Monitoring devices placed by foreign countries in Chinese waters pose a great danger to national and military security as the oceanographic data collected can be used to interfere with China's oil and gas exploration and military activities, a State security officer and experts said.

Since 2016, various foreign monitoring devices equipped with solar panels, sensors and communication modules have been fished out by fishermen in Jiangsu, Zhejiang and Hainan provinces. Ninety-one fishermen have been rewarded for bringing such devices to State security authorities, China Central Television reported on Wednesday night.

The devices placed in Chinese waters, including the South China Sea, can collect oceanographic data including currents, temperatures, salinity and pollutants, which are all classified as sensitive State security information, a State security officer from Taizhou, Zhejiang, told CCTV.

Such illegally collected data could be used by other countries to interfere with the activities of Chinese naval vessels and submarines as well as the launch of underwater missiles, the officer, who spoke on condition of anonymity, said.

Wang Suo, a fisherman from Yancheng, Jiangsu, recently fished out an unidentified device when drawing in a net in the Yellow Sea, CCTV reported.

The 3-meter-long black device had solar panels and propellers, Wang said, adding that he had never seen one before. Upon returning to port, he handed the device to the local State security authority.

Tang Jiansheng, an underwater acoustic engineer, said the device was an advanced wave glider equipped with many sensors to constantly collect oceanographic data.

Tang said he was certain the wave glider was an intelligence gathering tool secretly placed by another country. It was equipped with GPS and wireless communication modules, which meant it could send back information and receive orders. From the state of the device, it had been active in China's waters for quite a long time, he added.

Fishermen from Lianyungang, Jiangsu, caught two similar devices during fishing trips in the Yellow Sea in 2016 and 2020.

Chen Qingliang, another underwater acoustic engineer, said in an interview that they were airborne sonar buoys dropped by anti-submarine aircraft that could send oceanographic data they collected back to aircraft.

China's maritime security is facing great challenges, and placing monitoring devices at China's door step is clearly theft and against international law, said Lou Yuan, deputy secretary-general of the China Association of Military Science.