Common mechanisms for guidance efficiency of descending Atlantic salmon smolts in small and large hydroelectric power plants

Dams and turbines associated with hydroelectric power plants (HEP) disrupt connec- tivity by affecting fish movement and survival. There has been an increasing focus on measures to facilitate downstream migration at HEPs. The fish guidance efficacy (FGE) of downstream mitigation measures largely remains suboptimal and calls for development of knowledge on factors influencing FGE. In this study, we analyse 6 years of wild Atlantic salmon smolt passive integrated transponder (PIT)‐telemetry data (N = 1,498) from a neighbouring small‐ and a large‐scale HEP. Timing of the smolt migration period was significantly different between the two rivers. Thus, river‐ specific smolt‐run timing is imperative for proper measures management in regulated rivers aiming at maximizing smolt‐descent survival. A generalized linear model including additive effects of relative bypass discharge and scaled river discharge on the FGE for descending smolts received highest Akaike's information criterion support in the data and explained 74.2% of the FGE variation. This model, including no river effect, predicted high FGE (up to 90%) at low river flow (≤30% of HEP maximum capacity) when 7% of the water is allocated through the bypass. Many run‐of‐the‐river HEPs have highly variable river flow during the smolt‐run period. Our model suggests that these HEPs could utilize their manoeuvre flexibility to obtain water allocation routines between bypass and turbines that optimize both FGE and hydroelectric production. bypass, fish guidance efficiency, passive integrated transponders, run‐of‐the‐river, salmonids