Impacts of artificial flow fluctuations on cyprinid fishes

This project aims to assess the impacts of anthropogenic sub-daily flow fluctuations on two Austrian keystone cyprinid fish species, the barbel (Barbus barbus L) and the nase (Chondrostoma nasus L.). In contrast to run-of-river hydropower plants which regularly generate electric power, hydropeaking power plants can quickly turn the turbines on and off only when needed. This operation mode causes significant, quickly fluctuating water levels so-called hydropeaking. Recent studies have shown that hydropeaking can put fish at serious risk. It has been documented, for example, that larvae and juveniles of salmonid fish can be drifted or stranded during quickly fluctuating water levels. Although cyprinid fish such as the barbel and nase could potentially inhabit 77% of all hydropeaking rivers in Austria, no study has yet assessed the effects of hydropeaking on this fish family. This project will, for the first time, analyze the effects of hydropeaking on two keystone native cyprinid fishes, the barbel and nase. The project contains two work packages. For the first, we will consolidate existing biological, hydrological, and other pressure data in a unique, extensive database. We will conduct statistical analysis to detect the response of cyprinid fish to hydropeaking and undisturbed rivers in Austria. The second work package follows an experimental approach where the effects of hydropeaking will be tested in the outdoor flumes of the HyTEC (Hydromorphological and Temperature Experimental Channels) facility in Lunz am See (Lower Austria). This facility enables the assessment of direct effects on larvae and early juvenile life-stages of both species through accurate simulation of hydropeaks in two nature-like channels. We will focus on quantifying drift and stranding rates caused by single and multiple hydropeaks, whereby the following parameters will be considered: up- and downramping rate (change in discharge per unit time) of a hydropeak, time of day (hydropeaking during day or night), as well as different structural conditions of the gravel bar. By combining the results of the first work package, it will be possible, for the first time, to assess the effects of hydropeaking on individual native cyprinid fish as well as for their entire populations in combination with other stressors, thereby recognizing possibilities for improving their current state.

Hydropower plants, particularly storage power plants operated on demand, cause rapid and frequent fluctuations in discharge (hydropeaking) that threaten river ecosystems, especially fish larvae and juveniles. To date, hydropeaking research has primarily focused on salmonids, while the early life stages of cyprinids have been largely neglected. This project aimed to investigate the effects of short-term flow fluctuations on two cyprinid indicator species, the nase (Chondrostoma nasus) and the barbel (Barbus barbus). This project conducted research using two methodological approaches: field studies and hydropeaking experiments conducted in near-natural outdoor flumes. The analysis of Austrian fish sampling sites reveals that nase and barbel have experienced significant declines within their natural distribution ranges. The biomass of both target species is generally very low, particularly in rivers affected by hydropeaking. The hydropeaking experiments highlighted that fish length is a crucial factor in determining the risk of stranding, which decreases with increasing fish size. Nase and barbel larvae strand more frequently at night than during the day. River bar slope significantly affects stranding rates; low-sloped bars result in higher stranding compared to steeper bars, although the former provide more habitat. A faster down-ramping rate increases the risk of stranding, especially at night and on low-sloped gravel bars. Heterogeneous bank habitats (with depressions) can still present a significant stranding risk for larger juvenile fish due to a fish trap effect. Stranding is also strongly influenced by water temperature, which is critical for fish growth and development. First experiments with recurrent hydropeaking events indicate a reduction in stranding rates over successive peaks; however, a residual risk remains. It is still unclear how persistent this (learning) effect is over time. This project provides new insights into the effects of hydropeaking on the early life stages of cyprinids, contributing to the development of effective mitigation strategies.