As to the fish, in Canada the much larger northern pike (Esox lucius) is also often referred to as a "jack". I think the sportfishing record for that species is around 46-48 pounds. There was a 1960's recording of Jackfish, with words pretty similar to those in the second post way up at the top, by the Modern Folk Quartet. It may have been called "To Catch A Shad" on the album, but I don't remember for certain. None of this adds anything to the origins of the song.

rich r