result of flooding. These channels disappear after some time due to clogging or drought. A dynamic environment such as this is ideal for young fish. As our rivers are regulated for safety reasons and to accommodate shipping, this dynamic nature has disappeared, and humans must dredge the side channels to provide a suitable habitat for young fish.
The past few dry summers have shown the researchers that fresh water is becoming an increasingly scarce commodity. The field research had to be postponed several times because there was simply insufficient water in the channels. The value of ecologically healthy rivers cannot be emphasised enough. 'This study is a modest step', Stoffers says. 'But, hopefully, it contributes to better river management for humans and nature.
This is a study conducted by Wageningen University & Research in collaboration with Rijkswaterstaat.