ABSTRACT Rapid sub‐daily flow fluctuations caused by hydroelectric power generation (hydropeaking) pose a significant challenge to aquatic ecosystems in Alpine rivers, particularly for young‐of‐the‐year (YOY) fish. Hydropeaking reduces recruitment rates through reduced spawning success and habitat availability and persistence, as well as increased downstream drift and stranding rates. This study assessed the abundance of YOY brown trout ( Salmo trutta ) and European grayling ( Thymallus thymallus ) in multiple Austrian Alpine rivers sampled mainly in August and September. Hydropeaking was analysed alongside other factors, including land use, river morphology, hydrology and tributary influences. The results show that sub‐daily flow fluctuations, characterised by event frequency and down‐ramping rates, were the main predictor associated with lower YOY fish abundance. To capture their combined pressure, event frequency, down‐ramping rate, and amplitude were integrated into a hydropeaking index. Along the gradient of the hydropeaking index, the abundance of YOY brown trout and European grayling declined from median values of 10.6 and 4.8 individuals per 100 m, respectively, to less than 1 individual per 100 m. Consistent with this pattern, the literature‐based down‐ramping threshold (0.2–0.3 cm/min) became more common with increasing hydropeaking index and coincided with higher event frequency. Suitable tributaries showed only limited mitigating effects on conditions in the main river channel. From a management perspective, the findings highlight the need to reduce down‐ramping rates and event frequency, particularly during the critical emergence period of YOY fish, in order to mitigate the effects of hydropeaking. Furthermore, the findings show that YOY fish are a suitable indicator of hydropeaking.
Merl et al. (Wed,) studied this question.