Historically, this is incredibly important. Japan has no mineral, rock salt, so all of it has to be drawn from seawater. In the premodern, workers had to gather wood and brush to burn off the water... it’s incredibly labor-intensive.... https://twitter.com/unseenjapansite/status/1264697026572283904

But very necessary for good preservation. Salt allows for miso, fermented fish, and so much crucial to the food supply.
Salt-production is all over ancient poems in one form or another, if you’re looking. It was a precious thing.

Historians and archaeologists debate the introduction of salt flats to Japan. I attended a conference on specifically premodern salt production last year at Meiji. It’s very much A Thing. No one would put a date on salt flats, but they show up in some places in the medieval.

Japan’s humidity does make using salt flats to evaporate seawater difficult. But the theory is that people used the salt flat to make a concentrate that was then boiled. This cuts down on the amount of kindling, etc. to gather.

In some cases, seaweed was gathered and burned and added to the concentrate for additional salinity and flavor. You can find the modern descendant of this product in Japanese grocery stores under the name “Moshio.”

We forget about salt and take it for granted, but it really was the backbone of much of Japanese prepared food for a long time. The introduction of modern production methods and imported salt was an important turning point.