Nice cite, TriPolar, thank you. Seems to confirm what I conjectured. And I agree about the catfish thing. It just makes no sense as anything but a fisherman’s tale.
(And all us fishermen know what *that *means…)
Yeah, maybe, for expanding definitions of “shipped.” This cite describes live wells on fishing boats, not on shipping vessels. These fish were kept alive long enough to survive a fishing voyage. Once at harbor they were slaughtered before shipping, per your cite.
In addition to “well-boats” **TriPolar’s **cite also describes “floating… immense boxes”. It also notes (I’m re-typing this, the cite doesn’t seem to allow me to cut and paste)
but ice packed, presumably dead, fish became the norm. I’m guessing, as I did above, that live transport any significant distance for any commercially significant number/poundage of fish was uneconomic.
So yes, a variable definition of “shipped”. It wasn’t impossible, it wasn’t unknown or unattempted. It does though seem to have been impractical.
Read the rest of it. Those immense boxes were at port. Fish were taken from them and “felled.” The boxes were not transport vessels for live fish.
The fish shipped on railroad cars were not shipped in live wells. He refers to fish being shipped in live wells. They weren’t. They were kept alive in live wells while the *trawlers *were at sea.
They weren’t shipped in live wells at all, unless you consider a fishing boat that returns to port as “shipping” it’s catch. I don’t.
I didn’t disagree with you about the well boats and the holding tanks, merely pointing out that they existed.
My quote above though does seem to specify railway transport. The sentences before it speak of rapid expansion of rail “providing quick and efficient transport between the coast and inland…”. This expanded the market, causing rising prices. The very next sentence states “For a time live line-caught fish were carried in tanks of water mounted on railway carriages…”. That sounds like a rail-borne live well to me. And although it doesn’t explicitly state that these were operated to inland destinations, the implication from the preceding context certainly makes this a reasonable conclusion. So yes, it does appear that live fish were transported to markets in live wells on railway cars “for a time”. I still believe that the time was brief, but this cite does seem to say that it occurred.
The significance of this goes back to the OP and the question of catfish as a “product enhancer” or “preservative”. 
This requires as a prerequisite the live shipping of cod either across a continent (railroad) or across an ocean (in barrels on a ship), not in a fishing boat’s live well simply for the return to port from the fishing grounds. I think that the catfish tale is indeed merely a tale, but the background story (live shipping of cod) has at least a germ of truth.
This link says that adult cod are preyed upon only by fish such as sharks. This would seem to exclude catfish.
Additional Googling shows that saltwater catfish (as freshwater catfish) are scavengers. That would also exclude cod as their prey. Also, saltwater catfish in the Atlantic live in the south, are known as hardheads, and have very painful barbs, so I think if they were tightly packed with live cod, there would be injuries.
Also, hardheads live in shallow muddy water. This is not cod habitat, so this is another reason why catfish would not be their predators.