Workers sort kelp for processing at a factory workshop in Ningde in March. HU MEIDONG/CHINA DAILY

"More than 200,000 large yellow croakers are cultivated inside," said Lu Tongfeng, the general manager of Fujian Xinmao Fishery Development Co, the platform's owner.

Ten other similar platforms sit in the waters nearby, churning out a total of 2,000 tons of seafood each year.

As coastal waters become dotted with fish farms, fishing companies like Fujian Xinmao are turning their focus to areas farther offshore.

Lu, a former shipbuilder, attributes the move to deeper waters to engineering advances that have enabled the creation of controlled environments for fish cultivation in the open ocean. "Building the platform is much more complex than building a ship. As a fixed structure out in the open, the pen must be engineered to be strong enough to withstand extreme weather," he said.

Lu quit shipbuilding to develop the platform in 2003. Despite reaching out to several design teams, success eluded them until they collaborated with scientists from Wuhan University of Technology in Hubei province. A design that passed hydrodynamic testing was finalized in 2020, leading to the platform's debut in July 2021.