Eight patrol ships from three provinces that border the Beibu Gulf in South China set off this week for their annual fishery watch.
The inspection is part of a regular tour of duty that has been carried out each year since 2004, said the coastal armed police.
The patrol aims to protect Chinese fishermen and stop illegal fishing in the 128,000 sq km Beibu Gulf, which is shared by China and Vietnam.
The patrol also monitors and enforces the annual three-month moratorium on commercial sea fishing that is honored nationwide as a measure to conserve fish stocks. This year, the moratorium started on May 16.
Zheng Huiguang, commander of Beibu Gulf division of the coastal police, said more than 1,200 foreign vessels infringing China's maritime territory have been discharged since 2004. Fifteen of those ships were detained for some time by Chinese coastal police for violating fishing regulations.
The police vessels have also become involved in several sea rescues over the years, pulling more than 800 men from the turbulent waters.
China and Vietnam signed agreements in December 2005 on the demarcation of territorial waters, exclusive economic zones and the continental shelves of the two countries. They also streamlined guidelines for fishing cooperation in the Beibu Gulf, known in Vietnam as the Gulf of Tonkin.
The eight patrol ships come from the Guangxi Zhuang autonomous region, and Guangdong and Hainan provinces.
They will patrol close to the division line between either country's waters during the moratorium period, which ends on Aug 1.