BEIJING — Chinese fishing fleets, driven by plummeting catches close to home, are flocking to distant West African waters, where they engage in ecologically ruinous bottom trawling, subterfuge and other illegal activities that threaten marine resources in a region already under pressure from overfishing, according to a report issued on Wednesday by Greenpeace.

The study, the result of a two-year investigation, accuses hundreds of Chinese-owned or Chinese-flagged vessels of taking advantage of weak enforcement by African governments to indiscriminately net untold tons of fish off the coasts of Gambia, Guinea, Guinea-Bissau, Mauritania, Senegal and Sierra Leone.

Among the worst offenders, Greenpeace said, is the state-owned China National Fisheries Corporation, whose ship operators were said to have lied about their locations, underreported the amount of fish in their holds and used damaging fishing methods that are largely banned in Chinese waters. The report said that government regulators in Beijing had been lax in enforcing regulations that govern overseas fishing.

“China is exporting to Africa the kind of destructive fishing practices that depleted local fishing grounds off the Chinese coast,” said Rashid Kang, the director of the China Ocean and Forests campaign at Greenpeace. “At a time when China talks about win-win partnerships with African governments and is concerned with improving its international image, these kinds of practices damage marine resources, threaten local livelihoods and undermine China’s soft power.”