Abstract
Despite of structural deficits, highly modified water bodies (HMWB) contain remnant populations of endangered fish that have high conservation value. Restoration in HMWBs underlies different principles when compared to natural rivers because of impaired river dynamic processes. The objective herein was to assess the contribution of restored habitats of the River Günz, Germany to fish diversity and restoration success of target species. All habitat restoration types, including bank habitats, fast flowing habitats with gravel, structured shallow water zones, nature like fish passes, and a former river course below a spillway were assessed. None of them comprised the full set of 27 fish species or size classes, indicating that the diversity and interlinkage of the restored habitats is most crucial to sustainable fish populations and high biodiversity. When considering the rheophilic target species Chondrostoma nasus and Barbus barbus, only fish passes and the former river course with their high flow current contributed much to their population development, particularly to young specimens <20 cm. The results of this study indicate that measures inside the main channel are unlikely to mitigate deficiencies in HMWBs concerning specialized riverine fish. Instead, interlinkage of a diversity of restoration measures and investment of resources for restoration in HMWBs into nature-like fish passes or former river courses below spillways with dynamically managed discharges seem most useful.
Highlights
Many European rivers have faced fundamental structural changes throughout the last centuries and they are heavily fragmented and channelized
HB and SH habitats were deepest with a mean water depth >1 m, whilst JH, Fish Passes (FP), and SW habitats comprised a mean water depth
The findings of our study indicate that life stage requirements of highly specialised target species concerning their reproduction, juvenile growth or feeding habitats cannot solely be provided in deep slow flowing habitats of the main channel, but rather need a combination of different habitat restoration measures that are placed in appropriate local and interconnected arrangement
Summary
Many European rivers have faced fundamental structural changes throughout the last centuries and they are heavily fragmented and channelized. This often resulted in their recent classification as heavily modified water bodies (HMWB) in the European Water Framework Directive (WFD). HMWB should be considered in conservation management of highly protected fish species, as well as in the general conservation of fish diversity [3,4]. In addition to biological conservation aspects, restoration in HMWBs must take into account flood protection, hydropower generation, and the limited land availability adjacent to the rivers [3,6]. In HMWBs, habitat restoration mostly addresses measures inside of
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