The smoking of katsuobushi is traced to Usa in Tosa. A man from Kishu called Kadoya Jintaro is credited with working out the baikan smoke-drying method there in the 1670s. About a century later, Yoichi of Tosa improved on the method by deliberately introducing mold onto the surface of the fish, which sounds like the opposite of preservation and is not.

The mold was a problem before it was a technique. Bonito sent to Osaka would go moldy on the way.

What was discovered is that if you apply a good mold deliberately and dry the fish thoroughly, the bad molds cannot take hold. That turned an accident into a step. Then bonito shipped on from Osaka to Edo would bloom again, and it had to be wiped down repeatedly along the road, and someone noticed that the repeatedly wiped fish tasted better than the rest.

Honkarebushi came out of that observation. The mold is applied and the fish sun-dried, four times over or more, each cycle drawing out more water and more flavor. The tehiyama method of smoking, using bamboo steamers held over the smoke, belongs to this coast as well. By 1822 an Osaka ranking sheet of bonito from the provinces listed a hundred and twenty-two entries, from Hachinohe in the north to Yakushima in the south. A spoilage problem ended up as the name of a process.