Cold thermopeaking-induced drift of nase Chondrostoma nasus larvae

Abstract

Research on how intermittent water releases from hydropower plants affect the early life stages of fish has advanced in the last years, focusing not only on the direct impacts of rapid flow changes (hydropeaking), but also on the short-term fluctuations in water temperature (thermopeaking). Flow and thermal fluctuations caused by hydropeaking may affect fish movement patterns and migration at critical stages of a species’ life cycle, e.g., by inducing passive downstream drift. Using two experimental outdoor channels, we investigated how nase (Chondrostoma nasus, Cypriniformes) larvae respond to a rapid drop in water temperature during hydropeaking (simulating a cold thermopeaking event), reaching on average 5.5 °C under peak flow (maximum discharge) conditions, in comparison with a hydropeaking treatment with a constant water temperature regime. Responses of fish larvae were analyzed during acclimation, up-ramping (increase in discharge), peak flow and down-ramping (decrease in discharge) phases. Fish drift increased during peak flow in the cold thermopeaking treatment compared to hydropeaking. Higher drift rates were also negatively associated with pronounced water temperature drops during peak flow conditions. In addition, the starting temperature of the experiment influenced drift during up-ramping. Overall, the results suggest that cold thermopeaking may increase drift in the early life stages of cypriniform fish compared with hydropeaking with stable water temperature. Hence, monitoring and active water temperature adjustments following hydropower releases should be adopted as strategies to mitigate power plant-related impacts on aquatic organisms.

Salmonids elicit an acute behavioral response to heterothermal environments

“…While limited to laboratory conditions, these observations provide a framework for interpreting the thermal behavior of fish in nature. For example, a recent study observed a significant increase in downstream displacement for early life stages of a cypriniform fish in response to cold-water thermopeaking 62 . The authors hypothesized that this response was caused by reduced swimming capacity and cold shock.…”

Variation in hydropeaking‐induced stranding of Barbus barbus L. and Chondrostoma nasus L. larvae: Assessing the impact of daytime and down‐ramping rates

“…This would be relevant concerning natural year-to-year variations in river water temperature, with colder conditions potentially reducing the abundance of early life stages. However, more studies targeting temperature-related changes in cyprinid stranding, including tests on critical water temperatures and thermopeaking (e.g., Mameri et al, 2023), are required to define effective mitigation measures, such as introducing temperature-dependent restrictions on flow down-ramping. This may be an appropriate approach to reduce larval stranding, particularly for nase, which usually emerge from the riverbed substrate in the spring when water temperatures can still be cooler, for example, due to snowmelt.…”