Chinese fishing boats catch about 4.6 million tonnes of fish in foreign waters each year - a haul worth about US$11.5 billion - and substantially under-report their netted take, according to findings by overseas researchers.
The study, called "China's distant-water fisheries in the 21st century", was funded by the European Parliament's Fisheries Committee and conducted by 20 scientists and researchers at the University of British Columbia in Vancouver, Canada. Their findings were recently published in the academic journal Fish and Fisheries.
The estimated total haul in foreign waters is about 12 times the average of 368,000 tonnes a year reported to the United Nations' Food and Agriculture Organisation, which keeps track of global fisheries catches.
Experts say that, in light of rapidly depleting fish populations, drastic action is needed - namely, more accurate data on fishery catches.
Professor Daniel Pauly, a marine biologist at the university and the paper's lead author, said that global demand for fish is putting too much strain on supply.
"There is a race among countries for diminishing resources," he said.
"They're trying to get as many as they can while supplies last," Pauly said.
To gather what the team called conservative estimations, they accessed a newly assembled database of reported occurrences of Chinese fishing vessels in various parts of the world, as well as information about annual catches by vessel type.
"China hasn't been forthcoming about its fisheries catches," said Dr Dirk Zeller, co-author of the study and project manager of the university's Sea Around Us project.
However, the issue of under-reported catches in foreign waters is not limited to China, and the study suggested that Spanish fishermen may also be under-reporting how much fish they catch in foreign waters.
"We are not singling out China for any reason other than the fact that China has a very large distant-water fleet and is rather secretive about its fisheries and access agreements [with countries to fish in their waters]," Zeller said.
"While other distant-water fishing nations may also have under-reporting problems, the sheer scale of China's fleet makes it a point of consideration for us," Zeller said.
Of the 4.6 million tonnes of fish estimated to be caught annually by Chinese fishermen in the waters of at least 90 countries, the study found that a whopping 95 per cent comes from the waters off West Africa.
"This is an issue of equity and an issue of transparency," Pauly said. "China should not participate in international affairs in this fashion."
Pauly added that the situation was also affecting the livelihoods of West African fishermen.
Commenting on the study, Professor Chen Yong, with the School of Marine Sciences at the University of Maine in the United States, said the study was important and useful, but he questioned whether the estimates may have been exaggerated.
Chen also expressed dire concerns about the development of China's distant-water fishing.
"Most fishery resources are overfished along the coast of China, which means that many fish populations of commercial importance are at very low levels," Chen said. "The situation is very serious and drastic action is needed."
Chen said China should be able to generate more reliable and realistic data in the next few years, as it has been building a database of fisheries catches.
He also said it was important to achieve sustainability in the use of marine resources, by collecting accurate fishing data, but that China lacked the necessary expertise to carry out such research.
"I would say there are only two to three scientists in China working in the field, and they are not even working full time in the area," Chen said. "There should be at least 20 of them."