An Experimental Test of Hydropower Entrainment Risk for Subadult Sea‐Run Brown Trout (Aka Blenkje; <scp> <i>Salmo trutta</i> </scp> )

ABSTRACT Brown trout play an important role in the northeastern Atlantic aquatic ecosystems, moving between freshwater and marine environments to maximize fitness across their lives. The blenkje (aka finnock) is a life stage of migratory brown trout that returns to freshwater after its first summer migration to sea without maturing. Their survival in the river is critical for them to return to sea, mature and ultimately reproduce. In the river Aurland, a hydropower tunnel was installed in the lake Vassbygdivatnet, which has the potential to entrain blenkje through the turbine. To investigate risks to blenkje and their potential for successful escape, we experimentally displaced fish into the tunnel before and after the turbines were put into operation. We identified nine clear mortalities from accelerometer data among 44 fish that provided data. More fish (86%) exited the tunnel when they were released into the inactive power station than when released into the active station (56%) and passage was also faster for the group released during the inactive period (29 ± 13 days to exit compared to 44 ± 56 days); however, a survival analysis did not identify a significant difference ( z = −1.07, p = 0.28). Moreover, two individuals were observed to survive entrainment by the turbine and subsequently swim into the lower part of the river after entrainment. Accelerometer data from the tags provided evidence that swimming demands were increased when the tunnel was actively producing hydropower.