WHY JAPAN’S OYSTERS ARE DYING EN MASSE

Why Japan’s oysters are dying en masse. A death rate of up to 90%, linked to warming seas, is threatening the oyster industry in Hiroshima prefecture, which produces most of the country’s farmed oysters. At the Kure oyster festival, beer and grilled meat sell well, but the longest lines form at oyster stalls, where chefs cook shellfish until they open to reveal their contents. As reported in a recent article in The Guardian newspaper.

Nobuyuki Miyaoka, attending with his family, prefers oysters steamed with sake and served with ponzu sauce. He notes that local oysters were fine until this year but have become noticeably smaller.

It is not just their reduced size that concerns people in Kure and across Hiroshima’s coastal areas. At an event meant to promote the region’s signature product, oysters are in short supply. Mass die-offs are affecting the shellfish, which are central to both local cuisine and the fishing economy. Experts attribute this to rising sea temperatures and last year’s extreme summer heat, which reduced oxygen and food availability.

With concerns that such die-offs may become more frequent, the Japanese government has stepped in to support struggling fisheries.

Taketoshi Niina, who runs a small fishery in Kure, describes this season as a disaster. Around 80% of his oysters are dead when harvested, and many survivors are too poor in quality to sell. The situation is already affecting finances, and prospects for next year appear equally bleak.

Similar conditions are being reported along the Seto Inland Sea, from Hiroshima to Hyogo, though Hiroshima has been hit hardest. The prefecture produces nearly two-thirds of Japan’s farmed oysters, with fisheries in the inland sea accounting for about 80% of national production.

Niina first noticed the problem last October at the start of the harvest season, when unusually high numbers of dead oysters appeared. While typical annual losses range from 30% to 50%, mortality rates have reached up to 90% in some areas. Veteran oyster farmer Tatsuya Morio says he has never seen anything like it in over 20 years.

Japan experienced its hottest summer on record last year, with average temperatures 2.36°C above normal. Prolonged high temperatures weaken oysters and make them more vulnerable to disease, according to Shoichi Yokouchi of the Hiroshima prefectural government. Water temperatures along the coast during key cultivation months were also significantly higher than average.

In response, the fisheries agency introduced support measures, including low-interest loans and access to aid programmes for aquaculture businesses.

Local officials warn of wider economic impacts. Oyster production in Kure supports not only fisheries but also jobs in distribution and food tourism. Businesses are already feeling the strain: a popular oyster-themed restaurant will close early this year due to shortages, and a tax incentive programme will not provide its usual oyster gifts.

Kazuhiko Koike, a professor at Hiroshima University, points to multiple environmental factors linked to climate change, including high temperatures, low oxygen levels, lack of rainfall, and reduced nutrients. Warmer surface water prevents mixing with cooler layers below, limiting oxygen supply to the seabed.

While stopping climate change is difficult, Koike suggests possible adaptations, such as moving oyster rafts to cooler waters or placing them deeper to avoid heat stress.

After another poor harvest, Niina questions the future of his family business, which his father has run for decades. Although his son once planned to take over, he now worries whether the industry has a viable future.

Image: ©Fish Focus