LAUSR

Abstract In September 2013, hatchery reared sub-yearling steelhead trout, Oncorhynchus mykiss, juveniles were separated into three groups and were reared in 3 different water temperature regimes (ambient (14.4–16.9 °C), cool (10.6–11.8 °C), and warm (20.0–22.3 °C)) in circular hatchery tanks for approximately 1.5 weeks. In Experiment 1, an individual steelhead trout juvenile acclimated in ambient water was transferred into a two-choice stream tank where the fish received either ambient (n = 12) or cool (n = 6)/warm (n = 6) water. During a 30 min observation, steelhead trout juveniles clearly avoided both cool and warm water areas. In Experiment 2, we transferred an individual steelhead trout juvenile, which was acclimated in cooled water, into the cool two-choice stream tank and introduced ambient water (relative water temperature increase) (n = 6). Such an increase caused the juvenile to stay in cool water, suggesting that once steelhead juvenile have acclimated in cool water for 1.5 weeks, these fish would avoid relatively higher temperature (ambient) water and stay in a cooler area. In Experiment 3, we transferred an individual steelhead trout juvenile, which was acclimated in warmed water, into the warm two-choice stream tank and introduced ambient water (relative water temperature decrease) (n = 6). As a result, the relative decrease in water temperature did not influence swimming location choice of the juveniles, suggesting that juvenile steelhead trout do not avoid ambient water (relatively cooler water) even after they were acclimated in warm water. Based on these findings, we suggest that water temperature changes may promote swimming location choice in steelhead trout juveniles, depending on the changing patterns.