Management and monitoring of the return of riverine fish species following rehabilitation of Dutch rivers.
De Leeuw, J. J.; Buijse, A. D.; Grift, R. E.; Winter, H. V.
The Dutch lowland rivers Rhine and Meuse have lost much of their natural character and many species of riverine fish have become rare or extinct due to water pollution, habitat loss, and exploitation over the last century. Since the 1980s, management measures have been taken to improve water quality and to restore habitats and fish migration opportunities. Rehabilitation includes amongst others enhancing the active floodplain dynamics through (re)connecting isolated floodplain waters to the main river channel. Fish passages along weirs and sluices have been constructed and, at dammed estuaries, discharge regimes will be adjusted to facilitate fish migration. There are several lines of evidence that these measures have contributed to a rehabilitation of riverine fish populations. Studies in newly created side channels demonstrate the enhancement of nursery habitat. A cost-effective monitoring programme based on registration of by-catches from commercial (fykenet) fishermen carried out since 1993 in the Rhine and Meuse in the Netherlands reveals that the population levels of many rare (mainly red list) species show increasing trends, demonstrating the improvement of the ecological quality of Rhine and Meuse. Rehabilitation of the floodplain function and reducing the impact of hydromorphological barriers could further enhance fish populations at the river basin level. The response of different fish species to river rehabilitation has been variable and may have different causes.