China's fishing fleet in the South China Sea. [File photo]

Vietnamese fishing boats are often sighted in Xisha islands in the western section of the South China Sea, Chinese fishermen said.

In recent weeks, China beefed up patrols in the region to protect the country's maritime rights and interests as fishermen complained of being frequently harassed or seized by sometime armed ships of neighboring countries.

In another move to assert sovereignty, China in June announced to set up a prefectural-level city -- Sansha, which will administer over 200 islets, sandbanks and reefs in Xisha, Zhongsha and Nansha islands, covering 13 square kilometers in island area and 2 million square kilometers of water.

Hainan fishery officials estimate that Sansha hosts a fishery reserve of about 5 million tonnes. Annual sea catch lower than 2 million tonnes will be able to allow the environment to sustain. Hainan currently reports only 80,000 tonnes of catch every year.

Hainan aims to shift the focus of its fishery industry from near-shore fishing to far off-shore fishing. Fishermen are encouraged to "build big boats and explore the deep sea," said Ding Xiuhong, a fishery official of Sanya city who is on board the fleet.

"We are researching on the ways to explore the vast fishery resources in Sansha, in a bid to help Hainan fishermen raise their income," Ding said.

Big-fleet fishing, once proved successful, will be adopted for fishermen eyeing the catch in Sansha, Hainan officials said. The island province has 36 fishing cooperatives involving 3,000 fishermen.

"It is not a coincidence. I guess there will be regular big-fleet fishing in the future," said fisherman Huang Xiucheng.