Auswirkungen von Schwall-Sunk auf Nase (Chondrostoma nasus) und Barbe (Barbus barbus): ein Analyseansatz auf zwei Ebenen mit Fokus auf Larvenstrandung

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 article aims to investigate the effects of short-term flow fluctuations on two cyprinid indicator species, the nase (Chondrostoma nasus) and the barbel (Barbus barbus). This article presents the results of 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. Bank 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. Initial 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 research provides new insights into the effects of hydropeaking on the early life stages of cyprinids, contributing to the development of effective mitigation strategies.