Effects of river bank heterogeneity and time of day on drift and stranding of juvenile European grayling (Thymallus thymallus L.) caused by hydropeaking

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