Apart from the fishing, most of the mortality on the salmon when they're moving out into the ocean comes from a predation by other fish, like striped bass in the Miramichi, and birds like gannets, which take salmon smolt off of Newfoundland. Things like pinnipeds also eat them anywhere they are in the ocean.
That is all part of the original, natural mortality that took place on salmon stocks anyway. What happens is that you have natural mortality and then you have the commercial fishing impact over and above that. If you can manage the commercial fishery, that's fine, but if you can't manage it, like illegal fishing offshore, they can take whatever they want.