The idea of harvesting or removing problem seals at river mouths won't work at all. Seals kill adult salmon on returning, and that's highly visible. People have known about it. It's basically accounted for in the population dynamics analyses already.
The juvenile mortality that's causing the more severe problems with Chinook and coho salmon does not occur at the river mouth. It's not problem seals; it's the whole seal population through which the juvenile fish pass. The juvenile fish migration is a kind of gauntlet as they're working their way along the coast.
It's a diffuse problem, so the selective cull idea is what I call a lose-lose policy. It will cause huge public controversy and won't do any good.