[jonnathanson]

Atlantic salmon aren't "different" enough from the various Pacific salmon species to occupy their own niche in the Pacific Northwest. If the big ones are too big for you, there are also medium-sized species and smaller species.

The Atlantic salmon is a big, predatory fish that needs to eat a lot of smaller species to stay alive. In the Pacific Northwest region, pretty much every species that could sustain the Atlantic salmon is also preyed upon by an extant, native salmon variety of some stripe. If the Atlantic salmon possessed some sort of advantage in obtaining one prey species or another, then there you go, there's a niche it can adapt to. Thus far, we haven't seen that advantage materialize, or the niche appear.

I apologize if some of the nuance of this point was lost in my "bigger, faster" figure of speech. My tl;dr here is that exotic species don't just magically, automatically win in a new environment simply because they're exotic. To thrive, their exoticism needs to confer some specific competitive advantage within the local ecosystem. I'm struggling to see what that advantage is for the Atlantic salmon in the Pacific Northwest, simply because the oceans and waterways in that region are teeming with very, very similar competitors.