When you go out of state/country to fish or hunt you will need to prove, if asked, by WDFW, that the animal(s) were legally taken. So, if you fished idaho you better be able to show an Idaho license.

There are, or were, special rules that applied to fishing in Canada. There was a time, recently, where I believe you had to land the fish in Canada before taking it to the US. It was something like that so that one could not leave WA, fish Canada, and return to WA directly. I think, too, that even if had a Canada license that landing in WA, without proof that they were landed in Canada, that WA bag limits applied.

Complex.