Ayu, also called sweetfish, is a freshwater fish listed among the seafood used in tempura.
Ayu is the species Plecoglossus altivelis, a fish native to East Asia. It is associated with clear rivers and lakes and is especially prized in Japan for its subtly sweet flesh, which inspired the English name “sweetfish.” The fish is related to smelts rather than to the fish named in the other options.
Ayu is amphidromous in many populations: young fish move between coastal waters and freshwater rivers during their life cycle. Some populations are landlocked, including forms associated with Japan’s Lake Biwa. Many ayu live for about one year, although some freshwater-restricted populations can survive longer.
Grilling ayu whole with salt is especially famous in Japanese cuisine, but the fish also appears in culinary lists of tempura seafood. Iwashi means sardine, tai commonly means sea bream, and saba means mackerel, so none is the canonical sweetfish answer.