JD,
Virtually all salmonoid stocks feed and mature in the N Pacific ocean. Off the top of my head I can't give you the lats.& longs. but they are far off shore. Trawling in Alaska takes place almost exclusively in the relative shallow waters of the Bering sea. The foreign drift net fleets that primarily fish for squid (now) have been moved out of those areas except for an occasional poacher (relatively no impact). Longlines do not target and very infrequently catch salmomoids.
Even from the healthiest of river systems there is no uniformity of adult returns from year to year or cycle to cycle because of the fluctuating mortality rate both in the rivers and on the high seas. Biologists have proven that they cannot predict mortality rates or adult returns.
What is the answer? Very complex I would say.
More on the history of the best salmon hatchery (Minter Creek) in history on another post.