It depends on your fishing regulations.
Here in Missouri anything considered "bait fish" or "other fish" (defined as fish that aren't listed as game fish, endagered, or protected) can be taken by sein, castnet, or dip net, and you are allowed to take a certain number.
Game fish must be taken by hook and line and cannot be kept if under the length limit. Some game fish do not have a length limit and must be taken by hook and line but can be kept at any size.
So if the fish you are netting are considered game fish then yes, it is illegal to net them.
As far as the native american thing, that is not as stupid as it sounds. In some states native americans are allowed to break certain fishing regulations, but I dunno about MN law as far as that.