When the shogunate was established, Tokugawa Ieyasu moved fishermen from the village of Tsukuda in Settsu province to Tsukudajima, an island at the mouth of the Sumida river. That relocation turned the island into a center of the fishing trade for the new city.

Preservation at that time meant boiling things in salt, or packing them in salt. What changed things here was the arrival of tamari soy sauce from Kishu, and the practice of simmering small fish down in a heavily concentrated seasoning until they would keep for a very long time indeed.

Whitebait was the classic material for it, but nori, kelp, shrimp, and clams all go into it as well, and there are versions made using produce from the land rather than from the sea at all.

Simmered right down with soy sauce, mirin, sugar, and starch syrup, the result is sweet and salty and intensely concentrated, and it goes with plain white rice in a way that very few other things manage. It is eaten across the whole country now and hardly anybody thinks of it as regional food at all. But the name never moved anywhere. It is still, precisely, the name of the island those fishermen were relocated to.