I know it is illegal in Minnesota if you do not have the appropriate documentation allowing you to stock a manmade pond. Knew someone who did the same thing, mainly stocked his private pond with crappie, bluegill, and walleye (can't remember the size of the pond, but it was large and one area was at least 35' deep). Anyway, the MN DNR got a tip that he was doing this, so they checked it out. I believe the story goes that he did not have any documentation that he had purchased walleye fingerlings, which, I assume, is the proper way of stocking a private pond, not catching full grown walleye, transporting them, and then throwing them in your pond. They ended up draining 2/3rds of the pond and confiscating his truck and anythng that was deemed used for transporting these fish. He eventually got his truck back, only after paying a hefty fine - $3000.00 if my memory serves me right. (sorry for he long post)