Without knowing any true specifics about your situation, the MI Game Commission (or whatever is is called in MI) might play a role, especially if these guys didn't have fishing licenses.
In PA, the Fish & Game guys have a lot of authority.
Keep in mind though that you could alienate the cops before knowing who is who if you stir the pot.
When I had problems with poachers last year I was referred to the Department of Natural Resources (game wardens) by the Sheriff's department. We never did catch the theives, but the DNR certainly appeared to be very aggressive. Had they caught the poachers, they would have been charged with a number of crimes. They too brought up filing a civil case for damages afer the DNR got through with them.
Ken
