Sunrise is still a good hour away when the first batch of limp, lifeless sharks are winched ashore and dumped on to the portside at Kesennuma.

As daylight throws its first shadows on to the loading bay, fishery workers begin gutting the sharks before removing their fins with razor-sharp knives. It is a messy, blood-spattered business, and a study in industrial efficiency.

The fins are hurled into plastic buckets, and what’s left of the animals is scooped up by a forklift and loaded on to a truck. In contrast, the marlin, swordfish and bluefin tuna that share the port’s 1,000 metre-long bay are afforded almost reverential treatment.

Kesennuma, a fishing town on Japan‘s north-east Pacific coast, does a lucrative business in the staples of Japanese cuisine: tuna, flounder, octopus, crab, bonito, Pacific saury, seaweed and squid.

But the trade in shark fins is its commercial lifeblood. The port, 250 miles north of Tokyo, accounts for 90% of Japan’s shark fin trade and the promise of eating the country’s best shark fin soup draws busloads of tourists every day in summer.

In 2009, Kesennuma landed almost 14,000 tonnes of shark, worth just over ¥2.4bn (£17.9m): a decent-sized tailfin can fetch as much as ¥10,000.

The minimal threat sharks pose to humans is the overriding theme of the town’s shark museum, while stalls at the port’s market sell everything the animal has to give: dumplings, jerky, shark-skin bags and accessories, and salmon-shark hearts – a local speciality eaten raw.

Few people outside Japan are aware of Kesennuma’s contribution to the global trade in shark fins. And many among the town’s 2,000 fishery workers would rather keep it that way as the Guardian discovered during a recent visit. We were asked to leave the port and film from a gantry reserved for tourists, while local officials turned down requests for comment. Our guide suggested, only half-jokingly, that we had been sent by Greenpeace.

Workers, contending with near-freezing temperatures and noisy, hungry seabirds circling above, moved quickly along the lines of sharks removing their fins. Pools of blood were hosed away as quickly as they formed.

Most of the shark fins handled at Kesennuma are taken to a nearby drying area – whose location is a closely guarded secret – and sold to upmarket restaurants in Tokyo and other big cities. A much smaller quantity is exported to Hong Kong and China, where the newly affluent have acquired a taste for Kesennuma shark fin.

The fishery workers go to extraordinary lengths to pursue their prey. The biggest ships among the town’s 130-strong fleet spend up to 50 days at sea, casting baited lines several miles in length along a stretch of ocean between Japan and Hawaii.

But growing demand for shark fins, coupled with modern fishing methods, has caused a rapid decline in shark populations around the world, according to conservation groups. Many of the top catcher nations under-report their catches, in violation of international regulations.

In a report released to coincide with a meeting of the UN Food and Agriculture Organization last month, the Washington-based Pew Environment Group said at least 73 million sharks were killed every year, primarily for their fins.

“Sharks play a critical role in the ocean environment,” said Pew’s global shark conservation manager, Jill Hepp. “Where shark populations are healthy, marine life thrives. But where they have been overfished, ecosystems fall out of balance.

“Shark-catching countries must stand by their commitments and act now to conserve and protect these animals.”

The movement to turn shark fin soup into a culinary pariah is gathering pace. The British celebrity chef Gordon Ramsay highlighted the cruelty involved in finning – the practice of removing fins and discarding the body – in a recent documentary for Channel 4, while several Chinese restaurants in London have removed the soup from their menus.

The blue sharks that comprise 80% of the shark catch at Kesennuma are listed as “near threatened” by the International Union for Conservation of Nature. Its migratory habits make it difficult to gauge the exact population, but there is no doubt that catches are in decline.

Japan caught about 65,000 tonnes of sharks just over 40 years ago, according to the country’s fisheries agency; by 2009 that had almost halved to 35,000 tonnes.

“The number of sharks is definitely falling,” says Kokichi Takeyama, an expert on Kesennuma’s fishing industry who conducts tours of the port. “In the old days the fishermen used to throw them away as bycatch, but now they recognise their commercial value.”

Japan’s shark industry uses every part of the animal and so does not deserve to be targeted along with countries that catch them only for their fins, Takeyama argues.

But that does nothing to protect shark populations, says Mayumi Takeda, co-founder of PangeaSeed, a shark conservation group in Tokyo.

“Whether or not consumers use every part of the shark does not safeguard them against extinction,” she says. “Just walk through the massive piles of juvenile sharks in Kesennuma and the picture becomes quite clear that this is the genocide of a species.”

Like other members of the species, the blue shark is slow to mature and produces few offspring. “Should we wait to protect a species once it’s on the verge of extinction, or act responsibly while we still have the chance?” asks Takeda.

Several countries, including the US, have taken steps to conserve shark populations, but she holds out little hope that Japan will follow suit. “Many Japanese turn a blind eye to the problem and ocean conservation seems to be overlooked here,” she says.

“And because of the efforts of conservationists in the southern ocean and Taiji, the Japanese media have spun these issues to appear to be the actions of eco-terrorists. But Japan’s international scoresheet can’t handle much more negative press.”

The people of Kesennuma, meanwhile, fear that media coverage of Japan’s whaling and dolphin-hunting industries will put them under closer scrutiny. “We have seen what happened with the whaling issue, and don’t want the attention,” said one resident, who asked not to be named. “We just want to be left alone to get on with our jobs.”

Shark fishing worldwide

20 countries account for 80% of the global shark catch, according to the Pew Environment Group. The top catching nation is Indonesia, followed by India, Spain and Taiwan. Japan lies in 9th place, with an annual average catch of almost 25,000 tonnes.

The IUCN’s red list shows that 30% of all shark species are threatened or near threatened with extinction, and an internationally agreed plan to conserve sharks reached 10 years ago has had little effect.

“The fate of the world’s sharks is in the hands of the world’s top 20 shark catchers, most of which have failed to demonstrate what, if anything, they are doing to save these imperilled species,” says Glenn  Sant of the pressure group Traffic, which monitors the global trade in wildlife. guardian.co.uk © Guardian News and Media Limited 2011