Standing among coconut and mango trees near the coast of Mozambique, Fernando Nhamussua carefully prepares shark meat for a family meal – and contemplates a basket with a profitable haul of four dried shark fins.

"I want to sell them to the Chinese," the 33-year-old admits with disarming candour, estimating that a kilogram's worth will fetch around 5,000 meticals (£104). "We take them to town where there is a place for Chinese buyers. It's good money."

Nhamussua reckons he has sold 20 fins so far, boosting his normal income and his hopes of completing a modest concrete house that stands unfinished. But this burgeoning trade along the Mozambican coast is putting precious species such as manta rays in existential danger, according to local conservationists.

Fishing for sustenance has long been a staple here, with few alternative sources of income. But it is an open secret that Chinese syndicates are supplying improved fishing nets, buying shark fins and manta ray "wing" tips and shipping them back to Asia, where there is increasing demand for delicacies such as shark fin soup.

Nhamussua's nephew, Americo Gilamba, is 19 and has scant other career prospects. "We do it because we don't have a good job," he explained, standing in the small, sandy family settlement that includes huts made of reeds and coconut leaves. "We know it's not good and the Chinese are killing things that are not allowed to be killed, but we do it to survive and get some money. We don't want to have to steal from other people. If we were given an alternative, we would stop."

Inhambane's beaches and ocean are a diver's paradise with one of the most fabulous concentrations of marine life in the world. Tourism, a vital lifeline in one of the world's poorest countries, could be threatened.

Carla Victorino Guicome, who last year became the first Mozambican woman to qualify as a diving instructor, said: "I am sad, I am angry because if it continues like this it's going to kill tourism in Mozambique. No more tourists will come here."

Shark fin harvesting in Mozambique. Photograph: Alamy

She continued: "People come from all over the world to enjoy diving with sharks and manta rays, but if this goes on, they won't be there any more.

"The Chinese don't respect marine life and they're trying to destroy our heritage. We have tried and tried, but the government don't seem to be doing anything to stop it. If nothing is done, the animals will disappear."

Conservationists have called for legal protection of species such as sharks and manta rays, the banning of gillnets – which create a wall of netting to catch fish – and greater education of and alternative livelihoods for fishermen. But the fisheries ministry is powerful.

Andrea Marshall, director of the Marine Megafauna Foundation in Tofu, said: "I would argue that far more people benefit from tourism here than from the fishing industry. The economic argument for preserving these charismatic animals in the long term surely outweighs a one-off profit for a few fishermen and Chinese."

Time is running out for the manta ray, a beautiful fish with big, triangular pectoral "wings" that has a meagre reproductive cycle.

Inhambane has one of the biggest populations in the world, with 908 known to the foundation, but has witnessed an 87% drop in the past decade. This means that where visitors could once expect to see six or seven of the creatures in a single dive, now the average is less than one.

Marshall, principal scientist for the manta ray programme, said: "We're looking at decimation in the next decade or decade-and-a-half. Manta rays are in big trouble along the coastline. If current trends continue, I don't give this population more than a few generations."

Fishermen are more efficient than ever before thanks to bigger nets and more sophisticated equipment. "Sometimes the fishing nets are given by the Chinese, other times they're part of official schemes intended to benefit fishing communities," Marshall added. "We've been to the fishing camps and you can see the high-quality hooks and lines that they have now."

And when she followed the money, it led to China. "It's a very secretive operation, but we've had confirmation of the Chinese buying and shipping them out. There are containers that are just reeking. Everyone knows who owns those containers, but nobody does anything."

Many Chinese people are in Inhambane on legitimate business and working on government infrastructure programmes.

But the fishing controversy echoes wider concerns over what some in Africa regard as a Faustian pact with China. The Environmental Investigation Agency has said nearly half of the timber exported from Mozambique to China is done so illegally, costing the impoverished nation tens of millions of dollars a year.

Based on evidence given by fishermen, the Guardian visited a Chinese-run shop said to be a front for the illicit trade in marine life; an expensive car was parked outside. Its owners said they were aware of such trade in recent years, but claimed it had declined of late. Asked where shark fins could be obtained, a co-owner shook her head and said: "I don't know."

But Lon Chen, a resident of five years and owner of a supermarket in Inhambane, claimed that he been offered such items in the past. "The local people kill them and bring them," he said.

"They came here to ask if we needed it or not. I said no, but some Chinese are buying that kind of food. They don't eat it at all; they are buying it to export to Asia. It's their business and they're surviving on that."

Mozambican authorities are said to be taking the problem seriously, but are working with limited resources, including only one or two patrol boats to cover the immense coastline, although focusing on Inhambane with its abundant marine life would be a start.

Carlos Carvalho, an activist based in the capital, Maputo, claimed that sea turtles and dolphins are also being targeted for their flippers and organs, although observers in Inhambane could not verify this. Chinese traders are seeking to obtain boats to extend the fishermen's range, he added.

"The Chinese are gangsters and they have the protection of certain officials in Inhambane province," Carvalho said. "Every month it is escalating. Inhambane is out of control. It is the killing field of Mozambique and nobody is doing anything about it. It absolutely devastates me."

But the issues are complex and there is a danger that fishermen will be scapegoated. Timothy Dykman, director of Ocean Revolution, said: "It's about the markets, conditions and global impacts far beyond the control of local fishermen that are being run by organised criminal networks. Networks are also selling drugs and engaged in human trafficking."