Hybridization between historically allopatric Chinook salmon populations in the White Salmon River, WA

Chinook Salmon spawning in the White Salmon River consist of members of three historically distinct populations: spring Chinook Salmon, Tule fall Chinook Salmon and Upriver Bright (URB) fall Chinook Salmon. Previous work examined juveniles captured in 2006-2008 and reported hybridization between introduced URBs, and the native threatened Tules. Recent increases in nearby hatchery URB release numbers raised the question of whether hybridization rates were increasing. We estimated hybrid frequencies among juveniles collected in the lower White Salmon River between 2016 and 2019. We also evaluated the frequencies at which non-target fish and hybrids were incorporated into the broodstocks of adjacent hatcheries. We observed that frequencies of hybrids in juvenile samples from the White Salmon River were greater in 2017-2019 (17-32%) than they had been in 2006-2008 (4-15%), but that a few (2/9) comparisons exhibited overlapping confidence intervals, suggesting that the rate has increased over time, but also that more sampling is needed to understand the importance of year-to-year variation. Further, differences in the habitat following dam removal and in the sampling sites complicated interpretation of our results. Examination of broodstocks of nearby hatcheries revealed low rates (< 0.5%) of incorporation of non-target populations and higher rates (< 9.0%) of incorporation of hybrids into those broodstocks. The relative compositions of all hatchery and natural-origin collections were similar: most individuals were one of the two parental stocks, a very small fraction were F1 hybrids, and a larger minority fraction were back-crosses. This pattern, in the context of hybridization which we know has been happening for several generations, is consistent with a hypothesis of selection against hybrids in which F1 hybrids are less fit than backcross hybrids.