Sweetfish, also called ayu(Plecoglossus Altivelis), a delicately flavoured marine fish that migrates upstream to spawn in clear waters. The sweetfish is light yellow or olive-colored, about 20 cm (less than 1 foot) long, and similar to a small trout in appearance. It is distinguished by a ridged tongue, a sail-like dorsal fin, and teeth arranged on saw-edged plates at the sides of the jaws. Most of them would die after spawning. All is recorded in Shu Xincheng's (1936) Cihai ("Sea of Words"), a comprehensive dictionary of characters and expressions, and provided near-encyclopedic coverage in fields like science, philosophy, and history.

Tiantai Sweetfish is mainly produced in the streams of eastern Cangshan Mountain, which is only 40 li (20 kilometers) from the East China Sea. The fish will swim upriver to spawn in fall. After that, the baby fish would swim down when the water gets cold. Next spring the grown-up fish would swim upstream again. This is the uniqueness of the fish, as interesting as the swallow and wild goose.

According to the historical records, sweetfish will grow an inch every month and stops at the tenth months. It will die within a year, that’s why it’s also called yearly fish. Its flesh has a distinctive, sweet flavor with "melon and cucumber aromas". The special flavor is attributed to the boiling of its dorsal fin to fish oil. Value comes from scarcity; it’s said that the fish was served to the Emperor in ancient China. Sweetfish is also found in Lingjiang River years earlier, but these years due to water pollution, the Five Treasures of Yanshan Mountain, namely the sweetfish is disappearing. Therefore, the only place for the fish is Yongxi Stream in Tiantai Mountain, a paradise for the fish in the clear and wooded river.