Winter‐Rearing Temperature Affects Growth Profiles, Age of Maturation, and Smolt‐to‐Adult Returns for Yearling Summer Chinook Salmon in the Upper Columbia River Basin

We assessed the effects of rearing conditions in four hatchery programs from the upper Columbia River basin on the survival and demographics of yearling summer Chinook Salmon Oncorhynchus tshawytscha over four release years. Juveniles from each hatchery program were initially reared at Eastbank Hatchery near Wenatchee, Washington, (which uses groundwater for fish rearing) and experienced similar rearing temperatures until their first autumn in culture. Fish that were to be used for two of the programs were subsequently transferred to surface water acclimation sites, where they were reared until release the following spring (surface water winter rearing). Fish to be used for the other two programs were overwintered at the Eastbank Hatchery and then transferred to their acclimation and release sites 1 to 2 months before spring release (groundwater winter rearing). Fish from the two rearing strategies experienced contrasting temperature profiles, which in turn affected winter growth, age at maturation, and smolt‐to‐adult survival (SAS). Overall, the two release groups that were overwintered in colder surface water experienced reduced winter growth, reduced minijack rate, and smaller size at release, but achieved a two‐ to threefold higher SAS than did the two release groups overwintered in warmer groundwater at Eastbank Hatchery. In addition, based on migration data compiled from fish tagged with PIT tags, smaller juveniles tended to mature at older age‐classes than did larger smolts. We concluded that rearing yearling hatchery summer Chinook Salmon under more natural thermal regimes (surface water) may result in the return of larger, older adults that have a higher survival rate than would fish reared under constant or less natural thermal regimes (ground water). These results highlight the importance of the hatchery‐rearing environment in shaping the survival and life history of summer Chinook Salmon juveniles released into the Columbia River basin.