Juvenile torrentfish (Cheimarrichthys fosteri), which enter New Zealand rivers from the sea during spring through summer and into autumn, differ from post‐migratory juveniles and subadults in being tadpole‐like, with relatively longer but narrower heads and larger eyes, a highly compressed caudal region, and a spinous postero‐ventral preopercular margin; the fins appear larger. Migratory juveniles mostly lack the colour pattern of larger fish, and have extensive unpigmented areas on the margins of the dorsal and anal fins, most of the tail, and much of the pectoral fins. The condition found in post‐migratory juveniles and later growth stages develops soon after migration.