China's largest fishery patrol ship China Yuzheng 311 takes part in a drill to mark the 5th signing anniversary of the Demarcation Agreement on Beibu Bay, signed by China and Vietnam, in Sanya of Hainan Province, China on June 30, 2009. China will intensify patrols in the South China Sea to protect its fishermen amid a rising tide of disputes with neighboring countries, an official said Tuesday.