> In thirteenth-century England, Christians were expected to avoid meat every Wednesday, Friday and Saturday during Lent (when eggs and dairy products were also forbidden), Advent and on some saints’ days (or the eves of their feasts)

Every person (back then) could observe farm animals had sex. Since no one had seen fish having sex, it was believed that fish spawned spontaneously from rivers/lakes/oceans and therefore were free from the animal version of "original sin". That's why so much effort was put into catching and preserving fish.

Source:

https://www.amazon.com/Cod-Biography-Fish-Changed-World-eboo...

Also history of food:

https://www.amazon.com/Salt-World-History-Mark-Kurlansky-ebo...

The author has a fascination with Basques & Basque history. Or maybe they really had a large influence in fishing and trans-Atlantic navigation. And that maybe they discovered the huge cod banks by Newfoundland before John Cabot did.

https://en.wikipedia.org/wiki/John_Cabot

Medieval people were not stupid. They observed that pickled vegetables preserved the veggies for months. And they observed that people who ate pickled vegetables got scurvy (vitamin C deficiency disease) at a much lower rate. The recipes didn't travel too far, so while sauerkraut and kimchi are both pickled cabbage, with different spices they are wildly different.