[jonnathanson]

Right, but genetic diversity of the breeding population would be very low, making the population itself very fragile. And that's assuming the spawn survive their first season. (Most young salmon spawn do not survive, and those not native to the local waters face unique challenges.)

This doesn't seem to be a case where the invading species gains an immediate upper hand in its new environment. (Cf., the rampant python population in the Florida Everglades.) In this case, the invading Atlantic species faces stiff and probably superior competition from the various Pacific salmon species, which are every bit as big and fast, and which fill the same ecological niche. Hell, a Pacific Chinook will get twice as big as an Atlantic salmon. It also knows the territory, including where and what to hunt, and its wild instincts haven't been dulled in a fish farm.

If I were a betting man, I'd wager on the native Pacific populations over the scattered pockets of farm-raised exotics nearly 99 times out of 100. Barring human involvement, of course. If we hunt the Pacific species out of existence, or damage the environment beyond recognition, all bets are off.

[LeifCarrotson]

> Hell, a Pacific Chinook will get twice as big as an Atlantic salmon.

They don't have to be bigger and faster, with precise 'local knowledge' to be dangerous to an ecosystem. They merely have to be different.

Perhaps their small size will cause them to escape the attention of normal predators, or their lack of knowledge of what to eat causes them to decimate some species that Pacific salmon don't eat.

[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.