We experimentally studied the locomotor components of rheotactic response in three age cohorts (0+, 1+, 2+) of Altantic salmon and brown trout parr from, respectively, the Varzuga River main channel and tributary (Kola Peninsula, White Sea drainage basin). A specially designed hydrodynamic device permitting to regulate the current velocity from 0.05 to 0.9 m/s was used. There was no reliable difference between velocity thresholds for 0+ salmon and brown trout or for 1+ parr of the two species. For 1+ parr, however, the velocity threshold was lower than for 0+ parr in both species. Sensitivity to the current dropped sharply in 2+ parr, especially in brown trout. In brown trout aged 1+ and 2+, the index of active rheotactic response, meaning that locomotion is activated to keep the position in the current, is reliably higher than in salmon. The critical rheotactic response index also differed among age cohorts, and brown trout parr excelled, withstanding current velocities of 80–90 m/s for several seconds. Differences in rheotactic response indexes apparently ensue from the hydrological, thermal and trophic conditions of habitats in the main channel and the tributary, which influenced the size and weight of parr in both species. The main channel is mostly inhabited by salmon parr, whereas the tributary is home
to brown trout parr, which grow faster owing to the better foraging conditions.