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Abstract Hydropower is the world's largest renewable electricity source and will have an important role in the future energy system with increased requirements to integrate environmental and socioeconomic aspects of sustainability. One important field of interaction is between hydropower and fish. The aim of optimizing hydropower production as well as fish production via Research and Innovation in the context of the European policy framework was the topic of the workshop " Hydropower and Fish – Research and Innovation in the context of the European Policy Framework " organized in May 2017 in Brussels. This paper reports the main messages from the workshop sessions including future research needs, collaboration strategies and knowledge exchange. In particular, the workshop emphasized the need for standardized monitoring and mitigation approaches and of following a balanced approach in addressing challenges between renewable energy production and river and fish ecology. Future research in the area is needed. As perspective and primer for future discussions, the interrelations of hydropower and fish to the different spheres of the total environment are presented and discussed. Graphical abstract Unlabelled Image Highlights • Hydropower and fish is embedded in the different spheres of the total environment. • A sustainable and balanced approach is required for hydropower and fish. • Research on hydropower and fish is multidisciplinary and systemic. • Cooperation, information exchange and coordinated research are paramount. • Sustainable solutions can be found when research, industry and water management is brought together. [ABSTRACT FROM AUTHOR]

Flow regime alterations from hydropower regulation may impact fish populations. Impacts can be characterized using methods for mapping bottlenecks for different species life stages. Improving the understanding of such bottlenecks could provide a basis for mitigation measures. An integrated modelling approach coupling a salmon population model (IB-salmon), a hydraulic 1D model (HEC-RAS) and a temperature model (Air2Stream) was used to identify the bottlenecks constraining the salmon population in a hydro-regulated river (Ljungan River, Sweden). This integrated approach evaluated changes in the salmon population under different low flow scenarios, involving potential dewatering of spawning sites, and the effect of restored spawning sites. Model results demonstrated that low discharges in winter and summer were potential hydrological bottlenecks for the salmon population, particularly in winter due to potential dewatering of spawning sites. Restoration of spawning sites increased salmon production under all scenarios, posing a potential counter measure against low flows. • Salmon population constrains from low flows in a hydro-regulated river were studied. • Smolt production changes were evaluated using an integrated modelling approach. • Winter and summer low flows were potential bottlenecks for salmon production. • Winter low flow was found the main constraint for Baltic salmon in River Ljungan. • New spawning sites could mitigate low flows effects increasing smolt production. [ABSTRACT FROM AUTHOR]

The obstruction of fish migratory routes by hydroelectric facilities is worldwide one of the major threats to freshwater fishes. During downstream migration, fish may be injured or killed on the trash-racks or in the hydropower turbines. Fish-friendly trash-racks that combine both ecological and technical requirements are a solution to mitigate fish mortality at a low operational cost. This study presents results from an experimental investigation of head-losses and the hydrodynamic performance of six angled trash-rack types with 15 mm bar spacing, varying bar-setup (vertical-streamwise, vertical-angled and horizontal bars) and bar profiles (rectangular and drop shape) under steady flow conditions. The trash-racks were positioned at 30° to the wall of the flume and combined with a bypass at their downstream end. The impact of the different trash-rack types on the upstream flow field was characterized using Image based Volumetric 3-component Velocimetry (V3V) and at the bypass-entrance using an Acoustic Doppler Velocimeter (ADV). The results show that trash-racks with vertical-streamwise and horizontal oriented bars with drop-shape profiles have similar head-losses (13% difference), while trash-racks with vertical-angled bars provide 3–8 times larger head-losses compared to the remaining configurations. The velocity measurements showed that the highest flow velocities occurred for configurations with vertical-angled bars (0.67 m s −1 and 0.81 m s −1 on average, respectively). Turbulence related parameters (e.g. Reynolds shear stresses and Turbulent kinetic energy) were also investigated to evaluate the performance of the alternative trash-racks from both, engineering and ecological perspectives. [ABSTRACT FROM AUTHOR]

Much effort has been devoted to developing, constructing and refining fish passage facilities to enable target species to pass barriers on fluvial systems, and yet, fishway science, engineering and practice remain imperfect. In this review, 17 experts from different fish passage research fields (i.e., biology, ecology, physiology, ecohydraulics, engineering) and from different continents (i.e., North and South America, Europe, Africa, Australia) identified knowledge gaps and provided a roadmap for research priorities and technical developments. Once dominated by an engineering-focused approach, fishway science today involves a wide range of disciplines from fish behaviour to socioeconomics to complex modelling of passage prioritization options in river networks. River barrier impacts on fish migration and dispersal are currently better understood than historically, but basic ecological knowledge underpinning the need for effective fish passage in many regions of the world, including in biodiversity hotspots (e.g., equatorial Africa, South-East Asia), remains largely unknown. Designing efficient fishways, with minimal passage delay and post-passage impacts, requires adaptive management and continued innovation. While the use of fishways in river restoration demands a transition towards fish passage at the community scale, advances in selective fishways are also needed to manage invasive fish colonization. Because of the erroneous view in some literature and communities of practice that fish passage is largely a proven technology, improved international collaboration, information sharing, method standardization and multidisciplinary training are needed. Further development of regional expertise is needed in South America, Asia and Africa where hydropower dams are currently being planned and constructed. [ABSTRACT FROM AUTHOR]

Anthropogenic structures in rivers are major threats for fish migration and effective mitigation is imperative given the worldwide expansion of such structures. Fish behaviour is strongly influenced by hydrodynamics, but little is known on the relation between hydraulics and fish fine scale-movement. We combined 3D Computational fluid dynamics modelling (CFD) with 2D and 3D fish positioning to investigate the relation between hydrodynamics and the downstream movement of Atlantic salmon smolts (Salmo salar). We show that fish use fine-scale flow velocity and turbulence as navigation cues of fine-scale movement behaviour. Tri-dimensional swimming speed and swimming direction can be explained by adjustments of fish to flow motion, which are linked to fish swimming mode. Fish diverge from the flow by swimming at speeds within or higher than their prolonged speeds (0.38–0.73 m s−1). Flow direction plays a pivotal role on fish swimming performance, with high upstream and downwards velocities impacting swimming the most. Turbulence is also influential, by benefiting swimming performance at low TKE (< 0.03 m2 s−2) or constraining it at higher levels. We show that fish behaviour is affected by interactions of several hydraulic variables that should be considered jointly. Unlabelled Image • Development of a combined 3D hydraulic- individual fish -based behavioural model. • 3D movement of salmon smolts partly results from adjustment of fish to flow motion. • Fish can diverge from flow direction at speeds higher than their prolonged speed. • Scientific foundation for improving engineering solutions for safe fish migration. [ABSTRACT FROM AUTHOR]

The paper demonstrates the use of Bayesian networks in multicriteria decision analysis (MCDA) of environmental design alternatives for environmental flows (eflows) and physical habitat remediation measures in the Mandalselva River in Norway. We demonstrate how MCDA using multi-attribute value functions can be implemented in a Bayesian network with decision and utility nodes. An object-oriented Bayesian network is used to integrate impacts computed in quantitative sub-models of hydropower revenues and Atlantic salmon smolt production and qualitative judgement models of mesohabitat fishability and riverscape aesthetics. We show how conditional probability tables are useful for modelling uncertainty in value scaling functions, and variance in criteria weights due to different stakeholder preferences. While the paper demonstrates the technical feasibility of MCDA in a BN, we also discuss the challenges of providing decision-support to a real-world habitat remediation process. Image 1 • Multi-criteria decision analysis (MCDA) of environmental flows and physical habitat restoration measures in a hydropower regulated river. • Conditional probability tables used to model uncertainty in value scaling functions and criteria weighting. • Object-oriented Bayesian network used to integrate expert judgement and quantitative model simulations in MCDA. [ABSTRACT FROM AUTHOR]