"Of course, squid are not found in Minnesota lakes," reports journalist Al Edenloff. So true, and so charmingly put!
Squid are not found in any lakes. Neither are octopuses, cuttlefish, or nautilus. Class Cephalopoda is exclusively marine, with the closest approximation to a freshwater representative still only tolerating a wee bit of rain with its seawater. Other mollusc groups, like snails and mussels, have members in both salt and freshwater--but not cephalopods. What's the diff?
Cornell's Ask A Scientist addressed this question back in 2005, but I find the answer rather unsatisfying. Dr. Vawter just explains why freshwater snails are good at coping with freshwater, without explaining why cephalopods haven't evolved the same tricks. Commenters at TONMO and Why Evolution Is True have also weighed in, but to my knowledge science hasn't yet got a clear answer.
At least the Minnesota mystery has a satisfying resolution!