Looking through mass.gov and the Mass Fisheries and Wildlife Dept. there's nothing, either statewide or on the city level regarding non-native catfish ownership. I'd be happy to ask the customs people I deal with or the federal park services folks, but I'm pretty sure there isn't one, since I also can't seem to find a single person on MFK complaining about this "law" of yours. It's not like Springfield is a tiny little town.
Given that your first reaction, was to explode like that instead of oh, say finding somewhere online that mentions this law.. Well. I'm sticking with you got lied to, or are full of it.