Temporal changes in sensitive rheophilic fish communities: the effects of 30 years of floodplain management in the lower river Rhine, the Netherlands

Description

The extensive modifications in the river Rhine resulted in the severe decline, or even loss, of diadromous fish species typical for European lowland river systems, such as Atlantic salmon (Salmo salar) and European sturgeon (Acipenser sturio). In addition, the numbers of potamodromous, rheophilic species, such as European barbel (Barbus barbus), nase (Chondrostoma nasus) and dace (Leuciscus leuciscus) have significantly reduced. Since the 1990s, authorities in the Netherlands have reconstructed numerous floodplains to improve the ecological quality of the river systems. This resulted in positive changes to water quality and an increase in natural habitats, but not to the expected recovery of rheophilic fish species. We expect that the slow recovery of rheophilic fish populations is mainly caused by insufficient presence, accessibility, and/or quality of floodplain habitats that function as nursery areas for young fish. To study the effects of the reconstructed floodplain habitats on the population developments of rheophilic fish, and to assess why these developments are not as positive as expected, we compared species composition and abundance of juvenile fish from 1997, 2009, and 2017, for 4 floodplain systems in the Dutch part of the river Rhine. Temporal changes in the juvenile fish community were related to temporal changes in floodplain morphology (connectivity, size-length ratio, total bank length), habitat variability (vegetation cover, substrate, structural complexity), and river discharge and water flow dynamics. In general the same fish species were found during all periods, with the exception of alien gobies that were first recorded in the 2009 surveys. Rheophilic fish densities, however, all show a distinct decrease in all studied floodplain systems. We hypothesize that this decrease is related to deterioration of the nursery function of the habitats with time, probably related to silting of the systems after their construction. Silting leads to reduced water dynamics, less flow, and even detachment of the floodplain from the main channel, which makes these habitats less suitable for rheophilic fishes and may lead to an increase in eurytopic species, especially when there is also increased vegetation growth. As silting is a natural process and will almost always occur, this means that in order to maintain suitable nursery areas for rheophilic fishes, regular maintenance of these floodplain systems needs to be performed.

Period	8 Sep 2019 → 13 Sep 2019
Event title	6th Biennial Symposium of the International Society for River Science (ISRS), Vienna, Austria, 8-13 September 2019: Riverine landscapes as coupled socio-ecological systems
Event type	Conference
Location	Vienna, Austria