This document provides information on the taxonomy, biology, farming, and aquaculture of ayu sweetfish (Plecoglossus altivelis). It discusses that ayu are an amphidromous fish found in rivers and lakes in Japan, Korea, Taiwan and China. They spawn in rivers in spring, with larvae entering the sea and returning to rivers as juveniles. It also outlines their natural life cycle and details of their intensive aquaculture in Japan, where they are farmed commercially in ponds at densities of 150-200 fish/m2. Feeding practices and common health issues in aquaculture are also summarized.