China’s appetite of seafood is is so voracious that by 2030 the nation will account for 38% of the world’s fish consumed by people, the World Bank has predicted (pdf, p. 16). In the quest to satisfy this demand, Chinese fishermen are also catching vast quantities of fish too young or too small to be consumed by humans, or “trash fish,” and selling them for fish feed.

To boost its marine economy, China has in recent decades been encouraging unsustainable fishing practices. Chief among them is trawling, in which fishermen drag nets along the ocean floor, despite this method killing coral reefs and other living things not being targeted. According to a recent Greenpeace report, the practice accounts for about half of China’s total marine capture, which reached 13.1 million metric tons (14.5 million tons) in 2015. Much of that capture—more than 30%—is trash fish (pdf, p.3).

For the report, Greenpeace conducted research and surveys in China’s eight main fisheries provinces. It found that the northern province Liaoning, which borders the Yellow and Bohai seas, saw the highest portion of trash fish captured via trawling. The southern Hainan province, which borders the South China Sea, came in second.

The use of trash fish in fish farming in the Asia-Pacific region has been increasing because of the relatively low cost, according to the United Nations Food and Agriculture Organization. In 2014, fish feed consumption by China’s aquaculture industry was larger than (pdf, p. 3) Indonesia’s entire catch.

In Chinese fish farms, large yellow croaker, a popular fish in China’s southeastern Fujian province, eat some 442,489 metric tons (487,760 tons) of trash fish every year, according to Greenpeace.

China’s long-time trawling practice is “not sustainable,” said Rashid Kang, senior oceans campaigner at Greenpeace East Asia, in a press statement.

“In a healthy and sustainable marine fishing industry, fish species should be allowed to mature before being caught, otherwise they are unable to reproduce and fast reach the point where stocks collapse… With the emergence of a huge market for fish feed made from ‘trash’ fish, it is imperative that the government better regulate this type of fishing practices, before it is too late for China’s ocean ecosystem.”

In other words, today’s trash fish is tomorrow’s treasure—but only if it’s given a chance.