Nori Color Fading and Warming Seas: Farming Today and Food Loss

When nori does not turn black: what is happening in the sea?

Nori, a staple of rice balls, hand-rolled sushi and miso soup, is actually a "farmed crop" grown in the sea. The main species is susabi-nori (Pyropia yezoensis). Spores are attached to nets in autumn, the nets grow in the sea through winter, and the blades are harvested and dried. In recent years this farming has been shaken badly by rising water temperatures and nutrient-poor seas.

A poor nori harvest shows up as "color fading." Nori that should be dark, close to black, turns yellow or brown. Pale nori loses aroma and taste, and its commercial grade drops.

The yearly cycle of nori farming

Nori farming starts in autumn with "seeding" (saibyo). Spores released from thread-like seed stock (conchocelis) grown on oyster shells are attached to nets, the nets are set out in the sea, grown and harvested. Harvest is mainly from winter to spring; the picked blades are chopped fine, spread thin like paper and dried.

• Autumn (around Oct–Nov): seeding and early rearing. Spores are attached to nets and tiny sprouts are raised
• Winter (around Dec–Feb): main farming and harvest. Nori is picked repeatedly in nutrient-rich water
• Spring (around Mar–May): end of the season. Quality tends to fall as water warms and nutrients run low

Of these, the first stage, "seeding and early rearing," is now the phase most exposed to climate change.

Domestic nori output and prices in numbers

The long-term shrinkage of nori production is confirmed by several sources. The overseas fisheries outlet The Fish Site, citing Nikkei Asia, reports that Japan's nori harvest (wet weight) fell from a peak of about 480,000 tonnes in 1994 to about 288,000 tonnes in 2020.

In the 2022 season a red tide broke out in the Ariake Sea, the main production area. Reports say low rainfall and high water temperature lay behind it. River flow dropped, little nitrogen and phosphorus reached the sea, and the nori faded. An aging workforce and slow mechanization are also cited as challenges on the production side.

The head of the Osaka Nori Cooperative reportedly said at its general meeting in June 2026 that "last year's result was about 5.43 billion sheets." Because the report does not specify which season this refers to, we treat it only as a reference value.

How rising water temperature hits nori farming

The guideline seeding temperature is about 23°C

Nori seeding is done in autumn as the water begins to cool. Chiba Prefecture's nori bulletin gives 23°C as the guideline temperature to start rearing and says that judging the recovery of nutrients and the cooling trend is the key first point of the season. Miyagi Prefecture also starts setting nets only after confirming the temperature has fallen to 23°C or below.

A Kumamoto Prefecture document points out that in recent years water temperature has not fallen smoothly during the rearing period and sometimes even rises, which is one cause of later poor harvests.

Fish eat nori in warm seas

A March 2026 Yomiuri Shimbun report says that, according to the Fisheries Agency and others, the main cause of poor harvests is rising sea temperature. It also notes that in warm water fish such as black sea bream become active and eat nori sprouts (fish grazing).

Too cold is a problem too: research in Tohoku

Warming is not a one-way story. The KAKEN research project "Establishment of cold stress-resistant nori strains specialized for mariculture in Tohoku" looks for cold-tolerant strains against the background of production dropping in Tohoku because of cold seawater in early spring. That the trouble is the opposite depending on the region is what makes this problem hard.

The real cause of color fading: a shortage of nutrients

The direct cause of color fading is a shortage of nutrients in seawater, especially nitrogen. Nori absorbs nitrogen and phosphorus from seawater to make its pigments. Miyagi Prefecture materials say that color holds when the three forms of nitrogen (nitrate, nitrite and ammonium combined) are at 5 µg-at/L or more, and tends to fade below 3 µg-at/L.

Why do nutrients run short?

• Less river water: with little rain, supply of nitrogen and phosphorus from rivers to the sea falls
• Competition with phytoplankton: when diatoms increase, they use up nutrients before the nori can
• High water temperature: red tides and phytoplankton blooms occur more easily

A Hyogo Prefecture research report says that color fading caused by a shortage of dissolved inorganic nitrogen (DIN) has become a major issue in recent years, and that rivers, sewage treatment plants and industrial effluent are important nitrogen sources for nearby fishing grounds.

In the March 2025 case, water was cooler than normal yet color fading spread. This shows that the amount of nutrients, not only temperature, is decisive.

Substandard nori and food loss: what happens to faded nori?

In the processes before and after roasting, a certain amount of nori comes out misshapen, holed or unevenly colored. Faded nori also drops in grade and price. From a food-loss viewpoint, the pattern "edible but devalued because it does not meet the standard" is the same as with substandard vegetables.

On products made from nori offcuts, materials from the Kanto Regional Agricultural Administration Office of MAFF (FY2025 Green Strategy Student Challenge, Kanto Block) introduce the business research club of Chiba Prefectural Oyu High School, which is working on cutting food loss and revitalizing the community by developing and selling products made from nori offcuts. We could not confirm the product details or distribution, so we state only the fact that the initiative exists.

What you can do at home

• Eat it before it loses flavor to humidity: after opening, store it sealed with a desiccant
• Do not throw away torn nori or offcuts; use them as seasoning for furikake, tsukudani, soups and stir-fries
• Tear leftover roasted nori into small pieces, freeze it and use it like a seasoning

For food loss in seafood overall, see also Seafood food loss and Household food loss.

Producer measures: raise it earlier, stronger and more efficiently

Judging when to start seedling

Prefectural fisheries experiment stations frequently send producers nori bulletins with water temperature and nutrient measurements. The decision to wait until the water falls below 23°C before seeding is supported by this information.

Delivering nutrients to the sea

As the Hyogo Prefecture research report shows, rivers, sewage treatment plants and industrial effluent are important nitrogen sources for nearby grounds. How to balance cleaner water with delivering nutrients to the sea is a challenge for coastal water management.

Breeding better strains

The search for strains tolerant of heat and cold continues at research institutions. Besides the research project on cold-tolerant strains for Tohoku, breeding heat-tolerant strains is also considered a challenge.

For the spread of seaweed farming, see also Seaweed farming and recovery.

Links between ocean warming and fisheries as a whole

Nori is not special. Rising sea temperature is changing the species and distribution of fish caught. We discuss this in Ocean warming and fisheries.

Seaweed absorbs carbon dioxide in the sea and is also a place where fish grow. Declining seaweed beds are a similar problem involving warming and grazing. A poor nori harvest can be called one "sensor" of environmental change in the sea.

What we can do: how we eat and choose

• Understand why prices rose and use what you need with care
• Eat without waste, including torn, offcut and pale nori
• Take an interest in producing-area and season information (fishery cooperatives, prefectural announcements)
• Join activities that protect the sea, such as local beach cleanups and river conservation

Buying and supporting also supports the people who farm. We do not intend to recommend any specific product or company.