What Happened in the Year When 80% of Hiroshima’s Oysters Died

In 2024, oyster farming in Hiroshima Prefecture was struck by a record mass mortality event. One of the aquaculture operators stated, “80% died,” leading to a significant decline in the overall production in the prefecture compared to the previous year. The systems that have supported Japan’s top oyster-producing region are being shaken by environmental changes such as rising seawater temperatures and decreasing nutrient salts—if this were the only fact, it would be just another story of a “crisis in production areas.”

However, behind this crisis, a somewhat unusual movement began. Ainanchō, located about 200 km from Hiroshima, started procuring seedlings from Hiroshima and began cultivating them in its own waters. Meanwhile, the fishing cooperative in Etajima City, Hiroshima Prefecture, has been sinking the large amounts of oyster shells left after cultivation to the seabed, improving the bottom quality of fishing grounds. When we connect these two flows—seedlings moving west and shells returning to the seabed—a structure that could be called the “oyster circulation” in the Seto Inland Sea emerges.

This is not the result of someone drawing up a grand plan. It is a story of how each site began to move due to its own circumstances, leading to a system that started to fit together.

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Why Ainanchō Chose Hiroshima’s Seedlings

Ainanchō is located at the southernmost tip of Ehime Prefecture, facing the Bungo Channel. While it has traditionally been active in the farming of sea bream and yellowtail, in recent years it has also been putting effort into oyster farming. Notably, Ainanchō’s decision to select Hiroshima as its source for seedlings is significant.

In oyster farming, where to obtain seedlings—meaning juvenile oysters—is a crucial variable that affects survival rates. Seedlings harvested from Hiroshima Bay are believed to possess genetic traits adapted to the environment of the Seto Inland Sea. When the aquaculture operators in Ainanchō cultivated these seedlings in the waters of the Bungo Channel, they did not experience the mass mortality seen in Hiroshima Bay, and the mortality rate significantly decreased.

Why is this the case? In recent years, the number of days when seawater temperatures in Hiroshima Bay exceed 30°C during the summer has increased, and due to advanced sewage treatment, nitrogen and phosphorus levels in the sea have decreased, leading to a shortage of phytoplankton, which serves as food for oysters—a problem referred to as “too clean of a sea.” On the other hand, the waters of Ainanchō face the open ocean, resulting in relatively stable water temperatures and good water circulation. Even with the same seedlings, changing the growing environment leads to different results—this idea of separating the production area from the growing area is functioning here.

An aquaculture operator in Ainanchō stated, “Ainanchō oysters can survive.” This statement reflects not just mere optimism but a calm assessment of their own sea conditions while keeping an eye on the crisis faced by the massive production area of Hiroshima. For Ainanchō, oysters could become a “third pillar” following sea bream and yellowtail. While there are costs associated with procuring seedlings, it also means that for Hiroshima’s seedling producers, a new outlet for seedlings that can no longer be raised in the bay is created. The crisis has, as a result, fostered a complementary relationship between production areas.

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450 Tons of Shells Returning to the Seabed—The Design of Etajima

When you eat oysters, shells remain. Hiroshima Prefecture has produced around 100,000 tons of shelled oysters annually, leading to a vast amount of shells generated during the aquaculture and processing stages. Handling these shells has been a long-standing issue. Some have been reused as fertilizers, soil conditioners, and raw materials for construction, but it is impossible to process all of them on land.

In response, the fishing cooperative in Etajima City has been promoting the initiative of sinking oyster shells to the seabed. The scale of this effort is about 450 tons. The cooperative crushes the shells and sinks them to the seabed to improve the bottom quality.

The main component of oyster shells is calcium carbonate—essentially limestone. The shells that sink to the seabed serve several functions. First, they physically harden the soft muddy bottom, creating a foundation conducive to the settlement of benthic organisms. Second, the surfaces of the shells become substrates for microorganisms, promoting the decomposition of organic matter. Third, the gaps in the shells provide hiding places for small crustaceans and juvenile fish, contributing to the recovery of biodiversity.

For the fishermen in Etajima, this is not just a measure for oyster farming. Improving the bottom quality will also lead to the recovery of other marine resources such as clams and sea cucumbers. In fact, there have been reports that the number of benthic species increased in areas where oyster shells were introduced. What was once waste is transforming into a foundational infrastructure for the sea. This transformation has a dual economic rationale of reducing processing costs and enhancing the value of fishing grounds.

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A Nested Structure Perspective

Seedlings born in Hiroshima become oysters in the waters of Ainanchō, and shells produced in Hiroshima support the fishing grounds on the seabed of Etajima. When we layer these two flows, a “nested structure” mediated by the living organism of the oyster becomes visible.

Traditional oyster farming has been a vertically integrated model that concludes within a single production area. Seedling collection, farming, processing, and shell handling—all have been conducted within the single marine area of Hiroshima Bay. However, environmental changes are beginning to disrupt this premise. Rising seawater temperatures, decreasing nutrient salts, and the risks of red tides and hypoxic water masses. A structure that relies entirely on one marine area is vulnerable to change.

What is currently happening is a gradual shift from vertical integration to “horizontal dispersion.” Hiroshima takes on the production of seedlings, while areas like Ainanchō with different environmental conditions take on the cultivation. Shell handling is utilized in contexts of fishing ground regeneration, as seen in Etajima. Each region takes on roles suited to the conditions of their own waters.

The interesting aspect of this structure is that it was not designed by anyone. Seedling producers in Hiroshima were looking for new outlets for their seedlings. Aquaculture operators in Ainanchō were seeking new sources of revenue. The fishing cooperative in Etajima wanted to solve both shell handling and fishing ground improvement simultaneously. Each of these challenges has, as a result, shaped a single circulation. Individual decisions have, retrospectively, become a “system”—this is a type of order.

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Unseen Connection Points

Of course, this circulation is not yet complete. Several questions remain.

First, there is the issue of stable seedling supply. If the environmental degradation of Hiroshima Bay continues, there is a possibility that the production volume of seedlings themselves may decrease. How to diversify the risks of Ainanchō continuing to depend on Hiroshima-produced seedlings will need to be addressed. Comparative verification with seedlings from other production areas, such as Miyagi and Iwate, will also be necessary.

Second, there is the branding issue of oysters raised in Ainanchō. What kind of oysters are “oysters grown in Ehime from Hiroshima seedlings” to consumers? There may be discrepancies between the rules of origin labeling and consumer perceptions. It is essential to establish a system that aligns facts with labeling early on.

Third, there is the need to verify the effects of sinking oyster shells to the seabed. To what extent does the scale of 450 tons contribute to the improvement of bottom quality over a certain area? Quantitatively monitoring changes in the biological community after the introduction and accumulating data will be key to transforming this initiative from an “experiment” to an “institution.”

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When the System Begins to Turn

This movement surrounding oysters in the Seto Inland Sea has not yet made big headlines. While the mass mortality in Hiroshima has been reported, there are hardly any articles conveying the structure of seedlings moving to Ainanchō and shells sinking to the seabed of Etajima.

However, local activities often unfold in such a manner. There are no flashy turning points; rather, small decisions made at each site accumulate, and one day, when looking back, they form a “system.” It is a structure that begins to turn not through individual efforts but through the design of relationships—therein lies a glimmer of hope.

The small juvenile oysters born in the sea of Hiroshima are thickening their shells in the tides of Ainanchō. One day, those shells may become a foundation for other living organisms on the seabed of Etajima. The oyster, as a living organism, is transcending the boundaries of production areas and reshaping the seabed of the Seto Inland Sea. I wish to quietly observe this.