“From ‘Reducing’ to ‘Adding’ — The Common Sense Surrounding Ocean Nutrients Has Been Reversed

Relaxing sewage discharge regulations on purpose —.

In an era when the righteousness of water quality was defined by “cleaning up,” Hiroshima Prefecture chose the opposite. The mass death of oysters in Hiroshima Bay in the 2024 season was labeled as “disaster-level,” causing a significant drop in the local aquaculture production compared to the previous year. In response, the prefecture proposed relaxing discharge standards for 33 sewage treatment facilities, intentionally increasing the amounts of nitrogen and phosphorus flowing into the sea to boost marine nutrients. This is not merely an emergency measure; it could represent a turning point that fundamentally questions the water quality management of the Seto Inland Sea, which has been designed to “reduce pollution” for half a century.

This article explores the structural background of the mass oyster deaths, the changes brought about by the relaxation of discharge regulations, and the voices of producers who continue to engage with the sea, pursuing the question of “who designs the nutrients of the sea, and how?”

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Mass Death of Oysters — What Happened and What Coincided?

Hiroshima Prefecture accounts for about 60% of the national oyster production. A mass death event occurred in the aquaculture areas around Onomichi City in the 2024 season. Some producers reported that their shipment volumes dropped to less than half of the usual amount.

The immediate trigger is believed to be red tide — a phenomenon where phytoplankton in the seawater proliferate abnormally, changing the color of the water. As the plankton die in large numbers, oxygen in the water is rapidly consumed, leading to the formation of hypoxic water masses. Oysters are filter feeders, taking in large amounts of seawater to filter out plankton for food. If the seawater itself lacks oxygen and contains harmful metabolic substances, oysters can suffocate or die from toxicity.

However, the occurrence of red tide itself is not uncommon. In Hiroshima Prefecture, red tide warnings are issued annually, primarily during the summer. The problem lies in its frequency, scale, and timing. In recent years, rising seawater temperatures have led to earlier occurrences of red tide, increasingly coinciding with the oyster growth period. Additionally, there are concerns that the “oligotrophication” of the Seto Inland Sea — a state where the nutrients in the sea have decreased too much — is altering the species composition of plankton, creating an environment conducive to harmful species dominance.

There is a structural irony here. Since the enactment of the Special Measures Law for Environmental Conservation of the Seto Inland Sea (Seto Inland Sea Law) in 1973, the Seto Inland Sea has been managed as a “clean sea.” The total amount of nitrogen and phosphorus contained in factory and domestic wastewater has been gradually regulated more strictly, leading to dramatic improvements in water quality. Transparency has increased, and the number of red tide occurrences significantly decreased at one point. However, behind this “success,” the fundamental productivity of the sea — the ability of phytoplankton to produce organic matter through photosynthesis — has declined, leading to the depletion of marine resources such as oysters, nori, and clams that rely on it for food. The addition of the phrase “rich sea” in the 2021 amendment to the Seto Inland Sea Law was a response to this contradiction.

— In other words, the mass death of oysters did not occur in a “dirty sea.” It is the result of a “sea that has become too clean” breaking down in another form.

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Relaxation of Discharge Regulations — A Decision to Reverse the Mechanism

Hiroshima Prefecture is targeting a total of 33 sewage treatment facilities located in Hiroshima City, Hatsukaichi City, and Kure City. Specifically, the plan is to lower the removal rates of nitrogen and phosphorus in the treated water and intentionally increase the nutrient concentrations in the effluent. Traditionally, sewage treatment plants have been designed with the technical goal of “how to reduce nutrient salts.” Since this operational policy is being reversed, significant adjustments will be necessary for both on-site operations and legal frameworks.

This initiative is noteworthy not only for the result of “increasing nutrients” but for the act of intentionally redesigning the nutrient balance of the sea.

Conventional water quality management has been, in a sense, a “subtraction” design. It aimed to reduce pollutants, tighten discharge standards, and keep the sea clean. Within that framework, sewage treatment plants were seen as “barriers protecting the sea.” However, this relaxation redefines sewage treatment plants as “infrastructure delivering nutrients to the sea.” The same facilities, using the same pipes, will now play opposite roles. The meaning of the mechanism changes entirely with the shift in purpose — this nested structure is the essential intrigue of this policy.

Of course, there are risks involved. Increasing nutrient salts theoretically raises the risk of red tide as well. The ratio of nitrogen to phosphorus (N/P ratio) affects the dominant species of plankton, necessitating precise control over “which nutrients to increase, by how much, and when.” Similar initiatives have been advanced in Hyogo Prefecture, where seasonal operational management of sewage treatment plants has been attempted as a countermeasure against the discoloration of nori. How well Hiroshima Prefecture can incorporate insights from these precedents will be one key to its success.

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Producers’ Hands — Those Who Scatter Oyster Shells on the Seafloor

While the system is moving, producers on the ground are trying to restore the sea with their own hands.

The Onomichi Oyster Producers’ Association in Kure City continues its efforts to scatter oyster shells on the seafloor of fishing grounds. Oyster shells, primarily composed of calcium carbonate, function to stabilize pH and serve as a substrate for microbial attachment when sunk to the seafloor. The cost of about 1,500 yen per ton is not a light burden for individual producers. Nevertheless, the attitude of “we will take care of our own sea” supports this initiative.

In May 2025, a discussion meeting was held between young agricultural, forestry, and fisheries producers in Hiroshima Prefecture and the governor. What the producers sought was not only the relaxation of discharge regulations. They raised questions such as, “How will the results of increased nutrients be measured?” “Is there a monitoring system?” “Who will take responsibility when red tide increases?” — in other words, they were questioning the design of the “exit” of the policy.

This question carries weight. A policy of “adding” nutrients requires far more complex feedback than a policy of “reducing” them. In the case of reduction, management could be based on a single indicator of discharge volume. However, in the case of addition, multiple variables such as water temperature, tidal currents, sediment quality, and plankton species composition in each sea area must be considered simultaneously. What producers are intuitively seeking is precisely that “system for observation.”

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Who Designs the Nutrients of the Sea? — The Structure of the Question

The recent shift in nutrient management of the Seto Inland Sea involves three overlapping layers.

First, the institutional layer. The legal and administrative framework changes from the amendment of the Seto Inland Sea Law to the relaxation of discharge regulations. Second, the scientific layer. The accumulation and limitations of knowledge regarding oligotrophication and red tide, control of N/P ratios, and modeling of marine ecosystems. Third, the field layer. The experiential knowledge of producers who scatter oyster shells, manage rafts, and observe the color of the sea every morning to make judgments.

Only when these three layers interlock can the act of “designing the nutrients of the sea” be established. Conversely, if any one of them is missing, the policy will stagnate. If the institution moves ahead alone, the field will not catch up; if science becomes sophisticated but administration does not act, it will not be implemented; and if only experiential knowledge exists, broad-scale control cannot be achieved.

Hiroshima Prefecture’s recent decision has at least moved the institutional layer. In the scientific layer, institutions such as Hiroshima University and the Fisheries Research and Education Agency are working to strengthen marine monitoring. What remains is the field layer — how the voices and experiences of producers will be integrated between the institution and science.

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Future Points of Interest

In the short term, the greatest focus will be on the impact on oyster farming in the 2025 season. How will the species composition of plankton change in the areas where discharge regulations have been relaxed? Will there be changes in the frequency of red tide warnings? What is the growth status of the oysters? These data will serve as the initial material for judging whether the policy has been effective or not.

In the medium to long term, the question will be how this “adding” management will spread to other coastal prefectures of the Seto Inland Sea. The nori of Hyogo Prefecture, the yellowtail of Kagawa Prefecture, and the pearls of Ehime Prefecture — each of these marine resources depends on the nutrient balance of their respective sea areas. Will the experiment that began with Hiroshima’s oysters expand into a nutrient management model for the entire Seto Inland Sea, or will it remain an individual response?

Another important but often overlooked point is that changing the operational policy of sewage treatment plants means turning attention upstream — that is, to how we manage our domestic wastewater. The water flowing from our kitchens ultimately influences the lives of oysters. When we become aware of that circuit, the question of “designing the nutrients of the sea” is no longer just for fishermen and administrators.

— The sea of the Seto Inland Sea is beginning to question “what does richness mean” once again, after becoming too clean. The answer will depend not on institutions or science, but on the hands of those who observe the sea’s color every morning and the design of the systems that enable those hands to reach.