Japan is facing a major oyster die-off in parts of its top production region, with officials attributing the mass mortality to unusually warm sea temperatures. Coastal areas surrounding the Seto Inland Sea, which produces over three-quarters of the country’s farmed oysters, have reported up to 90 per cent of cultivated oysters dead.
Veteran oyster farmer Tatsuya Morio, who has over 20 years of experience, described the situation as unprecedented. Shoichi Yokouchi, an official in the Hiroshima area bordering the Seto Inland Sea, said elevated water temperatures combined with other environmental factors are likely responsible for the die-offs.
Data from Hiroshima prefecture shows that average water temperatures along the coast from July to October were 1.5 to 1.9°C higher than the 1991-2020 average, a critical period for oyster cultivation. Yokouchi explained that sustained high temperatures weaken oysters, making them more vulnerable to viruses and bacteria. Typically, oyster mortality rates range from 30-50 per cent, but mid-November surveys revealed rates of 60-90 per cent in central-eastern Hiroshima Prefecture.
Shinichiro Toi, a marine product technology researcher, added that low rainfall this July, combined with higher temperatures, increased salinity in the water, further harming the oysters. Even strains of oysters previously thought to be heat-resistant have been severely affected, with surviving oysters in Hyogo Prefecture reported to be about half the normal size, making them difficult to sell.
The Ministry of Agriculture, Forestry and Fisheries (MAFF) is working with local authorities to investigate the cause of the die-offs. Notably, oysters along the Sanriku coast in northern Japan, which account for about 17 per cent of production, have not been impacted. Rising ocean temperatures in the region, which have more than doubled in rate since 1993, are linked to broader global warming trends. Major importers of Japanese oysters include Hong Kong, Taiwan, Singapore, Vietnam, and Thailand.
