Sockeye do inhabit Puget Sound streams. Both the Stilly and Snohomish systems have them. My theory is that numerous watersheds formerly hosted annual runs of sockeye. What we see today in WA are remnants that have escaped the horrific commerical fishery of the late 1800's and early 1900's. Although I don't have scientific evidence to back it up, I have arrived at this conclusion as a result of personal observation as well as acquired knowledge about historic runs.
I would have to wonder what the impact of the new proposed Cedar River hatchery will have on available cutthroat spawning habitat. Should be quite a show if an increased return happens.... maybe the Cedar river will run red and we can walk on the backs of sockeye :p