Stray compositions of hatchery-origin Chinook Salmon and steelhead in natural spawning populations of the upper Columbia watershed

Objective: One of the biggest concerns of operating hatchery salmonid programs is high straying of returning adults into nontarget populations and the possible homogenization of genetic diversity among populations caused by spawning of stray fish. The composition of hatchery-origin stray Chinook Salmon Oncorhynchus tshawytscha and steelhead O. mykiss relative to the natural spawning populations, termed "recipient population stray composition," was evaluated at three spatial scales in the upper Columbia River basin, where stray target limits were 5-10%.Methods: Chinook Salmon carcasses were collected from 1999 to 2018 in spawning areas shortly after spawning, and carcasses were examined to determine origin. Steelhead strays and spawning escapement were evaluated using passive integrated transponder tags between 2013 and 2018. We calculated mean stray compositions for 1999-2018 and for 2014-2018.Result: The mean recipient population stray compositions ranged between 0.02% and 87.35% and increased with decreasing spatial scale. Recipient stray compositions of all taxa at the basin scale were <3%, and summer and fall Chinook Salmon were <0.5%. Stray compositions in subbasins for all taxa ranged between 0.07% and 33.04%. Spring and summer Chinook Salmon exceeded 5% and 10% (summer Chinook Salmon) in some periods in the Entiat and Methow River subbasins, but stray compositions for all Chinook Salmon were <5% in the Wenatchee, Okanogan, and Hanford Reach subbasins for all periods. All steelhead stray compositions in subbasins exceeded 5%, except for those in the Wenatchee subbasin. Stray compositions of spring Chinook Salmon in tributaries ranged between 0.61% and 87.35%. Only the Chiwawa, Icicle, and Twisp rivers were consistently below 10%.Conclusion: In cases where recipient stray management targets were exceeded, some were the result of single hatchery contributions but others were the result of cumulative contributions from multiple hatcheries. Some solutions to reduce recipient stray compositions will likely involve a combination of changes to hatchery, harvest, and habitat management.