China’s maritime militia, once a shadowy and ill-understood actor in the South China Sea disputes, has become visible to international observers in a very tangible way over the last year. Increasing numbers of militia vessels have been involved in collisions and tensions with the Philippines at Second Thomas Shoal, and their close coordination with the China Coast Guard has been well documented by photographs and video made available in mainstream media coverage. But while the dozens of militia deployed to Second Thomas Shoal to confront the Philippines have been made plain to see, the question of just how many militia vessels are active across the South China Sea, and where are they, remains approachable only through a quantitative review of remote sensing data.

Tracking the Militia

AMTI examined Planet Labs satellite imagery for the calendar year of 2023 at nine features across the South China Sea known to be frequented by Chinese militia ships. This study counted ships from both Chinese militia fleets active in the South China Sea: the professional militia that operate purpose-built vessels out of Hainan province, and the “Spratly Backbone Fleet” consisting of commercial vessels subsidized to operate in disputed waters to support Chinese sovereignty claims. Militia vessels were counted in imagery an average of 4 times per month at each feature, and data on days between observations was interpolated linearly between the two nearest observations to enable comparison across features.

Although by regulation militia vessels can be as small as 35 meters in length, only vessels falling between 45 to 65 meters were counted as militia for the purposes of this study. This is both the most common length for Chinese militia trawlers and a range rarely seen in other vessels active in disputed waters, such as Chinese coast guard or fishing vessels of other coastal states.

This data likely accounts for a majority of militia activity in the South China Sea, but it is not comprehensive: ships at other features in the South China Sea, or those in transit at the time of imagery, were not captured in this study.

The data shows that China’s militia is as active as ever. An average of 195 militia ships were seen across these features on any given day in 2023, an increase of 35 percent from AMTI’s last observation of the militia over a 12-month period in 2021-2022.

This overall increase was accompanied by a dramatic shift in vessels to Mischief Reef in the summer of 2023: over 180 militia ships were observed gathered in imagery from July after only a minimal presence in the months prior, a number confirmed by Philippine authorities in reporting at the time.

Mischief Reef, August 4, 2023

The reason for this increase, an anomaly compared to the previous year’s peak of only 37 vessels at Mischief, is unclear. While it is true that the militia vessels seen active at Second Thomas Shoal typically operate out of Mischief, deploying to Second Thomas in response to Philippine resupply missions, the summer surge at Mischief appears mostly unrelated to those efforts. Satellite imagery suggests that only a minority of the boats at Mischief were the professional type seen supporting Chinese blockade efforts at Second Thomas, and the peak occurred months before militia counts during those missions began to rise.

Apart from the surge at Mischief, the militia behaved similarly to last year. The largest consistent groupings of militia ships continued to be seen at Hughes and Whitsun Reef, showing that Union Banks remains a preferred anchorage, especially for the “Spratly Backbone fleet” of civilian ships that are subsidized to operate in disputed areas but seldom engage with foreign vessels. A persistent militia presence was also maintained near China’s outpost at Gaven Reef, and smaller groups could be seen at the reefs east of Philippine occupied Thitu and at Iroquois Reef. Like last year, a notable dip in militia numbers across the South China Sea occurred in late December and lasted through mid-February, as many ships likely headed home for the holidays.

While this data offers a glimpse into the pattern of life for the majority of Chinese militia vessels operating in disputed waters, a different pattern of activity was on display during tensions at Second Thomas Shoal in 2023. There, purpose-built professional militia ships from the “Qiong Sansha Yu” fleet that operate out of Hainan routinely worked with the China Coast Guard to physically oppose Philippine resupply missions to the BRP Sierra Madre. The contrast between the relatively low persistent militia presence at Second Thomas and the intensity of operations during resupply missions indicates that, while the militia may frequent certain locations, professional militia ships stand ready to support China Coast Guard operations at short notice at any point of friction. With recent incidents at Scarborough Shoal, it may be that the traditionally low militia presence there gives way to increased numbers of professional militia as interactions with Philippine vessels increase.