Ayu — 鮎, the "sweetfish" — is the delicate, pale flesh of Plecoglossus altivelis, a slender silvery river fish native to the clear mountain streams of Japan and East Asia. Prized above almost any other freshwater fish in Japanese cuisine, it is famous for a signature sweet, green aroma often likened to melon, cucumber or watermelon rind that rises from its skin, and for a soft, mild, faintly bitter flesh eaten whole — head, bones and viscera included — most classically threaded on a skewer and salt-grilled (shioyaki) beside a summer river.