Analysts say they started noticing the drop-off in shipping traffic toward the end of October, as China prepared to enact legislation governing data privacy.
Usually, shipping data companies are able to track ships worldwide because they are fitted with an Automatic Identification System, or AIS, transceiver.
This system allows ships to send information — such as position, speed, course and name — to stations that are based along coastlines using high-frequency radio. If a ship is out of range of those stations, the information can be exchanged via satellite.
But that's not happening in the world's second-largest economy, a critical player in global trade. In the past three weeks, the number of vessels sending signals from the country has plunged by nearly 90%, according to data from the global shipping data provider VesselsValue.
"We are currently seeing an industry wide reduction in terrestrial AIS signals in China," said Charlotte Cook, head trade analyst at VesselsValue.
New data law could worsen supply chain chaos
China's Ministry of Foreign Affairs told CNN Business that AIS stations along Chinese coastlines that are legally constructed in accordance with international treaties "have not been shut down" and "are operating normally."
Publicly available AIS platforms are also operating normally, it added.
The State Council Information Office, which acts as a press office for the country's cabinet, did not respond to a request for comment about why shipping providers were losing access to data.
But analysts think they've found the culprit: China's
Personal Information Protection Law
, which took effect November 1. It requires companies that process data to receive approval from the Chinese government before they can let personal information leave Chinese soil — a rule that reflects the fear in Beijing that such data could end up in the hands of foreign governments.
The law doesn't mention shipping data. But Chinese data providers might be withholding information as a precaution, according to Anastassis Touros, AIS network team leader at Marine Traffic, a major ship-tracking information provider.
"Whenever you have a new law, we have a time period where everyone needs to check out if things are okay, "
Other industry experts have more clues of the law's influence. Cook said that colleagues in China told her that some AIS transponders were removed from stations based along Chinese coastlines at the start of the month, at the instruction of national security authorities. The only systems allowed to remain needed to be installed by "qualified parties."
Not all of the data is gone: Satellites can still be used to capture signals from ships. But Touros said that when a ship is close to shore, the information collected in space is not as good as what can be gathered on the ground.
"We need terrestrial stations in order to have a better picture, a more high-quality picture," he added.
With Christmas approaching, a loss of information from mainland China — home to six of the world's 10 busiest container ports — could create more problems for an already troubled global shipping industry.
Supply chains have been under strain
this year as badly congested ports struggle to keep up with a rapidly rebounding demand for goods.
Shipping firms rely on AIS data to predict vessel movement, track seasonal trends and improve port efficiency, according to Cook from VesselsValue. She said the lack of Chinese data "could significantly impact ocean supply chain visibility across China." The country is one of the world's major importers of coal and iron ore, as well as a huge exporter of containers.
"As we move into the Christmas period, it will have a really big impact on [supply chains] and this is the most important element right now," said Georgios Hatzimanolis, media strategist for Marine Traffic. He expects the loss of "minute by minute" ship data from China to have "a great impact on the supply chain," since companies may lose crucial information about ship docking, unloading and leaving times.
The global supply chain is already under "great stress," he added. "It doesn't need another factor to make it more difficult."
China's desire to retain absolute control over all data and information within its borders isn't surprising, as President Xi Jinping continues to reassert the ruling Communist Party's dominance in every aspect of the economy and society.
The country has been pushing for economic self-sufficiency as it faces external threats, such as US sanctions on
Xi emphasized his self-reliance goals in the years before and during
a bitter trade and tech war
with former US President Donald Trump. That's the point, for example, of
"Made in China 2025,"
an ambitious plan to push China's manufacturing sector into more advanced technological fields.
Some top officials in Beijing have recently tried to quell concerns among global investors that the country is isolating itself from the rest of the world as it prioritizes national security.
Chinese Vice President Wang Qishan, considered a trusted ally of Xi, told the Bloomberg New Economy Forum in Singapore that China would not "develop isolated from the world." Speaking via video, he also called on countries to keep supply chains "stable and smooth."
But China has embraced policies during the coronavirus pandemic that often appear to do otherwise.
For example, during the pandemic Xi has doubled down on his push for self-reliance, stressing the need to create
"independent and controllable" supply chains
to ensure national security.
And the country's sweeping clampdown on tech
extended this summer
to foreign IPOs, when the Cyberspace Administration of China
that major companies with more than a million customers seek approval before listing shares overseas. As with the recent data privacy law, the agency cited concerns about whether personal data held by those companies could be exploited by foreign governments.
China's actions this year may come at a cost, though, if the country goes too far in its attempt to protect itself from perceived foreign interference.
-- CNN's Beijing bureau contributed to this report.