One other possibility (as wild as it might sound) is if that waterway was connected by a creek, stream or brook, to a laker waterway and during one of our flooding rainy periods a juvenile laker made its way down stream.
Remember, that's how the Asian carp got released into the Mississippi River system from a fish farm that got flooded.
I fished a pond in NE Mass that is stocked with trout, and caught a salmon.
Not ignoring the ever-present bucket brigade, there are other ways, regardless of how off-the-wall they may seem...
