Be curious to find out how they negotiate the shared fisheries with streams flowing out into the panhandle from northwestern BC such as the Stikine, Taku, Alsek, Tatshenshini.
they don't. They are mostly harvested by US commercial anglers though there is FN economic fisheries. I think attempts to have in river commercial fisheries in the Stikine etc mostly failed despite government support. You can include the upper Yukon Fish in that too. According to Wikipedia 35 to 50% of the Yukon's chinook return spawns in Canada including some in BC.
Unlike Canada, salmon in the US are managed by the state of origin. A State like Alaska has to agree to enter an international treaty and have the US government negotiate on their behalf. Alaska has mostly declined to do this so are no bound by most salmon treaties.