The interactive effect of river bank morphology and daytime on downstream displacement and stranding of cyprinid larvae in hydropeaking conditions

Downstream displacement and stranding of fish are key impacts in hydropeaked rivers, adversely affecting the organisms and, subsequently, the population. While these phenomena have been extensively studied for salmonid fish, few studies have been conducted on cyprinids, particularly at heterogeneous river banks that provide nursery areas for the earliest life cycle stages. This study examines the effects of rapid flow decreases (i.e., down-ramping) on downstream displacement and stranding of common nase, Chondrostoma nasus, larvae at two river bank morphologies (a flat gravel bar and a heterogeneous river bank consisting of a sill and ditch) and daytime periods (day and night) in a mesocosm facility. For a subset of the sill and ditch experiments, stranding location was noted to quantify the trapping effect of the structure. The results show that nase larvae’s downstream displacement and stranding rates were higher at the sill and ditch than at the flat gravel bar. In addition, the effects of down-ramping were more visible at night than during the day. The displacement and stranding rates were the highest for the sill and ditch structure at night. In this case, the stranding rate was four times higher in the ditch area compared to other available areas of the mesocosm. Riverbank depressions can increase the stranding risk when disconnected and dewatered from the main channel after down-ramping, constituting potential ecological traps for fish larvae. These findings highlight the need to integrate riverbank morphology and flow schemes in hydropeaking mitigation frameworks to reduce fish displacement and stranding risks.