Purely speculative: Could it be the burbot?
"The burbot (Lota lota) is the only gadiform (cod-like) fish inhabiting freshwaters. It is also known as mariah, the lawyer, and (misleadingly) eelpout. It is closely related to the marine common ling and the cusk. It is the only member of the genus Lota.
The genus and species name "lota" comes from the old French "lotte", fish named also "barbot" in this language. The Inuktitut–Iñupiaq word for burbot was used to name the recently discovered extinct transitional species Tiktaalik."
From the photos in the article and other locations I've found online, it looks like some sort of cross (appearance only, not scientific) between an eel and a catfish.
According to the Wisconsin Department of Natural Resources, the Burbot has very fine scales. http://dnr.wi.gov/fish/documents/catchsnakehead.pdf Again, purely speculative, but perhaps this is why it was unclear whether or not it was Kosher?
Are there any experienced frum fishermen reading this who can speculate as to the Kashruth of other fish of a similar nature? I know there was (is?) some controversy regarding swordfish for reasons relating to the existence/disappearance of its scales during its development. Maybe something similar happened here?