If its closed in the regs and you get caught by anyone they could swear a complaint against you in city, county or state courts. They do not have to be an offical just a citizen. If its open but on private property it's trespassing only. The same above still applies.
This has been tested in Watersheds for hunting and fishing many times.

So the Tribal police could swear a complaint against you in the local court. It also depends how close the tribe and the local law enforcement work together.
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