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

Abstract

One of the biggest concerns of operating hatchery Salmon and steelhead programs is high straying of returning adults into non‐target 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 Basin, where stray target limits were 5‐10%. Chinook Salmon carcasses were collected from 1999‐2018 in spawning areas shortly after spawning and carcasses were examined to determine origin. Steelhead strays and spawning escapement were evaluated using passive‐ integrated transponder (PIT) tags between 2013‐2018. We calculated mean stray compositions for 1999‐2018 and for 2014‐2018. The mean recipient population stray compositions ranged between 0.02‐87.35% and increased with decreasing spatial scale. Recipient stray compositions of all taxa at the basin scale were <3%, and summer Chinook and fall Chinook salmon were <0.5%. Stray compositions in subbasins for all taxa ranged between 0.07‐33.04%. Spring and summer Chinook Salmon exceeded 5% and 10% (summer Chinook) in some periods in the Entiat and Methow subbasins, but stray compositions for all Chinook Salmon were <5% in the Wenatchee, Okanogan, and Hanford Reach 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%‐87.35%. Only the Chiwawa, Icicle, and Twisp rivers were consistently below 10%. 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.

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

“…In general, adjacent populations are genetically more similar than those that are geographically separated by longer distances (Hillman et al 2019), so adjacent populations are also less likely to dramatically influence local adaptation. The genetic risks of straying are better evaluated by estimating recipient population stray percentage than donor stray percentage because recipient population stray percentage also incorporates the size of the recipient population relative to the abundance of strays (Keefer and Caudill 2014;Bett et al 2017;Pearsons and Miller 2023). For example, high donor population stray percentages may pose low genetic risks to large recipient populations but high genetic risks to small recipient populations.…”

“…This work focused on donor population strays, but recipient population stray evaluations should also be considered and prioritized when making management decisions (Keefer and Caudill 2014;Pearsons and Miller 2023). Recipient population stray rates are influenced by factors such as the size of the hatchery program, the size of the recipient population, and the donor stray rate (Bett et al 2017).…”

“…Straying can be estimated as either the percentage of a source spawning population that strays (i.e., donor stray percentage) or the percentage of a recipient spawning population that is composed of nonnatal spawners (i.e., recipient stray percentage) (Keefer and Caudill 2014;Pearsons and Miller 2023). Stray fish that spawn with nontarget populations can pose risks to both donor and recipient populations.…”