Hydropeaking can implicate severe hydrological and hydromorphological consequences on river systems, affecting fish populations e.g. by drift and stranding of young live stages of riverine fish. Many fish-stranding experiments have been done in the past, but until now very little is known about influences of time of day or gravel bank morphology.
We performed experiments with juvenile European graylings (∅-length: 53 mm) in a nature-like channel enabling hydropeaking simulations. In the first test series (n=21), we stocked 250 fish equally distributed into 5 channel segments; 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 within two segments stocked with same densities.
In the daytime and during decreasing water level, we observed low median drift rates of 15 % and median 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 around seven times higher than on the homogenous gravel bank.
This information can be used to modulate hydropower management in a manner more suitable for vulnerable life stages of fish. Reducing the downramping rate or shifting peaks to daytime can mitigate negative effects of hydropeaking in accordance to morphological character of affected rivers.