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The Kure oyster festival is doing a brisk trade in beer and grilled meat on sticks. But the longest queues are in front of the oyster stalls, where chefs shuffle piles of mottled shellfish across griddles, waiting for their hinges to ease and reveal their fleshy interiors.

Nobuyuki Miyaoka, who is attending the festival with his son, daughter-in-law and their young children, likes his oysters steamed with sake and served with a few drops of tangy ponzu sauce. “The local oysters were fine until this year,” he says. “They used to be a lot bigger … look how small they are.”

It is not only the oysters’ modest size that worries businesses and consumers in Kure and other locations in the coastal Hiroshima prefecture. For an event held to promote consumption of the area’s most acclaimed contribution to Japanese cuisine, the shellfish are noticeably scarce this year.

• Chefs prepare oysters at the Kure oyster festival. This year, local businesses and consumers say the shellfish have been scarce and smaller than usual

The region’s oysters – a popular Japanese dish and the lifeblood of fisheries in Hiroshima – are dying en masse, with experts blaming a combination of rising sea temperatures and, last year, a brutally hot summer that deprived the delicate bivalves of oxygen and food.

Amid warnings that mass die-offs could become more common, Japan’s government has had to step in to support struggling fisheries that say their livelihoods are being threatened.

Taketoshi Niina looks out at the oyster beds near his fishery in Kure and pronounces this season’s harvest a “disaster”.

Niina, who runs a small fishery in Hiroshima prefecture, says about 80% of his oysters are dead when they are brought to the surface. “This is something out of the ordinary. And a lot of those that do survive are in poor condition … they are not of a high enough quality to sell to shops and restaurants.

• Oyster farming in Mitsuguchi Bay in the Seto Inland Sea

• Niina shows one of his oysters; he says this year about 80% are dead, while those that survive are poor quality

“This is beginning to hit us financially. The season isn’t over yet, and next year is also looking bad. We’re all exhausted. If this happens again next year then it’s going to threaten businesses.”

His predicament is being repeated in prefectures along the coast of the Seto Inland Sea, from Hiroshima in the west to Hyogo in the east.

But the most serious damage has occurred in Hiroshima, which accounts for almost two-thirds of Japan’s supply of farmed oysters, producing 89,000 tons of the shellfish in 2023. Together, the fisheries dotted around the inland sea produce 80% of Japan’s oysters.

Niina noticed something was wrong last October, when he and his fellow fishers encountered unusually high numbers of dead oysters at the start of the annual harvest, which ends in May.

In a typical year, between 30% and 50% of oysters die, but the rate this season has reached as high as 90% in parts of Hiroshima, according to the fisheries ministry. “I’ve never experienced this in my whole career,” says Tatsuya Morio, who has farmed oysters in Hiroshima for more than 20 years.

Last year Japan suffered an intense heatwave, when the average summer temperature was 2.36C higher than normal, making it the hottest summer since records were first kept in 1898.

“If higher temperatures remain for a few weeks, that weakens oysters and makes them more susceptible to viruses and bacteria,” says Shoichi Yokouchi, the head of the marine products division at the Hiroshima prefectural government.

The average water temperatures along Hiroshima’s coast from July to October last year – an important period for oyster cultivation – were between 1.5C and 1.9C higher than the 1991-2020 average, according to local government data.

In response, in December, the fisheries agency announced measures to help struggling oyster famers, including five-year government loans at virtually zero interest and access to mutual aid programmes for aquaculture businesses.

“Kure’s oyster production is among the top in the country, and it supports a wide range of sectors – not only fisheries but also local employment in distribution and food tourism,” says Tomonori Uemoto, director of the fisheries promotion office at the Kure municipal government. “So [the oyster deaths] are likely to have a significant impact.”

• Kure’s oyster production affects more than just local fishers; jobs in distribution and food tourism are also tied to the success of the fisheries

At Kure Oyster Land, a pop-up restaurant where diners are invited to steam buckets of oysters at their table, a song played on a loop reminds them that the city’s famous delicacy is “delicious”. This year, however, the restaurant will close earlier than usual due to a shortage of produce; people who donate a portion of their income tax to Kure as part of Japan’s home town tax support programme will not receive the usual return gift of raw oysters.

Kazuhiko Koike, a professor in the graduate school of integrated sciences for life at Hiroshima University, says last year’s hot, dry summer in Japan was a factor in the mass die-off. “It is possible to say that various abnormal environmental conditions caused by global warming and climate change – such as high air and water temperatures, low oxygen levels, insufficient rainfall, nutrient and food shortages are causing oysters to die en masse,” Koike says.

When shallow water becomes abnormally warm, it struggles to mix with colder layers near the seabed, says Koike. That, in turn, reduces the supply of oxygen from the surface to the seabed.

• A woman shucks oysters at Niina Suisan

“It’s difficult to put the brakes on climate change,” he says. “But if the rainy season ends early again with little rainfall, and is followed by prolonged high temperatures and hot weather, this could mean that low oxygen levels and food shortages will occur again.

“If that happens, it might be possible to move the oyster rafts to areas with slightly lower water temperatures and more food, or to suspend the oysters at bigger depths to avoid high water temperatures.”

After another day of lean harvests, Niina, who started farming oysters a decade ago after quitting his job as a company employee, is unsure how long his family will survive in a sector that his father has worked in for half a century.

“My son decided a couple of years ago that he wants to take over the business after I retire,” he says. “But this year I’ve begun to really worry whether there is a future for him.”