Local government officials said that the training includes exercises at sea and requests that fishermen obtain information related to foreign vessels.
"The maritime militia is expanding because of the country's need for it, and because of the desire of the fishermen to engage in national service, protecting our country's interests," Reuters quoted an unnamed adviser to the Hainan government as saying.
The adviser added that fishermen get basic military training from the city-level branches of the People's Armed Forces Department and that the training encompasses search and rescue operations, which he said help contain disasters at sea and safeguard Chinese sovereignty.
With "a particular mission in safeguarding sovereignty" due to come up, the government will coordinate with the fishing militia by notably asking them to gather information on foreign vessels' activities at sea, according to the advisor.
Chinese Foreign Ministry Spokesman Lu Kang, for his part, said that his country does not use its fishing fleet to help establish sovereignty claims in the South China Sea, adding that "this kind of situation does not exist."
According to him, Beijing had taken measures to ensure that the country's fishing fleet carries out business on a legal basis.
China has been in the process of constructing an artificial island on the disputed reef in the South China Sea for about a year. In 2014, Beijing began building a 3,000-metre airstrip there, causing outrage from neighboring countries with overlapping territorial claims. China completed its first testing of the facility in January, with a civil aircraft landing on the reef.