Unnatural changes in river flow patterns resulting from peak-operating hydropower plants adversely impact freshwater ecosystems. In particular, the rapid dewatering of shoreline habitats during artificial flow down-ramping puts early fish life stages at a high risk of becoming stranded if they fail to follow receding water levels in time. While extensive research has been conducted on the effects of hydropeaking on salmonid species, there is limited knowledge on the diverse cyprinid family, particularly on vulnerable early life stages. Hence, this study aims to compare the larval stranding of two cyprinid species, the common barbel (Barbus barbus L.) and common nase (Chondrostoma nasus L.), in response to bank dewatering. We conducted larvae experiments in near-natural mesocosms, simulating single flow down-ramping events with varying down-ramping rates (0.3-1.8 cm & sdot;min(-1)) during the day and at night to quantify stranding rates, also including water temperature and fish development. Our results reveal distinct diurnal patterns for both species, with higher stranding rates during the night than during the day in all experimental scenarios. The data also show higher stranding rates at faster down-ramping, with interaction effects between down-ramping rates and time of day. The stranding rates between the two species are similar across most of the scenarios. Scenarios with colder water temperatures show that nase larvae tend to strand more frequently than with warmer temperatures. In conclusion, the study results contribute to the ongoing discourse on hydropeaking mitigation by providing new perspectives on flow-reduction effects on early cyprinid life stages. Mitigation measures should prioritize the periods during early larval development and factor in prevailing water temperatures. Lowering down-ramping rates, especially during nighttime, will help minimizing negative impacts on aquatic ecosystems, particularly when combining flow rules and habitat restoration measures.

Research and management of hydropeaked rivers largely overlook the ecological impacts of recurring flow fluctuations, such as fish stranding, on ecosystem health. This article synthesizes scientific and grey literature, field studies, and experiments to assess the effects of frequent hydropeaking on fish. Findings show that hydropeaking frequency significantly affects the ecological integrity of alpine rivers, with an average of three daily down-ramping events. Despite some evidence of behavioral adaptation of fish to recurrent flow fluctuations, this adaptation appears insufficient to counter the cumulative effect of a series of single hydropeaking events. Larval and juvenile fish are particularly vulnerable, with stranding impacts extending to the population and community levels. Effective mitigation should prioritize reducing the cumulative impact of recurring hydropeaks while ensuring single-event ramping rates and flow amplitudes remain within ecological limits. To effectively safeguard sensitive habitats, targeted mitigation efforts informed by an understanding of habitat dynamics are critical. Furthermore, maintaining lateral connectivity within river systems is essential for supporting resilient fish populations, especially where hydropeaking mitigation possibilities are limited. Finally, this study identifies future research directions on hydropeaking frequency and its ecological effects.

Wasserkraftwerke beeinflussen Flussökosysteme erheblich, insbesondere Speicherkraftwerke, die bedarfsabhängig betrieben werden. Diese diskontinuierliche Betriebsweise führt zu schnellen und häufigen Abflussschwankungen (Schwall-Sunk), die Fische, dabei vor allem Larven und Jungfische, gefährden. Bisher lag der Schwerpunkt der Schwallforschung auf Salmoniden, während frühe Lebensstadien der Cypriniden (Karpfenfische) kaum untersucht wurden. Ziel dieses Artikels ist es, die Auswirkungen kurzfristiger Abflussschwankungen auf zwei karpfenartige Zeigerarten, die Nase (Chondrostoma nasus) und die Barbe (Barbus barbus), zu untersuchen. In vorliegender Arbeit werden die Ergebnisse zweier methodischer Ansätze – Freilanduntersuchungen und Schwallversuche in naturnahen Fließrinnen – beschrieben. Die Analyse österreichischer Befischungsstellen zeigt, dass Nase und Barbe in ihrem natürlichen Verbreitungsgebiet signifikante Rückgänge erlitten haben. Die Biomasse beider Zielarten ist insgesamt sehr gering, insbesondere in Schwallstrecken. Die Schwallversuche zeigen, dass die Fischlänge ein entscheidender Faktor für das Strandungsrisiko ist: Mit zunehmender Größe nimmt das Risiko ab. Nasen- und Barbenlarven stranden nachts häufiger als tagsüber. Die Uferquerneigung beeinflusst die Strandungsrate stark; flachere Ufer erhöhen das Risiko im Vergleich zu steileren, bieten jedoch mehr Lebensraum. Eine schnellere Abstiegsrate erhöht ebenfalls das Strandungsrisiko, besonders nachts und auf flach geneigten Kiesbänken. Heterogene Uferhabitate (mit Mulden) können auch für größere Jungfische aufgrund eines Fischfalleneffekts ein signifikantes Strandungsrisiko darstellen. Die Strandung wird zudem maßgeblich von der Wassertemperatur beeinflusst, die entscheidend für das Fischwachstum und die -entwicklung ist. Erste Mehrfachschwallversuche zeigen eine Abnahme der Fischstrandung bei aufeinanderfolgenden Schwallereignissen, wobei jedoch eine Reststrandung bestehen bleibt. Wie nachhaltig dieser (Lern‑)Effekt ist, bleibt noch ungeklärt. Diese Arbeit bieten neue Einblicke in die Auswirkungen kurzfristiger Abflussschwankungen auf die frühen Lebensstadien von Cypriniden und trägt somit zur Diskussion über die Schwall-Sunk-Sanierung bei.

Artificial sub-daily flow fluctuations caused by peak-operating hydropower plants are considered one of the most significant impacts on riverine ecosystems downstream of dams. These rivers have, therefore, been subject to numerous studies in recent decades. However, cyprinid fish, in contrast to salmonids, have hardly been addressed in hydropeaking studies yet, and extensive knowledge gaps remain. Therefore, our experimental study aims to assess the effects of rapid flow reductions on the early life stages of two European cyprinid indicator species, the common barbel (Barbus barbus L.) and the common nase (Chondrostoma nasus L.).We conducted mesocosm experiments (2.25×2 m) under semi-natural conditions at an outdoor experimental facility (http://hydropeaking.boku.ac.at), simulating different hydropeaking scenarios with varying down-ramping rates during day and night. At each trial, 100 fish from one species (body length <20 mm) were stocked at peak flow (80 L.s-1). After an acclimation time (15 min.), the flow rate was reduced with variable ramping rates (0.3–1.8 cm.min-1) to constant low flow conditions (10 L.s-1). As a response parameter, larval stranding on a gently sloped shoreline mimicking typical nursery habitats was quantified during day and night.The results reveal distinct diurnal patterns for both species, with increased stranding rates at night for all experimental scenarios. In addition, the data indicate differences between the tested down-ramping rates and show interaction effects between both parameters. The difference between species may result from water temperature and ecological factors. The study outcomes will benefit the ongoing discussion on species-specific hydropeaking mitigation by providing first insights on the direct effects of artificial flow down-ramping on early life stages of cyprinid fish.

Intermittent water releases from hydropower plants, called hydropeaking, negatively affect river biota. The impacts mainly depend on hydrological alterations, but changes in physical habitat conditions are suspected to be co-responsible. For example, hydropeaking accompanied by a sudden change of water temperature in the downstream river-called thermopeaking-is also presumed to impair aquatic ecosystems. Still, knowledge about these thermopeaking impacts on aquatic species and life-stages is limited. We performed flume experiments under semi-natural conditions to fill this knowledge gap, simulating single hydropeaking events with a change in water temperature. As response parameters, we quantified the drift and stranding of early life-stages of European grayling (Thymallus thymallus L.), a key fish species of Alpine hydropeaking rivers. Hydropeaking events with a decrease in water temperature ("cold thermopeaking") led to significantly higher downstream drift (mean = 51%) than events with increasing water temperature ("warm thermopeaking", mean = 27%). Moreover, during cold thermopeaking, a comparably high fish drift was recorded up to 45 min after the start of peak flows. In contrast, drift rates quickly decreased after 15 min during warm thermopeaking. Remarkably, the spatial distribution of downstream drift along gravel bars during cold thermopeaking showed the opposite pattern compared to those triggered by warm thermopeaking events indicating different behavioral responses. Furthermore, the stranding rates of the cold thermopeaking trials were twice as high (mean = 31%) as those of the warm thermopeaking experiments (mean = 14%). The outcomes present vital information for improving mitigation measures and adapting environmental guidelines.

Downstream displacement and stranding of fish are key impacts in hydropeaked rivers, adversely affecting the organisms and, subsequently, the population. While these phenomena have been extensively studied for salmonid fish, few studies have been conducted on cyprinids, particularly at heterogeneous river banks that provide nursery areas for the earliest life cycle stages. This study examines the effects of rapid flow decreases (i.e., down-ramping) on downstream displacement and stranding of common nase, Chondrostoma nasus, larvae at two river bank morphologies (a flat gravel bar and a heterogeneous river bank consisting of a sill and ditch) and daytime periods (day and night) in a mesocosm facility. For a subset of the sill and ditch experiments, stranding location was noted to quantify the trapping effect of the structure. The results show that nase larvae's downstream displacement and stranding rates were higher at the sill and ditch than at the flat gravel bar. In addition, the effects of down-ramping were more visible at night than during the day. The displacement and stranding rates were the highest for the sill and ditch structure at night. In this case, the stranding rate was four times higher in the ditch area compared to other available areas of the mesocosm. Riverbank depressions can increase the stranding risk when disconnected and dewatered from the main channel after down-ramping, constituting potential ecological traps for fish larvae. These findings highlight the need to integrate riverbank morphology and flow schemes in hydropeaking mitigation frameworks to reduce fish displacement and stranding risks.

Fish protection at hydropower plants is important for the sustainability of hosting ecosystems and the acceptance of hydropower. On their way downstream, fish are exposed to hydropower plants and various related negative effects, ranging from a delay in downstream movement to being injured or killed by a turbine. Understanding the behavior of fish in close proximity to protection devices is essential in order to establish efficient fish protection facilities. In this study, physical (horizontal steel cables) and behavioral barriers (electric field) for fish protection were developed (Flexible FishProtector) and their effectiveness was investigated. The behavior of brown trout (Salmo trutta fario), rainbow trout (Oncorhynchus mykiss), grayling (Thymallus thymallus) and chub (Squalius cephalus) at the Flexible FishProtector was analyzed using video evaluation. The experimental setup was a non-scaled section model of a runoff river power plant. The used electric field induced a flight reaction at a corresponding distance to the Flexible FishProtector that significantly increased the protection rate. Furthermore, an increase in guiding efficiency was achieved with the use of a physical as well as a physical and behavioral barrier, supporting safe downstream migration with the narrower cable clearance (30 mm versus 60 mm).

Rapid water level decreases due to hydropeaking are known to negatively affect riverine biota, mainly due to the stranding of organisms in the river bank area that becomes regularly dewatered. Even though studies of the last decades have focused on salmonid fish, also cyprinids may be affected. However, limited knowledge is available of this fish family. Therefore, we conducted mesocosm experiments under semi-natural conditions, simulating single hydropeaking events at two different lateral bank slopes (2% and 5%) with varying down-ramping rates (0.7–3.0 cm min−1) during day and night. As a response parameter, we quantified stranding rates of different larval stages (III-IV and V) of common nase (Chondrostoma nasus L.). The experiments revealed that lower sloped banks exhibited distinctly higher stranding rates than steeper ones. Daytime revealed a similar pattern, with more fish becoming stranded at night than during the day, and this was consistent for all down-ramping rates. The data also indicate increased stranding with higher down-ramping rates, particularly at low sloped riverbanks, and interaction effects between the tested parameters. Overall, this study, for the first time, quantifies the consequences of flow down-ramping on nase larvae, also revealing differences between larval stages. The gained information will, therefore, advance the ongoing discussion on hydropeaking mitigation by providing a deeper understanding of the effects of artificial sub-daily flow fluctuations on the early life stages of cyprinid fish. Our results can inform management and policy to sharpen existing mitigation concepts and fine-tune hydropower operations to reduce negative effects on riverine ecosystems.

Rivers worldwide have been transformed into fragmented, impounded, channelized, and flow-regulated ecosystems. These anthropogenic transformations can reduce fish distribution and population status, especially of those species belonging to medium- or long-distance migratory guilds and those dependent on free-flowing rivers and intact sediment and habitat conditions. Here, we aim to understand how different hydro-morphological pressure types affect the distribution and population status of key potamodromous fish species of the rheophilic and lithophilic fish guilds, the barbel ( Barbus barbus ) and the nase ( Chondrostoma nasus ). We also assess the status of chub ( Squalius cephalus ) to include a species less sensitive to habitat degradation. For the first time, we assembled an extensive Austrian-wide GIS-based fish sampling database with hundreds of biological surveys, allowing us to analyze quantitatively >4,000 river kilometers for presence/absence of target fishes and to assess population status. The data reveal that the distribution range of target species decreased by around 40–60% compared to their natural ranges according to the reference standard (Leitbild). Hydro-morphological pressures affect target species’ population biomass, and trends between impact types can be detected. Chub and barbel exhibit the highest median biomass in free-flowing rivers and residual flow reaches, followed by reservoir sections. Of all pressure types, population biomass is lowest in hydropeaked river stretches. Nase biomass has a grand median of 0.0 kg/ha across all sites, showing hardly any differences between hydro-morphological pressure types. Overall, our results show a drastic shrinkage of the distribution range of three cyprinid fish species previously prominent in Austria. By linking current population vitality to hydro-morphological stressors and ecological status assessments, this study sets a baseline for data-based conservation actions of (Red-listed) species as well as policy and management frameworks.

The FishProtector, an Electric Flexible Fish Fence is investigated in ethohydraulic experiments. Fish protection potential is tested using an outdoor, experimental channel. To find optimum parameter settings, cable clearance, approach flow angle, and the size of the electric field vary within predefined limits. Due to the combination of both, the physical barrier (tensioned steel cables) and the behavioral barrier (pulsed electric field in the low voltage range), fish protection no longer depends solely on cable clearance. Generally, all investigated setups with combined physical and behavioral barriers show mean fish protection rates of higher than 97%. Cable clearances of 60 mm (largest cable clearances investigated within this study) show the same fish protection rates as cable clearances of 30 mm, each in combination with an electric field. The setup was a non-scaled section-model of a runoff river power plant. The behavioral experiments were performed with wild and PIT-tagged brown trout (Salmo trutta), rainbow trout (Oncorhynchus mykiss), grayling (Thymallus thymallus), and chub (Squalius cephalus). Measured body lengths were between 100 and 285 mm. The FishProtector with wide cable clearances and moderate electric fields in both size and voltage significantly improves the fish protection rate, showing significant potential for operating hydropower plants.

A new, integral fish protection system for medium to large sized hydropower plants, which combines the advantages of a physical and a behavioral barrier, is introduced in this paper. The FishProtector is based on horizontally arranged steel cables. These are installed in front of the turbine inlet in a way that fish are protected and guided to a bypass system. At the same time, the cables are used as electrodes to create an electric field in the adjacent waterbody. The fish protection efficiency as well as the guiding efficiency of the system were quantified and assessed in ethohydraulic experiments for various potamodromous species. Further, the behavior of fish in close proximity to the FishProtector was observed and classified using underwater cameras. The results of the ethohydraulic experiments show, that the electric field around the steel cables acts as a behavioral barrier causing very high fish protection rates and enables a larger spacing between individual cables while maintaining the fish protection rates. Further, fish are guided along the FishProtector, showing a typical behavior pattern in close proximity to the hybrid barrier. Verified by a huge number of empirical data, the knowledge stock in terms of downstream fish migration and fish behavior in close proximity to a barrier could be increased.

FRANZ GREIMEL1, JÜRGEN NEUBARTH2, BERNHARD ZEIRINGER1, DANIEL S. HAYES1, MELANIE HASLAUER1, SIMON FÜHRER1, STEFAN AUER1, NORBERT HÖLLER3, CHRISTOPH HAUER4, PATRICK HOLZAPFEL4, MARTIN FUHRMANN4, MARKUS PFLEGER5, PETER MATT6, VERONIKA KOLLER-KREIMEL7, STEFAN SCHMUTZ1 1Institute of Hydrobiology and Aquatic Ecosystem Management, University of Natural Resources and Life Sciences, Vienna, Austria 2e3 consult GmbH, Innsbruck, Austria 3Center for IT Services, BOKU University, Vienna, Austria 4Christian Doppler Laboratory for Sediment Research and Management, Institute of Water Management, Hydrology and Hydraulic Engineering, University of Natural Resources and Life Sciences, Vienna, Austria 5VERBUND Hydropower GmbH, Vienna, Austria 6Vorarlberger Illwerke AG, Bregenz, Austria 7Austrian Ministry for Agriculture, Forestry, Environment and Water Management, Vienna, Austria

High-head storage hydropower is deemed to be the ideal renewable energy source in Alpine regions to meet the increasing demand for daily peak electrical energy. However, this mode of operation - called hydropeaking - can imply severe hydrological and hydromorphological consequences for river ecosystems, affecting fish populations by e.g. drift and stranding of young life stages. Several fish-stranding experiments using physical models have been performed in the past, but until now very little is known about influences of time of day or gravel bank heterogeneity. We performed experiments during late summer 2013 with juvenile European grayling (Thymallus thymallus) (mean length: 53mm) in a nature-like experimental channel enabling hydropeaking simulations. In the first experiments (n=21) we observed relative drift and stranding rates for a single hydropeaking event focusing on the effect of time of day on a homogenous gravel bank. The second test series (n=15) focused on two dewatering potholes installed as potential traps. Additional experiments (n=6) were done with a reduced downramping rate to gain information about potential mitigation effects on stranding risk. During daytime and decreasing water level, we observed low drift rates of 15% and stranding rates below 5% in dewatering potholes and on homogenous gravel banks. However, in the presence of dewatering potholes, nighttime drift rates were about three times and stranding rates about ten times higher than on the homogenous gravel bank. A lowered downramping rate reduced drift to about a quarter and almost eliminated nocturnal stranding risk. These results might be used to effectively regulate water releases from high-head storage hydropower plants in a more suitable way for sensitive life stages of fish. Reducing the downramping rate or shifting peaks to daytime can reduce negative effects of hydropeaking in consideration of the morphological character of affected rivers.

We analysed the effects of hydropeaking in terms of drift and stranding of juvenile graylings ( Thymallus thymallus L.) with an average length of 53.3 mm. Single peak experiments were investigated in 2 outdoor flumes each of 20 m length with the main focus on time of day and gravel bank morphology ( absence or presence of dewatering potholes). By daytime we observed low median drift rates of 15 % and median stranding rates below 5 % in dewatering potholes and on homogenous gravel bank during decreasing water level. Although, in the presence of dewatering potholes, the nighttime drift rates were about three times and stranding rates around seven times higher than on the homogenous gravel bank.