Abstract
Fish passes facilitate fish movement in fragmented river systems, yet they can also provide important habitat functions. This study investigated the fish community composition of different constructed habitat types (fluvial habitats, floodplain ponds) within fish passes in relation to habitat characteristics in order to deduce recommendations for fish-friendly designs of such structures. Fish community structures within passes differed significantly from those in the main river, comprising a high number of rheophilic species in fluvial habitats (Thymallus thymallus, Hucho hucho, Salmo trutta, Cottus gobio, Chondrostoma nasus, and Barbus barbus), and of stagnophilic species in floodplain ponds (Rhodeus amarus, Scardinius erythrophthalmus, Misgurnus bipartitus, and Tinca tinca). During summer, floodplain ponds also provided important juvenile habitats for the target species C. nasus and B. barbus. Differences between the two habitat types in fish abundance were mostly explained by differences in macrophyte coverage, gravel, boulders, temperature, and current speed. The findings of this study stress the important habitat functions of fish passes. They also suggest that integration of diverse habitat structures, especially of currently hardly considered constructed floodplain ponds into fish passes, can greatly enhance their fish communities and contribute to the restoration of several declining target species of conservation.
Highlights
Habitat degradation and fragmentation of river systems caused by dams, weirs, and other barriers is considered a major challenge in restoring European fish populations [1].For instance, impoundments that restrict river dynamic processes, such as sediment transport or other matter fluxes, have been identified as one of the major factors related to the decline of riverine fish species in the headwater areas of Elbe, Main, and Danube rivers [2,3]
We investigated the value of fluvial habitats and newly created and connected small floodplain ponds as habitat for the fish community in two nature-like fish passes in relation to their characteristics
This study underlines the important habitat role of fish passes, which can largely contribute to overall fish diversity in rivers, comprising typically rheophilic species in fluvial habitats and stagnophilic species in stagnant waters, such as the created floodplain ponds
Summary
Habitat degradation and fragmentation of river systems caused by dams, weirs, and other barriers is considered a major challenge in restoring European fish populations [1].For instance, impoundments that restrict river dynamic processes, such as sediment transport or other matter fluxes, have been identified as one of the major factors related to the decline of riverine fish species in the headwater areas of Elbe, Main, and Danube rivers [2,3]. The restoration of fish communities in degraded rivers seems to be highly challenging and dependent on restoring connectivity, due to the manifold other restrictions in heavily modified water bodies (HMWB; [14]) In these waters, the natural river dynamic processes are strongly impaired [15,16,17], limiting the chances of restoring complete life cycles of specialized riverine fish species, such as Thymallus thymallus L., Hucho hucho L., Salmo trutta L., Chondrostoma nasus L., and Barbus barbus L. It has been proposed to restore fish habitats in highly degraded rivers by designing separate, parallel flowing river courses or channels outside the degraded systems using a portion of their natural discharge [19,20] as it is required for nature-like fish passes that initially were built to restore fish migration (reviewed in [21]). First studies on the contribution of nature-like fish passes to overall biodiversity of fishes indicated that these structures can be highly valuable refuges for rheophilic fishes [13,19,22], e.g., providing key habitats for reproduction [11,12]
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