I've been reading Ian Mortimer's excellent Time Traveler's Guide to Medieval England and stumbled onto a question that was outside the scope of his book. Mortimer discusses the prevalence of fish in the medieval diet for religious reasons, especially among those who could afford it, and the complexities of making fish widely available inland. He mentions salting of course, as well as river fishing and fish ponds, but he also stresses that in England few places were more than 2 day's ride from a port and fish could be shipped by boat live in barrels. I'm curious, then, how did other, more inland, Catholic countries keep a steady supply of fish? Were saltwater fish less available? Was salted fish more common? Did the nobility, which apparently in England put a high prestige value on rare fish, find ways to work around the transportation problems?
Salt fish and dried fish were shipped in barrels, usually making the first part of their trip on river barges. Butchers, unable to sell meat during Lent, sold preserved fish, instead. Fish sellers still sold fish from the large rivers, plentiful enough still to be shipped a day or two in from the banks. Many manors and castles in less frigid climes had fishponds. Of course, fish will stick to their breeding cycles, and that meant even the better classes ate a lot of dried or smoked fish.
Carp showed up in Europe around 1000 from the Black Sea and were adopted relatively quickly as a good fish for farming, because they like shallow, muddy water. Carp farming spread across central Europe during the following centuries and became a huge industry on aristocratic estates and monastery grounds. The farms were really elaborate, with multiple ponds and sluices and canals allowing the fish to be moved around as they grew, and then transported to markets still alive. The whole process was extremely labor-intensive and, apparently, mostly went out of fashion after salting and curing techniques improved and cod fishing expanded. Some ponds from the sixteenth century are still in use in southern Bohemia, though.