Hundreds of Chinese vessels, which the Philippines claim are linked to China’s navy, have been moored inside Manila’s 320-kilometre exclusive zone since last month.
The CSIS tracked the automatic identification system broadcasts of the Chinese vessels including 14 monitored by the Philippines coastguard.
The 14 ships were first tracked around Whitsun Reef in early 2019 and have shown little sign they are fishing there, CSIS researchers said.
Satellite imagery from Planet Labs used in the CSIS report showed that significant numbers of vessels have been at Whitsun since at least February 2020.
Chinese officials have offered little comment on the purpose of the fishing vessels and when questioned, refer to them as a “so-called maritime militia.”
“As with other known militia deployments, the behaviour of these vessels defies commercial explanation. Most have remained in the area for weeks, or even months, riding at anchor in clusters without engaging in any fishing activity,” the CSIS report said.
It also said that recent fine weather around Whitsun Reef “debunked the initial excuse from the Chinese embassy in Manila that they were riding out a storm”.
The Philippines maintains that Whitsun Reef falls inside the country’s exclusive economic zone.
A 2016 ruling by a United Nations tribunal dismissed China’s claim to virtually all of the South China Sea, though Beijing has refused to recognise the decision.
China has been accused of using its vast fishing fleets to help assert Beijing’s territorial claims throughout the 2.1 million square kilometre South China Sea, though China has dismissed the accusations as groundless.