| Abstract | Variation in rearing conditions across hatcheries and basins can affect the performance of hatchery salmonids in the wild. In 2008, the Shoshone-Bannock Tribes began planning an out-of-basin hatchery facility on the Eastern Snake River Plain for rearing threatened Chinook Salmon for release in tributaries of the upper Salmon River basin, Idaho, USA. To help determine the viability of the planned out-of-basin hatchery program, we reared 100,000 juvenile Chinook Salmon from the same genetic stock at an in-basin and out-of-basin site in Idaho, USA characterized by significant differences in water hardness and temperature regime. In October 2018 and April 2019, we tested whether fish condition, stress physiology, acute mortality, and downstream survival differed between the two groups at the parr and smolt life stages, 49 respectively, upon release in the Yankee Fork Salmon River, which is characterized by low water hardness. At the parr life stage, we observed low acute mortality rates during the 48 hrs after release, but the estimated survival through the unimpounded portion of the migration corridor of the out-of-basin group was an order of magnitude lower than that of the in-basin group. During the smolt release, the out-of-basin group showed signs of extreme physiological stress, acute mortality rates of 40-80%, and immeasurably low survival in the unimpounded portion of the migratory corridor. Meanwhile, the in-basin group recovered from the stress of release, had no acute mortality, and survived through the unimpounded migratory corridor at a rate comparable to previous years’ releases. Based on results of this comparative study, co-managers are now evaluating alternatives to the original proposed hatchery program. This case study highlights the potential impact of differences between rearing and release conditions, as well as the value of conducting preliminary evaluations prior to implementation of large scale hatchery supplementation programs.
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