Su Chengfen has spent all his life fishing in the reef-filled South China Sea, guided by a handwritten book more than 600 years old that depicts routes to various remote islands from Hainan province.

A book belonging to Chinese fisherman Su Chengfen depicts sailing routes from Tanmen county in Hainan province to Huangyan Island in the South China Sea. LIU XIAOLI / CHINA DAILY

The former fishing vessel captain, who lives in the town of Tanmen, cherishes the book, wrapping it in layers of paper even though at 81 it is impossible for him to return to the sea.

He has always known it is precious, as it contains detailed information handed down over the generations, but at first he had not realized its true significance.

Specialists say the information the book contains is undeniable proof of China's sovereignty over Huangyan Island.

"Unlike other versions, it depicts the exact route to Huangyan Island. It clearly proves that generations of Chinese fishermen have worked on the island," said Zhou Weimin, a retired professor at Hainan University.

Zhou wrote An Arcane Book About the South China Sea, the first book in China studying genglubu, a term used for various editions of ancient handwritten books recording sailing routes in the South China Sea. Zhou's book was published last year.

Gao Zhiguo, director of the China Institute for Marine Development Strategy, who used to serve as a judge on the International Tribunal for the Law of the Sea, said, "One book on genglubu beats a thousand words.

"It is ironclad proof. ... We can deduce China's historic fishing and sailing rights in the South China Sea, as well as ownership."

Su inherited the book from his father when he became a boat captain at 23. He said his father was given the book by his grandfather.

"I relied on it for many years until I got a modern map of the South China Sea in 1985," he said.