This belongs to the river valleys where sweetfish run, the Naka and the Kinu. Tochigi is landlocked, and river fish were a valuable source of protein here in a way that they simply were not on the coast.

Ayu were never everyday food in these valleys. They appeared on festival days and on the rest days between spells of agricultural work, and whatever could not be eaten at once was skewered, pushed into a bundle of straw, and hung up beside the hearth to keep. That is a considerable amount of care for a small fish, and it tells you what the household thought it had.

The dish itself does not reflect any of that. Grill the ayu plain over a flame, cook it together with the rice, then break the flesh up and fold it back through the grains. Among the many ayu preparations of this region it is the easy one, the one you could do without thinking about it.

So the fish that people had gone to the trouble of hanging up in the smoke gets treated, in this particular recipe, almost casually. A dish for a special day, cooked in the plainest way available. I suspect that is precisely why this one survived in the valleys while the more elaborate preparations quietly did not.