There are certainly documented cases every year of foreign fishing boats targeting all sorts of fish off our coast. The ocean is a big place so it is impossible to judge exactly how big the problem is, but we know it happens. A report I read indicated the most violations are found in the central Pacific islands or at least thats where we catch them. Not for salmon though. The most salmon violations are found in Alaska where it is much easier to find salmon.

The difficulty I have in your theory is that I doubt that these boats would have as much sucess targeting Spring Chinook bound for Washington rivers. Those particular fish make up such a small percentage of the salmon present in the ocean at any one time that I would think it would be difficult to isolate them, unless these boats have learned to track this particular run of fish, also unlikely.