Mass Oyster Die-Offs Confirmed in Japan’s Seto Inland Sea; High Water Temperature Cited as Primary Cause

The Yomiuri Shimbun

7:00 JST, November 23, 2025

Mass die-offs of farmed oysters have been confirmed in the Seto Inland Sea, causing oyster supplies to drop.

More than 50% of farmed oysters were found to be dead in many production areas, including Hiroshima Prefecture, which is Japan’s No. 1 producer of oysters. Poor growth due to rising sea temperatures from the extreme heat this summer is believed to be a primary cause.

On Nov. 5, Hiroaki Takata, an oyster farmer in Kure, one of Hiroshima Prefecture’s major production areas, brought in a haul of 40,000 oysters but found that nearly all of them were dead with their shells open. He stopped taking new orders in late October and does not know when he will resume.

“It’s tough to run the business,” Takata, 47, said.

According to the Kure oyster promotion council, 80%-90% of oysters in the city died off.

“Some businesses may be forced to close,” the council’s secretary general Koso Tanaka, 63, said.

The Fisheries Agency and other sources found that 60%-90% of oysters have died off in central-eastern Hiroshima Prefecture, about 50%-80% in Hyogo Prefecture, about 10%-70% in Okayama Prefecture and about 50%-90% in Kagawa Prefecture.

The rise in sea temperatures due to the extreme heat is considered the cause of mass die-offs.

The Hiroshima Prefectural Fisheries and Marine Technology Center in Kure said the average temperature of the water near the facility was 25 C in July, 27.2 C in August and 27 C in September, all of which were 1.5 C to 1.9 C higher compared to an average year.

Hiroshima University Prof. Kazuhiko Koike, who specializes in marine environmental biology, said mature oysters experience stress when water temperatures exceed 25 C.

“[This year], oysters were exposed to high water temperatures for an extended period of time,” Koike said. “They also were not able to get sufficient nutrients, as the seawater could not circulate properly due to the rainy season ending early.

Save businesses

From the beginning of November through Tuesday, 118 kilograms of Hiroshima-produced oysters that still had their shells were handled daily, according to the Hiroshima central wholesale market. That is less than half of last November’s average, which was 241 kilograms.

The average price per kilogram surged to ¥1,687, up more than ¥300. Similar trends were observed for shucked oysters.

To help oyster farmers and processing businesses, the Japan Finance Corporation set up consultation desks on Wednesday at its five branches in Hiroshima and Yamaguchi prefectures to provide financial advice. Hyogo Prefecture has considered financial support for affected fishermen.

Agriculture, Forestry and Fisheries Minister Norikazu Suzuki inspected oyster farms in Hiroshima Prefecture on Wednesday and said, “We will work with municipalities to support the businesses.”