One day Wang Haifeng would like an opportunity to talk to Yao Ming, but not about sport. Wang, a businessman, thinks the retired basketball star and idol of Chinese youth has it all wrong about shark fishing.

At Puqi, a small town 500km south of Shanghai, the local fish factory employs about 500 people. They are far from happy that Yao has come out against eating shark fin soup. "It has had an impact on our business," says Wang, head of Haideli Shark Products, which markets about 1,000 tonnes of shark meat a year. "It's discouraging young people unfamiliar with this dish from eating it, maybe for ever."

Inside the factory they hose down the floor, awash with shark blood. The skins lying on the ground give off a powerful fishy smell. Crates are piled high with the heads of blue sharks. Thousands of fins are spread out to dry in the open air on wire mesh resting on trestles, taking up most of the factory yard.

No one can say when Puqi first specialised in butchering sharks. "Long before the liberation of China," says Li Weijie, the head of a neighbouring factory, referring to the foundation of the People's Republic in 1949. In the old days methods were more rudimentary, he recalls: "Just a knife and a bag of salt."

But "business is no good any longer," Li adds. Both he and Wang blame the adverts featuring the former National Basketball Association player, the first campaign having launched in 2009, with a follow-up a year ago. Yao wraps up the clip with a stern warning: "Remember: when the buying stops, the killing can too."

"Society will turn against us; it's because of all the media coverage," Wang claims. "They're trying to make us believe sharks are protected, which is not true." He is right. The Convention on International Trade in Endangered Species (Cites) only covers three shark species: whale shark, basking shark and great white shark. Japan, Indonesia and China, among others, are against adding other species to the list.

But according to the International Union for Conservation of Nature (IUCN), out of 270 species surveyed, 55% are endangered or critically endangered by over-fishing. Hammerhead sharks all over the world are under threat. In March 2010 the species almost made it onto the IUCN red list. With 75 votes for, and 45 against, the hammerhead failed to obtain the two-thirds majority required for a ban on trade.

Wang consequently has no qualms about showing us a vacuum-pack from a freezer. It contains the foetuses of three hammerhead sharks, each about 20cm long. Li assures us that this species is highly prized for its superior flavour.The Puqi factory owners feel they have been abandoned by the Chinese authorities, who, they assert, have populist motives, rather than any concern for the environment. "Our industry is pretty small and Yao Ming is highly respected, so the government doesn't support us. It's really frustrating," Wang laments.

The factories here deny buying fins cut off sharks out at sea. The poor animals are thrown back into the water to die in agony, a practice condemned by environmental campaigners. To prove this, the factory-owners point out the many carcasses littering the shopfloor. They go so far as to deny that the practice exists at all. "It's impossible, the other parts of the shark are valuable too," says Li. Wang maintains that nothing is wasted: the stomach is fried; calcium powder is extracted from the spinal cord; the meat is consumed locally, or salted prior to export to Sri Lanka; the teeth end up as pendants.

But the fins command the highest price. They only represent 5% of body mass, but at least a third of revenue for Li's firm. The limited availability explains the eye-watering price on restaurant menus. Starting with 5kg of raw produce, the end result, ready for delivery, only weighs 500g. A kilo of blue-shark fin, processed by Haideli, costs 1,000 yuan ($160).

Neither Wang nor Li believe the shark population is dwindling. "Resources will carry on increasing," Wang asserts. "Yao Ming doesn't get it. The numbers in the sea have not dropped. There are still enough sharks."

• This article appeared in Guardian Weekly, which incorporates material from Le Monde