Author(s): Halvor Kjaeras; Henrik Baktoft; Ana T. Silva; Karl O. Gjelland; Finn Okland; Mohammadreza Maddahi; Ismail Albayrak; Leif Lia; Robert M. Boes; Torbjorn Forseth

Linked Author(s): Halvor Kjærås, Ismail Albayrak, Leif Lia

Keywords: Acoustic telemetry; Downstream migration; Fish guidance rack; Mitigation measures; Salmo salar

Abstract: Downstream passage remains a major challenge at hydropower plants (HPPs), where turbine entrainment causes high mortality of Atlantic salmon (Salmo salar) smolts. For large discharges, fine-bar racks are often infeasible, and behavioural guidance structures such as modified bar racks (MBRs) may offer an alternative, but their field-scale performance has remained untested. A floating, partial-depth MBR (1.5 m deep, 89 m long) was installed in 2021 at the Laudal HPP intake (River Mandalselva, Norway) to guide smolts toward a safe migration corridor. Using 2D/3D acoustic telemetry, 179 tagged smolts were tracked and migration routes determined for 109. Overall downstream passage efficiency was 72.5%, compared with 50.5% predicted by a site-specific model without a rack. Guiding efficiency among smolts interacting with the rack was 63%, increasing to 76% when excluding fish approaching below the rack. Depth data showed that 43% of smolts migrated deeper than 1.5 m, challenging the common design assumption of near-surface migration. Results demonstrate that MBR systems can reduce turbine entrainment at large intakes, but also show that performance depends strongly on vertical overlap between fish depth use and rack coverage. Future designs should consider deeper or modular guidance where smolts do not migrate predominantly near the surface.

DOI: https://doi.org/10.64697/iahr.proc.ise2026.abs.116

Year: 2026