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Plecoglossidae (ayu; subclass Actinopterygii, order Salmoniformes) A monospecific family (Plecoglossus altivelis) of fish that spend some time in the sea but return to freshwater streams to spawn. Large numbers of juveniles are caught during their annual migration, and are stocked in ponds for cultivation, making them commercially important in Japan, and to some extent China. Ayus occur in northern Asia.