Mitigating Hydropower Impacts Evaluating Fish Passage Improvements for Atlantic Salmon and Brown Trout

Abstract

Hydropower operations frequently obstruct the downstream migration of anadromous salmonids, exposing smolts to physical barriers, delayed passage, and increased mortality. In 2023, structural modifications were implemented at the Fosstveit Hydropower Plant (HEP) in southern Norway to improve passage conditions for juvenile Atlantic salmon (Salmo salar) and anadromous brown trout (Salmo trutta). A 15 mm angled trash rack and low-level bypass system were installed to reduce turbine entrainment and facilitate safer migration. This study evaluated how smolts responded to these modifications under varying environmental conditions, with a focus on behaviour, passage timing, route selection, and survival. In total, 359 smolts (207 Atlantic salmon, 152 brown trout) were tagged with PIT and radio transmitters. However, only 238 individuals were included in the final analyses due to post-tagging mortality, release-related losses, forebay mortality, possible tag failure, expulsion, predation, or undetected bypass exit. PIT antennas and manual tracking were used to monitor passage events, while water temperature, discharge, and light intensity were logged continuously. The majority of surviving smolts passed through the bypass, with no individuals detected in the turbine intake. Nevertheless, undetected mortality in the forebay could not be ruled out. Atlantic salmon migrated more rapidly than brown trout, exhibiting significantly shorter forebay hesitation times (mean: 41.5 min vs. 112 min) and bypass passage durations (mean: 8.5 min vs. 13.7 min). Passage was strongly synchronised with crepuscular periods, and both species preferred migrating under light intensities between 5 and 10 lux. Generalised Additive Models (GAMs) showed that hesitation time was significantly influenced by water discharge (EDF = 6.5, p 12 °C but plateaued above 16 °C, suggesting thermal constraints on movement readiness. Zero-inflated negative binomial models revealed that heavier individuals—particularly brown trout—made significantly more passage attempts (r = 0.52, p < 0.01), while fish length was only weakly associated (r = 0.24, p = 0.08). Principal Component Analysis (PCA) and k-means clustering identified three behavioural phenotypes differing in morphology, flow exposure, light conditions, and passage timing. Cormack-Jolly-Seber survival models estimated post-passage survival at 0.97 for salmon and 0.96 for trout, indicating overall successful mitigation. Smoltification is not only an energetically demanding developmental stage but also a physiologically constrained and behaviourally sensitive window. Delay or stress during this period can impair osmoregulatory performance and reduce marine survival. The redesigned intake structure at Fosstveit effectively reduced turbine risk and promoted functionally meaningful passage for both species. However, the broader inter- and intraspecific variation in brown trout behaviour suggests that structural mitigation should be complemented by behavioural guidance cues. This study underscores the need for species-specific and ecologically grounded design of fish-friendly hydropower systems