Atlantic salmonSalmo salarL. and brown troutSalmo trutta farioL. are species of high socio‐economic and ecological value. Declining populations make them target species of fisheries management. This paper reviews the direct effects of deficient longitudinal connectivity, changes in discharge, high water temperatures, oxygen depletion, changes in water chemistry and increasing loads of fine sediment on the critical life stages of spawning, egg incubation and emergence. It further provides an overview about the basic autecological requirements of Atlantic salmon and brown trout and summarises important thresholds of physico‐chemical tolerances. This collection of information provides important baselines for assessing historical, ongoing and new threats relevant for the management of both species in fresh waters. Critical early‐life stages of both species are almost identical, creating synergies in conservation and restoration. Seaward‐migrating forms are exposed to further stressors, but improving starting conditions can also greatly improve their resilience.

Resolving the controversy about hydropower is only possible based on reliable data on its ecological effects, particularly fish welfare. Herein, we propose a comprehensive assessment of conventional and innovative hydropower using a dataset of 52,250 fish. The effects of hydropower on fish were most harmful at sites with Kaplan turbines, showing ≤83% mortality. Innovative hydropower, often termed ‘fish‐friendly’, caused ≤64% mortality. Our findings suggested that the runner peripheral speed, number of turbine blades and turbulence at turbine outlets were the most important factors. Synthesis and applications. To reduce the impact of hydropower on fish, site‐specific characteristics such as head drop, bypass options and river‐specific species composition need to be more intensively considered in optimal turbine technologies and operation modes.

Understanding stream thermal heterogeneity patterns is crucial to assess and manage river resilience in light of climate change. The dual acquisition of high-resolution thermal infrared (TIR) and red–green–blue-band (RGB) imagery from unmanned aerial vehicles (UAVs) allows for the identification and characterization of thermally differentiated patches (e.g., cold-water patches—CWPs). However, a lack of harmonized CWP classification metrics (patch size and temperature thresholds) makes comparisons across studies almost impossible. Based on an existing dual UAV imagery dataset (River Ovens, Australia), we present a semi-automatic supervised approach to classify key riverscape habitats and associated thermal properties at a pixel-scale accuracy, based on spectral properties. We selected five morphologically representative reaches to (i) illustrate and test our combined classification and thermal heterogeneity assessment method, (ii) assess the changes in CWP numbers and distribution with different metric definitions, and (iii) model how climatic predictions will affect thermal habitat suitability and connectivity of a cold-adapted fish species. Our method was successfully tested, showing mean thermal differences between shaded and sun-exposed fluvial mesohabitats of up to 0.62 °C. CWP metric definitions substantially changed the number and distance between identified CWPs, and they were strongly dependent on reach morphology. Warmer scenarios illustrated a decrease in suitable fish habitats, but reach-scale morphological complexity helped sustain such habitats. Overall, this study demonstrates the importance of method and metric definitions to enable spatio-temporal comparisons between stream thermal heterogeneity studies.

To promote the sustainable management of hydropower, decision makers require information about cost trade-offs between the restoration of fish passage and hydropower production. We provide a systematic overview of the construction, operational, monitoring, and power loss costs associated with upstream and downstream fish passage measures in the European context. When comparing the total costs of upstream measures across different electricity price scenarios, nature-like solutions (67–88 EUR/kW) tend to cost less than technical solutions (201–287 EUR/kW) on average. Furthermore, nature-like fish passes incur fewer power losses and provide habitat in addition to facilitating fish passage, which presents a strong argument for supporting their development. When evaluating different cost categories of fish passage measures across different electricity price scenarios, construction (45–87%) accounts for the largest share compared to operation (0–1.2%) and power losses (11–54%). However, under a high electricity price scenario, power losses exceed construction costs for technical fish passes. Finally, there tends to be limited information on operational, power loss, and monitoring costs associated with passage measures. Thus, we recommend that policy makers standardize monitoring and reporting of hydraulic, structural, and biological parameters as well as costs in a more detailed manner.

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.

European nase (Chondrostoma nasus) is a specialist riverine fish, characterised by a complex life cycle making it vulnerable to habitat degradation. Recent findings indicate that, analogously to salmonids, the interstitial zone quality may pose a serious bottleneck for successful recruitment of this species. In this study, nase eggs were exposed to different substrate qualities. First, standardised substrate mixtures with differing fine sediment additions were used. Second, we tested different homogenous gravel fractions for their influence on egg development and emergence success. In both setups, substrate composition significantly affected emergence success, timing of emergence and larvae size at emergence. In the substrate mixtures, emergence was most successful in substratum with no fine sediment addition (98%) and decreased to 55% in substratum with 20% fine sediment addition. Emergence was most successful in the coarsest fraction (93%) and decreased to 47% in the finest fraction. Over all treatments, the time between hatching and emergence from substrate differed by up to 156 degree days, thereby indicating that free embryos of nase use the shelter of the interstitial zone for early ontogeny. These results suggest that a loose and porous stream bed can positively contribute to the development success of eggs and larvae and thereby potentially improve the recruitment of nase populations. It is thus important to consider the substrate and interstitial conditions in the conservation and restoration management of this rheophilic cyprinid.