The chief issue in fasts was to avoid all animal products, so most obviously, meat was right out. Fish, on the other hand, was okay for some reason. I don’t think there’s any real logic to it, though maybe others disagree. The stipulation that fish was the only thing to use in a meat course at castles and monasteries meant that year round, and even more at Advent and Lent, there was a steady market for fish. Some fish could be caught fresh in lakes or rivers, but most fish was obtained through much more planning and work.
Monasteries knew in advance that they’d be fasting for every possible fast day year round, so they put a lot of effort into growing fish. Some monasteries figured out that letting their sewage flush into the fish pond was actually good for the fish. They tried to build near or over a stream that could carry water in and sewage out, and then they dammed it up on the outflow side to make the fish pond.
Mill ponds were great for fish and for eels, too. The miller’s landlord often asked for eels as part of the rent. Eels could be found in streams and lakes, too. Peasants could spear or trap them. But they rarely ate eels; the cash value was too great. Eels went to the town, monastery or castle.
During the Medieval Warm Period, carp began to move up the Danube River to regions that previously had been a few degrees too chilly for them. The cash value of a five-foot carp was so great that many entrepreneurs invested in long-term fish ponds. Carp farming was so popular that the increased standing water at fish farms made malaria a newly dangerous disease in France and Germany. Carp were moved to a new pond each year, so that in a chain of ponds, you always knew the age and size of its inhabitants. It took about five years for a carp to grow to table length, but then the Abbot or Baron would pay a lot.
By far most fish at table was not fresh: either dried or salted/pickled. The fish preserving industry was one of the largest enterprises of its time. By the late Middle Ages, it was really a huge international business that spurred the creation of the Hanseatic League, an armed monopoly in Northern Germany and Southern Sweden. At Hanse ports, no other ships were allowed. The amassing of barrels, salt, and fish took hundreds of people in different cities. The Hanse may be considered the first global corporation that wielded more power than a nation by itself.
Cod came from the far north Atlantic, including Canada. During the late medieval decades, Basque fishermen were fishing the cod banks near the St. Lawrence River without disclosing that they’d found a New World. (You know how fishermen are! They will never disclose their personal spot to rivals.) Cod also came from Iceland and all around Norway. Cod was most often flayed, perhaps at sea, and then hung to dry in the cold wind. Prepared this way, it was called stockfish—-and “stock” referred to a stick of wood.
Cod stockfish were sometimes smoked, and frequently salted. When caught at sea, they were usually salted so that they would not spoil during the two weeks the ship stayed out. Stockfish had to be soaked for a long time, sometimes with many changes of water to remove salt, then pounded with a hammer to make it possible for teeth to chew it.
It was tasteless, once reconstituted. Cooks had to find ways to season or fry it, but stockfish was never very good. On the other hand, Norwegians still eat Lutefisk as a holiday food: it’s basically re-hydrated stockfish. Lutefisk is soaked in water for two weeks, some of that time in a lye solution. It becomes gelatinous.
Herring came from the north Atlantic and the Baltic, brought to port in huge nets by the millions. In Hanseatic League cities, hundreds of women gutted many herring per minute. The herring were packed firmly in salt and the barrels closed tightly. Ships were specially built to accommodate as many barrels as possible. Monasteries and armies bought barrels and barrels of the stuff.
Preserved herring, as you see, was salty. Monks ate so much herring, it felt like penance just to swallow another one. More creative cooks soaked it to remove some of the salt and hid it in other dishes. The poor saw herring simply sitting there, looking back at them in all their salty glory, next to a piece of rough bread or a pease porridge.