Increased Survival of European Eel ( <i>Anguilla anguilla</i> ) After Retrofitting a Hydropower Plant Intake

ABSTRACT The critically endangered European eel ( Anguilla anguilla ) faces major threats from hydropower turbines during downstream migration, which is a critical phase in the species' life history. To mitigate impacts on eel, engineering solutions can be implemented that enhance passage success around turbines. This study assessed the effectiveness of retrofitting the intake trash rack at Fosstveit Hydropower Plant in the River Storelva, Norway, to improve safe passage for silver eels. In 2012 and 2013, PIT‐tagged eels showed high turbine mortality, with survival rates of 52% and 69%, and dead eels were observed in the tailrace. A conventional trash rack with a bottom bypass was evidently ineffective during these years. In 2024, the intake was retrofitted with a 15 mm bar spacing, low‐inclined (29°α) rack guiding fish to a surface bypass. In the following years, 2024 and 2025, acoustically tagged eels released upstream the facility showed 100% survival to the river mouth. Most eels migrated during the dark hours and primarily on days with high or rising discharge, consistent with known cues. Although migration past the hydropower plant remained slower than in downstream sections, the retrofit greatly improved survival. These results support the use of inclined low bar spacing rack and surface bypasses at hydropower plants and underscore the need to address migration delays and explore adaptive flow management to support eel recovery.