Comparisons of donor stray percentages between hatchery- and natural-origin Chinook Salmon and steelhead in the upper Columbia watershed

Objective: Artificial propagation of Pacific salmon Oncorhynchus spp. is commonly used to achieve conservation and harvest goals. However, unintended effects of artificial propagation, such as high donor stray percentages, can reduce numbers of adults that return to target areas and also contribute spawners to different populations where they are undesirable. Until recently, it was difficult to assess if hatchery-origin fish stray rates were atypical because few estimates of stray rates of natural-origin fish were available. We sought to fill that gap by comparing donor stray percentages of hatchery- and natural-origin Chinook Salmon O. tshawytscha and steelhead O. mykiss from the same locations. Methods: We used 29,885 last passive integrated transponder tag detections of adult fish to estimate and compare donor stray percentages of hatchery-origin and natural-origin Chinook Salmon and steelhead in the upper Columbia River watershed between 2002 and 2018. Result: Donor stray percentages of hatchery-origin spring, summer, and fall Chinook Salmon and steelhead were <0.3% at the upper Columbia basin scale and similar to natural-origin donor stray percentages. Only steelhead donor stray percentages were higher for hatchery-origin adults at the subbasin scale than natural-origin counterparts and were <22% for all taxa. The percentage of hatchery-origin steelhead and spring Chinook Salmon released in tributaries were detected at nontarget areas at higher percentages (<63%) than natural-origin counterparts born in these tributaries. Returning hatchery-origin and natural-origin Chinook Salmon and steelhead generally strayed in an upstream direction in similar proportions. Juvenile spring Chinook Salmon that were moved 14-391 km from centralized hatcheries to tributaries for overwintering or final acclimation strayed at a much higher rate than those that completed their incubation, rearing, and acclimation at a single location, but this did not occur with steelhead. Other adaptive management actions that were implemented to reduce straying produced mixed results. A high proportion of stray hatchery-origin spring Chinook Salmon initially homed to tributary weirs. Conclusion: Donor stray percentages of hatchery-origin Chinook Salmon and steelhead were generally not higher than natural-origin donor stray percentages at larger spatial scales (e.g., upper Columbia basin and subbasin) but were higher at smaller spatial scales (e.g., tributaries). A variety of approaches can be considered to reduce undesirable production of strays, but most of them involve difficult management trade-offs.