A couple of things.

Once upon a time (appropriate start) all fish were wild and there were plenty for all. Now, that will never, ever again be the case. Even if the rivers under study were shut down completely to accomplish a "wild fish only" arena, the runs will not run to the numbers they once were because the life cycle exposure to "take" prior to river-return cannot be policed.

Secondly, Tribes make their own rules and their interest alone is "a fish is a fish", and "If you don't kill 'em I will. It's my right."