The Sea Water, Garden Water, and Table Water — Three Disturbances Converge This Autumn

Autumn in Hiroshima begins with water.

Oysters from the Seto Inland Sea line the storefronts, while koi swim beneath the autumn leaves in Shukkeien. Food, scenery, and culture all spring to life in the same season, supported by different water systems — this is how the city of Hiroshima operates.

However, in 2024, all three of these water sources are breaking down simultaneously.

Mass deaths of farmed oysters. The weakening and emergency evacuation of koi from Shukkeien. And the oyster season, which has been delayed by a month. While each event might be dismissed as a result of “the heat” when viewed individually, together they raise a singular question: — where has the system that supports us faltered?

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Mass Death of Oysters — The Reality of “70% Have Died” on the Rafts

Hiroshima Prefecture accounts for about 60% of the national oyster production. This foundation was shaken by record high water temperatures last summer.

In the summer of 2023, surface water temperatures in the Seto Inland Sea consistently exceeded the average by 2 to 3 degrees Celsius. The optimal growth temperature for oysters is generally between 15 and 25 degrees Celsius, but if temperatures exceed 30 degrees for an extended period, both juvenile and adult oysters cannot survive. According to Hiroshima Prefecture, approximately half of the farmed oysters in the region have perished. Reports from some operators indicate that as much as 70% of their stock has been lost.

The damage cannot be conveyed by numbers alone. Oyster farming takes about a year and a half to two years from seeding to harvest. This means that the individuals that died last summer were nurtured since the autumn of the year before. Imagine the weight of the task of pulling up the ropes of the rafts and checking each oyster, now just a shell.

As a result, the lifting of the oyster catch ban in 2024 has been pushed back from early October to November 2, nearly a month later than usual. The Hiroshima Prefectural Fisheries Cooperative explains, “We are waiting for the remaining oysters to grow, so we can ship them in the best possible condition.” While this is a desperate measure, there are no prospects for a recovery in shipment volume.

Some operators have begun relocating their rafts to deeper waters offshore or temporarily sinking oysters to deeper layers during the summer to avoid high temperatures. However, moving them incurs costs. Fuel expenses, labor, and permit applications — each of these is a heavy burden for small-scale operators.

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Behind the Lifting of the Ban — The “Price Barrier” Faced by Restaurants and Tourism

The decrease in oyster supply is not just a problem for farmers.

For oyster specialty restaurants in Hiroshima City and eateries around Miyajima, oysters are a pillar supporting sales during the autumn and winter seasons. With the delayed lifting of the ban, the peak sales period becomes shorter. Moreover, if shipment volumes decrease, procurement prices rise. Last season, the wholesale price of Hiroshima’s shucked oysters was around 2,000 to 2,500 yen per kilogram, but this season is expected to see further increases.

If restaurants raise prices, the barrier to the experience of “eating oysters in Hiroshima” for tourists becomes higher. On the other hand, if they keep prices steady, their profits are squeezed. In either scenario, someone bears the pain.

One restaurant owner stated, “I don’t want to raise prices, but if I can’t predict my procurement, there may be days when I have to take items off the menu.” The day that fried oyster set meals disappear from the menu — that is also a day when Hiroshima’s food culture landscape changes slightly.

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Koi in Shukkeien — The Garden’s Water Rejected “Living Things”

At the same time as the disturbances in the sea, water issues have also surfaced in Shukkeien, a famous garden in Hiroshima’s Naka Ward.

Nearly half of the koi raised in the garden’s Taku-ei Pond have died from weakening. The city of Hiroshima has taken measures to remove the remaining koi from the pond and temporarily evacuate them to another location.

The city has stated that the cause may be a combination of “changes in water quality and high water temperatures,” but detailed investigation results have not yet been released. It is necessary to separate fact from speculation here. It is still unclear whether high water temperatures were the direct cause of death, whether water quality deterioration occurred first, or if there were issues with the pond’s circulation system.

However, there are structural insights that emerge. The pond in Shukkeien is close to a closed water system. Because the inflow of water from outside is limited, when temperatures rise, water temperatures also increase, and the amount of dissolved oxygen decreases. The environmental conditions that previously allowed the koi to “survive the summer” may have been exceeded due to recent extreme heat.

Shukkeien is a garden with a history of 400 years, having begun construction in 1620. This place, which was devastated by the atomic bomb and later restored, symbolizes the resilience of the people of Hiroshima, who have a spirit of “rebuilding even after destruction.” The absence of koi in Shukkeien — while the loss of this landscape may not be reflected in tourist satisfaction metrics, it undoubtedly casts a shadow on the hearts of the citizens who visit the garden regularly.

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Who is Responsible for “Water Management”? — The Structure of Dispersed Responsibilities

The sea of oysters is managed by the prefectural fisheries cooperative and farmers. The pond in Shukkeien is managed by Hiroshima City’s Urban Development Bureau. Rivers are overseen by the Ministry of Land, Infrastructure, Transport and Tourism and the prefecture. Drinking water is managed by the city’s waterworks bureau. — Hiroshima’s “water” is divided into detailed jurisdictions based on its use and location.

This dispersed structure is a rational system where each specialization functions during normal times. However, when a single cause, such as “high water temperatures,” simultaneously affects both the sea and the pond, the walls of jurisdiction may lead to delays in response. It is unlikely that information about the damage to oyster farming and the water quality data from the garden would normally be cross-referenced.

The issue is not about “who is to blame.” Each manager is addressing issues within their own jurisdiction. What needs to be questioned is whether there is a system in place to share information across jurisdictions and take proactive measures when the premise of “continuing to rise in water temperature” changes.

Hiroshima City is set to begin revising its “Climate Change Adaptation Plan” in 2024. Whether a framework for cross-sector monitoring of aquatic ecosystems will be included remains a pivotal point.

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The “Quiet Adaptation” Initiated by Operators

Before the system catches up, the field is already moving.

Among the aforementioned oyster farmers, some have begun experimenting with changing the depth at which their rafts are set. They are measuring temperature differences at depths of 3 meters and 5 meters and comparing the survival rates of juvenile oysters. It will take several years to accumulate data, but the judgment that “if we continue using the same methods as last year, the same problems will occur next year” is clear.

Reports indicate that discussions are also underway at Shukkeien to improve the water circulation in the pond. However, maintaining the landscape as a cultural asset while installing new equipment presents both technical and budgetary constraints.

These on-site initiatives may not make flashy headlines. However, it is always the people on the ground who are the first to take action after a system breaks down.

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Future Points of Interest — What Lies Beyond the “Lifting of the Ban”

The lifting of the oyster ban on November 2 serves as both the starting gun for Hiroshima’s autumn and winter season and a day that will visualize the depth of the damage. When shipment volumes, sizes, and prices are all revealed, the full extent of the damage will finally come into view.

Here are some key points to watch:

• Trends in Shipment Volume and Prices: The initial auction prices and supply volumes throughout the season will reflect the reality of the damage.
• Water Quality Survey Results from Shukkeien: How far the city can identify the causes and publicize countermeasures. The next steps after the “temporary evacuation” will be questioned.
• Incorporation into Climate Change Adaptation Plans: Whether a system for protecting aquatic ecosystems across sectors will be integrated into the framework.
• Success or Failure of Operators’ Adaptation Measures: How changes in raft depth and farming methods will affect survival rates in the coming seasons.

All of these are themes that should be tracked over a span of months to years. It is not possible to present “solutions” in a single article. However, it is possible to leave the shape of the questions.

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Hiroshima’s waterways nurture food, create landscapes, and attract people — all three functions have simultaneously been shaken this season. Individual responses have begun. What is lacking is the perspective that connects them.

The hands pulling up the raft ropes and the hands scooping koi from the pond. Though they are doing different things, they are facing the same question — “Can we still nurture living things with this water?”