Water velocity preferences of Coho Salmon during the parr-smolt transformation

Juvenile Coho Salmon undergo many physiological changes during their springtime transformation from a freshwater parr to a migratory, seawater-capable smolt. Although field observations indicate smolts moving towards the surface and across the breadth of their streams to either swim or drift downstream with the current, water-velocity preferences of these developing cohos are unknown. Using video analysis of their swimming patterns in a calibrated, laboratory flow table with a velocity gradient, groups of three cohos generally increased their preferred water velocity through the springtime study period, to a late-May peak (daytime data, change-point regression analysis, p < 0.05) and over the entire period (nighttime data, regression analysis, p < 0.05). Moving to swifter currents should facilitate the downstream movements of these young cohos, as they develop through the parr-smolt transformation period. This information should assist managers of regulated watersheds and salmon hatcheries in optimizing juvenile salmon survival (e.g., with timely, late-spring water releases producing 0.1–0.3 m s⁻¹ downstream water velocities).