Got some interesting info when I fed this prompt into ChatGPT:
"The WDFW has just shut down fishing on the entire river system under the pretext that angling for other, more abundant species will pose too great of a threat to the wild fall chinook spawning in the Skykomish river, but I personally suspect that all of the supposedly wild chinook in that system show evidence of significant hatchery introgression, if not conclusive data that they derive from out-of-basin hatchery stock, and that there are not actually any genuinely wild fall chinook in the Snohomish river system to conserve. Is this a fair conclusion that can be supported by evidence or simply uninformed conjecture on my part?"
"NOAA’s biological opinion for Snohomish-basin hatchery programs is especially relevant. It says the Wallace River Hatchery Chinook stock was founded in the early 1970s from native Skykomish River summer Chinook returning to Sunset Falls, and that current programs were intended to propagate the native Skykomish stock, with hatchery- and natural-origin aggregations described as genetically identical in cited WDFW/PSTRT materials. That phrase — “genetically identical” — does not prove absence of hatchery introgression; it more likely reflects that an integrated local hatchery program and the natural spawners are not distinguishable with the markers/data being used.
The same NOAA document found meaningful hatchery presence on spawning grounds. For 2006–2015, it estimated that 28.7% of spawners in Skykomish natural spawning areas were hatchery-origin, with most of those attributed to Wallace River Hatchery and a smaller out-of-basin component from Soos Creek/Green River; it also cautioned that coded-wire-tag expansions captured only part of the estimate and may have substantial error. For Snoqualmie, NOAA estimated 18.9% hatchery-origin spawners over the same period, with a more diverse hatchery-source pattern and major out-of-basin contributors.
That is a strong basis for saying: these are not pristine, hatchery-free populations. It is not a strong basis for saying: all natural-origin fish are merely hatchery fish, or there are no native/wild genetic lineages left."