Sea Water Temperature: What Breaks and What Emerges When It Rises by 1°C

The mechanisms supporting the dining tables of the Setouchi region are currently shaking simultaneously.

The artificial seedling cultivation project for oysters being advanced by Hiroshima University and Higashihiroshima City. The annual release of juvenile sea bream and flounder by Fukuyama City every summer. And the series of Campylobacter food poisoning cases in Mihara, Onomichi, and Fukuyama—these three seemingly separate events all raise the question of how much the food safety network in Setouchi can withstand under the same strain of “extreme heat.”

What is noteworthy is that alongside the crumbling mechanisms, a new system is quietly emerging.

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Protecting the “Seeds” of a 20 Billion Yen Market—The Structure of Artificial Seedling Cultivation for Oysters

Oyster farming in Hiroshima Prefecture accounts for about 60% of the national annual production, with a market size of approximately 20 billion yen. The starting point of this massive industry lies in the tiny “seed oysters” just a few millimeters in size.

During the extreme heat of summer 2023, the sea water temperature in the Seto Inland Sea remained 1-2°C higher than usual for an extended period, leading to reports of mass die-offs of farmed oysters in various locations. The traditional method of natural seedling collection—allowing oyster larvae to naturally attach to scallop shells placed in the sea—is significantly influenced by natural conditions such as sea water temperature and tidal currents. If these conditions deteriorate, the seed oysters themselves cannot be secured. Here lies a structure where a 20 billion yen market is shaken at its roots by a single variable: sea water temperature.

The artificial seedling cultivation project being jointly pursued by Hiroshima University and Higashihiroshima City is a structural response to this vulnerability. By selecting parent oysters with high survival rates in high-temperature environments and artificially fertilizing and raising them, the project aims to create a seed supply line that does not rely on natural conditions. Selective breeding requires validation over multiple generations, and it will take several years for the results to reach the field. The funding involved is said to be in the tens of millions of yen, but what is being protected is the industrial foundation worth 20 billion yen. Before discussing cost-effectiveness, this is fundamentally about the most basic safety net: “not allowing the seeds to go extinct.”

It is important to pause here and note that this project is not merely about improving varieties. Transitioning from natural to artificial seedling cultivation is an attempt to switch the “mechanism itself” of aquaculture, and if successful, it will allow for planned management of seed oyster supply. The process, which has relied on personal experience and natural conditions, will be replaced with reproducible technology. It is a modest yet potentially transformative initiative for the industrial structure.

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The Uncertainty Beyond the Juvenile Fish Released by Children—The Release of Sea Bream and Flounder

On July 15, 2023, about 100 children released juvenile sea bream and flounder into the sea at a beach in Fukuyama City. This release project, conducted annually as part of aquaculture, serves two functions: maintaining fishery resources and educating the next generation.

The cost of the release project, including the purchase and cultivation of juvenile fish, is estimated to be several million yen annually. Based on data regarding the unit price and survival rate of juvenile fish produced by aquaculture centers nationwide, the survival rate of released juvenile sea bream until they are caught as adults is generally a few percent. In other words, if 100 fish are released, only a few are expected to return.

This “few percent” could decrease further due to rising sea water temperatures. While sea bream is considered relatively resilient to high temperatures, flounder tends to prefer cooler waters, and multiple researchers have pointed out that if summer water temperatures in the Seto Inland Sea continue to rise, it will impact the settling rate of the released fish. Although the act of releasing fish remains unchanged, the marine environment that serves as their “receptacle” is changing.

The sight of children releasing juvenile fish is beautiful. However, whether the sea will continue to be a place where these juvenile fish can survive is a problem that lies outside the release project. Continuing the release as a mechanism and maintaining an environment where it functions are two separate challenges, and there is a reality where investment in the latter is lagging behind.

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66 Cases of Food Poisoning Reflecting the “Last Mile” of Distribution

In July 2023, at least 66 people in Mihara, Onomichi, and Fukuyama developed food poisoning caused by Campylobacter. The common food implicated is chicken. Campylobacter is a bacterium that resides in the intestines of chickens and attaches to the surface of meat during processing. It can be killed by heating to over 75°C for more than one minute, but conversely, if heating is insufficient, the likelihood of illness is high.

It is necessary to clarify the relationship with the extreme heat. Campylobacter food poisoning cases tend to concentrate from June to September each year, which is due not only to the proliferation of bacteria caused by rising temperatures but also to the increased demand for chicken dishes in summer and more opportunities for outdoor cooking such as barbecues—these are complex factors at play. According to statistics from the Ministry of Health, Labour and Welfare, Campylobacter consistently ranks among the top causes of bacterial food poisoning cases each year. In other words, the extreme heat does not serve as a “cause” but rather amplifies existing risks.

The core of the problem lies in the “last mile” of the food safety management chain, which encompasses production, distribution, and cooking—specifically in the kitchens of restaurants and households. The safety of food ingredients can collapse if the final heating process is inadequate, regardless of how well it is managed during production. Health department suspensions and guidance are reactive measures, and as a preventive mechanism, continuous hygiene education for food handlers and accurate information provision to consumers are essential.

For the Setouchi region, which has a thriving tourism industry, frequent food poisoning incidents directly undermine trust. The brand image of “Setouchi seafood is safe and delicious” can be significantly damaged by a single incident. The economic impacts, such as decreased restaurant sales and tourists avoiding the area, spread as an “atmosphere” before they are reflected in numbers.

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Three Events Pointing to One Question

The artificial seedling cultivation of oysters is an attempt to shift the seed supply mechanism from natural dependence to technological management. The release of juvenile fish continues to maintain the resource conservation mechanism while facing changes in the prerequisite marine environment. Food poisoning countermeasures indicate that repeated cracks are occurring at the end of the safety management chain from production to consumption.

What is common among the three is the fact that “the mechanism exists, but the environmental conditions it anticipated are changing.”

Mechanisms function under the assumptions present at the time of their design. Sea water temperature remaining within a certain range. The sea being suitable for the released juvenile fish to grow. Ingredients being properly cooked. When these assumptions break down, one is faced with the choice of either redesigning the mechanism itself or making additional investments to maintain the prerequisite conditions.

The project at Hiroshima University has chosen the former path: redesigning the mechanism. On the other hand, the release of juvenile fish is at a stage where it maintains the traditional mechanism while struggling to keep up with environmental changes. Food poisoning countermeasures face the most challenging issue: they will not function unless human behavior changes at the end of the mechanism.

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Who Does This Make Easier?

When discussing the food safety network in Setouchi, there is a tendency to rely on grand terms like “the entire region” and “sustainability.” However, the value of a mechanism should be measured by whose burdens it specifically alleviates.

If artificial seedling cultivation succeeds, aquaculturists who have been at the mercy of the uncertainties of natural seedling collection each year will be able to manage their operations more systematically. If the effectiveness of juvenile fish releases is verified, limited budgets can be concentrated on more effective species and marine areas. If the food poisoning countermeasure mechanisms reach the field, restaurants that have diligently maintained hygiene management will suffer less from reputational damage.

Mechanisms exist to make someone’s life easier. And that “someone” is often the behind-the-scenes individuals who do not appear in the spotlight.

Extreme heat will come again next year. Sea water temperatures will not drop. Under this premise, how should we restructure the mechanisms that support Setouchi’s food? There is no single answer, but the way to frame the question is clear—does this mechanism truly make life easier for those working on the ground?

Only the regions that honestly confront this question will acquire the mechanisms to survive the next summer.