Having used up all the non-treaty Chinook impacts, as GodLovesUgly suggested could be a reason to not open. By the same token, I suspect the tribes modeled low Chinook impacts based on limited coho fishing as well. Maybe not. If they left Chinook impacts on the table that could be used for the terminal coho fishery (in case of a larger coho run), then there would be no reason for them to not fish.