There isn't much difference. IF you used local native stocks for your pens then strays would not be that big of a problem. I know that the SSNP's had coho from Wallace River; that was more of a problem.

The big fear, which has so far (a century plus of trying) proven rather baseless is that the Atlantic Salmon, or other imported stock, could escape, stray, spawn, and ultimately outcompete the native salmonids. So far, I think that century plus of rearing Atlantics in the PNW and trying to get them established have resulted in one successful spawning once. But that's the fear. Especially now when we have reasonably vacant anadromous salmonid habitat for somebody to move into.