The river Vecht fish community after rehabilitation measures: a comparison to the historical situation by using the river Biebrza as a geographical reference

Setting ecological goals for restoring fish communities in larger rivers is hampered by a lack of knowledge of the natural reference conditions. The lowland river Vecht has become highly regulated since 1850. Since the 1970s, measures have been taken to improve water quality followed by the construction of fishways along weirs in the 1990s to rehabilitate migration. We aim to assess the degree of deviation of the current fish community in the river from its pre-regulation state. The assessment is based on a comparison with the less impacted river Biebrza in Poland as a geographical reference, involving a semi-quantitative stepwise reconstruction based on available historic evidence. Electrofishing was used to describe the current quantitative species compositions of the fish communities in the rivers Vecht and Biebrza and historical records of the Vecht region were used to crosscheck the reconstructed fish community around 1850 after correcting for zoogeographical differences. Despite various rehabilitation measures, the deviation from the natural reference is still large. Currently, perch Perca fluviatilis and bream Abramis brama are much more abundant, and bleak Alburnus alburnus, white bream Blicca bjoerkna and most rheophilic species are far less abundant, than before. The main reasons for this deviation appears to be the present quality of the habitats and, for some species, the poor connectivity with the sea as well as with small tributaries. The approach used should be widely applicable to derive natural references, assess rehabilitation success or improve the ecological assessment of integrity as imposed by the European Water Framework Directiv