I think that fermented fish may be a very ancient part of the human diet. To me, it seems like and offshoot of methods used for preserving meat (especially salted meat). My guess is that some ancient human decided that if salting was good for meat, then it MUST be good for fish. Then, the idea spread from there.
When 1st hubby was in the AF, we were friends with a couple-the wife was Japanese-once when she and I were sampling each others’ ethnic foods, I gave her some marinated and grilled rattlesnake and she gave me a container of some brownish gooey stuff-fermented soybean sauce-explaining that it is made by cooking the soybeans in saki, putting in a pottery container and allowing it to ferment (she used the words-”you let it rot”) for 2 weeks. I thanked her-it smelled like sour milk left in the sun-I didn’t eat any, but my husband loved the stuff...