Ayu Rosui

Ayu rōsui is a rice porridge built around a whole sweetfish, its name said to be a slurred form of "zōsui" (rice porridge). Potato, onion, eggplant, and other summer vegetables are simmered with rice in a dashi made from the fish, then seasoned with miso. The sweetfish's flavor runs through the whole pot, giving the dish the aroma of river fish balanced against the sweetness of the vegetables. Sweetfish from the Katsuura River are said to feed on algae, which makes even their innards edible, and they are typically added whole, guts included, for their distinctive flavor.

The sweetfish is scaled and simmered in dashi together with cooked rice and firmer vegetables like potato and onion. Once those soften, more delicate vegetables such as eggplant, shimeji mushrooms, and garlic chives go in, followed by miso stirred in to season the broth, and the dish is finished with thin slices of sudachi. The innards are sometimes removed for those who prefer it that way. Using freshly caught sweetfish from the clear Katsuura River is said to be the traditional approach.

The Katsuura River is known for sweetfish, amego trout, eel, and carp, and the sweetfish season opens with the start of ayu fishing in June. The dish appears to have begun as a way of turning fish caught with shallow-water gill nets into a porridge on the spot, together with summer vegetables. A closed season from late October through around May the following year is said to protect the fish population, and the dish looks like a way of making the most of that limited seasonal catch.