Physiological stress response to hydropeaking in rainbow trout (Oncorhynchus mykiss)

In recent years, there has been growing interest in investigating hydropeaking's impact on rivers. In the case of fish, literature documents stranding, loss of spawning beds, and behavioural changes, while the physiological stress response is less understood. In this study, a natural flow scenario and five hydropeaking scenarios were simulated in a fluvial mesocosm named Greenchannel. Scenarios were characterised through hydraulic/hydrological (water velocity and level, water level fall rate, and frequency and number of inundations) and water quality (temperature, dissolved oxygen levels, turbidity) variables. Different test groups of 15 rainbow trout (Oncorhynchus mykiss) (n = 90) were subjected to these scenarios for 24 h each. Cortisol, creatine phosphokinase (CPK), lactate dehydrogenase (LDH), triglycerides (TGC), lactate, non-esterified fatty acids (NEFA), and histological pigmentation were measured at the end of the tests. The variation in physiological variables in response to the environmental variables characterising each scenario was assessed. The levels of cortisol, lactate, TGC, a pigmentation parameter (i.e. lightness), and water turbidity varied significantly with increasing intensities of hydropeaking, mainly velocity and rate of change of water levels. The mobilisation of metabolites suggests a prolonged stress response extending beyond the acute phase as a result of hydropeaking. Developed environment-physiology models pointed to specific environmental thresholds for rainbow trout (for instance, 0.1 m·s^-1 (velocity) and 0.13 m·h^-1 (rate of change (fall))) above which physiological impacts would lead to irreversible welfare problems. This study provides information on how and to what extent hydropeaking impacts fish communities, guiding the sustainable management of large hydropower plants and the restoration of affected rivers.