Basically they do it because they need to justify their actions if sportsmen grumble, especially if there are restrictions applied or if on the off hand a lawsuit are filed. Scientific facts then are used to how to best analyze what has happened and what needs to be done and are the basis to their actions.
They still are traveling through MN, but the ones going through Mn are not getting the food supplies needed to bulk up for the next leg of the journey. Basically the bills moved West into the Dakotas because of alot better food sources and less disturbances from boats. BTW, divers preferr very shallow wetlands. 20 to 100 acres in size and 4' or less in depth according to Ray Norrgard of the DNR. I got to listen to an excellent presentation from him yesterday about ducks in MN