Besides anago, nitsume is brushed onto sushi toppings that have been simmered in a flavorful liquid, like shako and hamaguri.
In Japanese culinary terms, “nitsumeru” (verb) refers to the process of reducing the liquid content of a sauce containing seasonings by boiling off the moisture, thereby concentrating the flavor and increasing its thickness. Both “eel sauce” and “conger eel nitsume” are sauces made by reducing the liquid content through boiling.
First, you need to understand the difference between eel and conger eel.
In Japanese, the word “unagi” generally refers to a freshwater eel. The scientific name is Anguilla japonica Temminck & Schlegel, 1846. On the other hand, the word “anago” generally refers to the conger eel. Its scientific name is Conger myriaster (Brevoort, 1856).
While both eel sauce and conger eel nitsume are made by reducing liquid through boiling (the process known as nitsumeru), only the latter is traditionally referred to as “nitsume” in sushi restaurants.
So, what is the fundamental difference between them?
Eel sauce (known as unagi sauce or unagi no tare) is a rich, sweet soy sauce-based sauce that is ideal as a dip or drizzling sauce. It does not actually contain any eel ingredients, but it is a sweet, sticky sauce that is perfect as a topping for sushi rolls and other dishes.
“Eel sauce” can refer broadly to both mass-produced sauces and artisan sauces made by eel restaurants. The basic method for making eel sauce is to mix mirin and soy sauce and heat it. Various ingredients such as sake, sugar, tamari soy sauce, water amber, honey, dashi, and potato starch (commercial products often contain thickening agents) are added to create a unique flavor.
Incidentally, most traditional eel restaurants make their eel sauce using only high-quality mirin and soy sauce. This gives the eel a beautiful glaze and subtle sweetness when grilled, as well as a clean aftertaste, which is why it is considered a craftsmanship.
When making kabayaki, the process of applying eel sauce is repeated multiple times. The components of the eel sauce combine with the umami of the eel to create a deeper, more complex flavor. In addition, applying the sauce and grilling it causes chemical reactions such as the Maillard reaction and caramelization, which further enhance the appetizing flavor.
While you can easily buy eel sauce at the market, it’s incredibly easy and tasty to make at home. Eel sauce can be used as a seasoning for other dishes if there is any left over, but it is primarily a sauce specifically made for making kabayaki. By the way, kabayaki is a fish dish where the long-bodied fish is filleted, the central bone removed, skewered, grilled, and then coated with a thick sauce before being grilled again. In Japan, when people say “kabayaki,” they are usually referring to “unagi kabayaki.” Other ingredients used include hamo and anago.
Nitsume is brushed onto nigiri sushi made with anago, shako, shellfish, and other toppings. In sushi restaurants, it is often abbreviated as “tsume.” There are several methods for making nitsume, including reducing the cooking liquid from simmered conger eel, or simmering the head and bones of conger eel to extract the broth, then adding seasonings and reducing it further. In other words, nitsume contains components of conger eel.
Compared to anago, unagi is more flavorful, fatter, and meatier. Moreover, unagi is considered a higher-quality delicacy in Japan and is therefore more expensive. In terms of taste, unagi has a richer, more intense umami flavor. The common opinion is that anago, because of its lighter taste, harmonises better with soured sushi rice and is therefore the preferred choice for making nigiri sushi.
As a side note, while nigiri sushi with eel is common in the Kansai region, eel is not used as a sushi topping in Edomae sushi. Therefore, Edomae sushi restaurants do not serve unagi nitsume.
In summary, unagi (eel) sauce is made by eel restaurants, while anago (conger eel) nitsume is made by sushi restaurants. Eel sauce does not contain eel components, while conger eel nitsume contains conger eel components.
Sujime (酢締め) is a traditional preparation method in which fish is first salted to draw out moisture (Shiojime) and then lightly marinated in vinegar. It is widely used as a sushi topping.
However, the techniques used for Sujime today differ significantly from those of the Edo period. Differences include the amount of time the fish is left with salt and the duration it is soaked in vinegar. Until around the 1980s, it was common practice to soak fish such as mackerel and horse mackerel in vinegar for an extended period. In modern times, thanks to advanced refrigeration technology, fish such as horse mackerel (aji) can now be eaten raw without concern. As a result, the time the fish is salted has been shortened to just a few minutes. Once moisture begins to appear on the surface, it is carefully rinsed off with water. Rather than soaking in vinegar, the fish is quickly dipped, more like a gentle rinse. The goal is to retain the natural flavor of the fish, requiring a delicate and refined touch.
Fish suitable for Sujime are those with a robust umami flavor and a distinctive aroma that can stand up to the acidity of vinegar. Typical examples include blue-backed fish like mackerel, Spanish mackerel, sardines, and gizzard shad. Depending on the case, white-fleshed fish such as barracuda, red seabream, and filefish may also be prepared as Sujime.
There are three main reasons why Sujime is used in sushi restaurants.
The first is preservation. Vinegar has strong antibacterial properties, and before the advent of refrigeration, Sujime was a way to prevent spoilage and extend the shelf life of fish. Vinegar kills bacteria on the surface of the fish, helping to keep it hygienic. Even today, in sushi where raw consumption is the norm, Sujime remains valuable for enhancing food safety.
Second, Sujime plays a key role in reducing unpleasant odors and refining flavor. Blue-backed fish are rich in fat, particularly unsaturated fatty acids found between the skin and flesh. These fats are prone to oxidation, which can cause a strong fishy smell. Additionally, the flesh of fish contains trimethylamine oxide, a naturally odorless compound that, over time and through bacterial action, turns into trimethylamine, which has a strong odor. Eventually, ammonia is also produced, intensifying the smell. Since Trimethylamine is alkaline, treating the fish with acidic vinegar neutralizes it, effectively reducing odor. A quick dip in vinegar removes unwanted smells and balances the umami and acidity, resulting in a more refined taste.
The vinegar causes the proteins in the fish to denature.
Finally, Sujime enhances both the flavor and texture of the fish.
When vinegar is added to fish, the proteins are denatured. The pH of fresh raw fish is around 6, but when it is into vinegar and the pH becomes more acidic. The gaps between the myofibrils decrease because of vinegar, tightening the flesh. This creates a texture and mouthfeel distinct from raw fish.
As the pH continues to drop below 4, the myofibril protein starts to dissolve in the acid, so the flesh becomes soft and mushy. But if the fish is salted and tightened before putting it in vinegar, the myofibrils do not dissolve and remain intact, so the flesh continues to tighten. This is why vinegared- marinated fish looks white.
Furthermore, the acidity of vinegar stimulates the taste buds on the tongue, creating a synergistic effect that brings out sweetness and umami. Even with a short vinegar marinade, a mature aroma and depth are added, giving blue-backed fish such as kohada and mackerel a deep, “mature flavor.”
Sujime is not merely a preservation technique; it is a skilled process that brings out the best qualities of the fish. It is one of the key steps in refining the sushi experience to perfection.
Namero (なめろう) is a traditional local dish from the coastal areas of the Bōsō Peninsula in Chiba Prefecture, which faces the Pacific Ocean.
This region has long been blessed with an abundance of fresh seafood, especially horse mackerel (aji), available year-round.
Originating as a fisherman’s meal, namero was devised to be easily prepared on the unsteady decks of fishing boats, making the most of freshly caught fish.
Main Ingredients and Preparation
The basic recipe involves finely chopping horse mackerel together with miso, green onions, ginger, and sometimes shiso leaves,
then mincing everything together until it becomes sticky and paste-like.
While horse mackerel is commonly used, other blue-backed fish such as sardines, pacific saury, flying fish, yellowtail, and even squid or bonito
are also popular depending on the season and locality.
The use of miso instead of soy sauce has a practical origin—it’s less likely to spill on a rocking boat.
The miso also helps neutralize the strong aroma of oily fish, enhancing only their natural umami flavors.
It is said the name “namero” comes from the phrase “so delicious you’ll want to lick the plate clean.”
Another theory is that the sticky texture makes it hard to scrape off the plate, so you’d have to “lick” it off to finish it.
Misconceptions and Variations
Namero is sometimes confused with “tataki (タタキ),” another chopped fish dish, but they are quite different.
Tataki is usually lightly chopped and often uses soy sauce and vinegar, whereas namero is more thoroughly minced and flavored with miso,
creating a smoother and more cohesive texture.
Over time, regional variations and creative adaptations of namero have emerged.
One notable example is sanga-yaki (山家焼き), a grilled version of namero.
Legend has it that fishermen used to pack leftover namero into abalone shells and grill it in mountain huts while working inland—hence the name “sanga,” meaning “mountain house.”
Another beloved version is “mago-cha (まご茶)” (literally, ‘grandchild tea’), a rustic ochazuke-style dish where namero is placed on hot rice and doused with warm dashi broth.
The name has two possible origins: one being that the dish could be eaten quickly without hesitation (“mago-mago” meaning to fumble),
and the other being that it’s so tasty you’d want to share it with your grandchildren.
The addition of dashi slightly cooks the fish, offering a delightful change in texture and flavor.
Conclusion
Namero is a dish that truly captures the culinary heritage of coastal Chiba, transforming simple, fresh fish into a flavorful and nutritious meal.
With its wide adaptability, regional roots, and deep umami taste, it continues to charm both locals and visitors alike.
Today, it is served as a tsumami (appetizer) in izakayas and sushi restaurants.
Whether enjoyed raw, grilled, or as a comforting bowl of ochazuke, namero remains a timeless symbol of Japan’s rich seafood culture.
Arai is a traditional Japanese sashimi technique in which raw seafood is rinsed in cold or ice water. Also known as “洗鱠” or “洗魚”, this method is especially popular during the summer months for its refreshing taste and texture.
How Arai Is Prepared
In this technique, thinly sliced or shredded fish is placed in a strainer set over a bowl of ice water and washed until the flesh firms and warps. Some chefs also use running water for the rinsing process. The purpose of this rapid chilling is to stimulate the muscle fibers, causing them to contract and tighten, resulting in a firmer texture and improved mouthfeel.
Benefits of the Arai Method
Washing the fish removes surface proteins and lipids, effectively reducing unwanted odors—often caused by the oxidation of fats—and producing a cleaner, lighter taste. Arai is especially well-suited for seafood that has a strong aroma, high fat content, or soft flesh.
Common Seafood Used in Arai
Freshwater fish: Carp (koi) is the most well-known example.
Shrimp: Live kuruma shrimp are sometimes rinsed instead of served raw, offering a firmer texture than odorigui (live sashimi).
Saltwater fish: Japanese sea bass (suzuki), black sea bream (kurodai), and redspotted grouper (akou or kijihata) are commonly prepared as arai to remove their pungent coastal aroma (known as iso-kusasa).
Historical Context
During Japan’s period of rapid economic growth in the postwar Showa era, marine pollution led to concerns about fish having a “petroleum-like” odor. Suzuki was especially affected and often prepared as arai to mask the smell. This historical context helped establish Suzuki no Arai as a quintessential seasonal summer dish in Japanese cuisine.
Freshness Is Essential
Fish used for arai must be extremely fresh and ideally still alive at the time of preparation. After death, fish rapidly lose ATP (adenosine triphosphate), the energy molecule responsible for muscle contraction. Once ATP is depleted, rinsing in cold water no longer causes the muscle fibers to contract, rendering the technique ineffective. The act of washing forcibly removes ATP and induces contraction, resulting in a firmer texture.
A Seasonal Delicacy
The brief rinsing also helps wash away surface fat, balancing the richness of fatty fish and resulting in a crisp, clean flavor. This quality makes arai not only a method of preparation but also an expression of culinary seasonality and refinement, offering a moment of cool relief during Japan’s hot summer months.
Tataki refers to a Japanese cooking method or the dish itself. Among its many variations, Katsuo no Tataki (seared bonito) is by far the most well-known. While the term tataki is also used for dishes made with other ingredients—like meat or vegetables—the preparation method and meaning can vary slightly depending on the context.
The appearance of Katsuo tataki
A Specialty of Kochi Prefecture
Katsuo (bonito), the official fish of Kochi Prefecture, is a beloved local staple. The origin of Katsuo no Tataki is said to lie in the humble meals eaten by fishermen aboard their boats. In times before refrigeration, they developed this method to make slightly aged bonito palatable. Lightly searing the fish helped reduce its fishy odor and enhanced its flavor.
Today, tataki has become a regional specialty. In Kochi, it’s not limited to bonito—local versions include moray eel, meats, shiitake mushrooms, pumpkin, and eggplant, all prepared in a similar style.
How It’s Made—and Why It’s So Good
The key to Katsuo no Tataki lies in its bold yet balanced preparation. Fresh bonito is scaled and filleted into five pieces, then quickly seared on the outside—traditionally over a strong flame fueled by straw. This high-heat method chars the skin, adding a smoky aroma, while keeping the inside rare and tender, like sashimi. After searing, the fish is rapidly cooled in ice water to stop the cooking and firm up the texture.
It’s then sliced and served generously topped with aromatic condiments like green onions, grated ginger, or shiso leaves. Ponzu, a citrus-based soy sauce, is often poured over the top. The contrast between the crisp, smoky exterior and the soft, raw interior creates a mouthwatering harmony of flavors and textures.
“Shio Tataki”: A Kochi Original
The appearance of shio tataki
In Kochi, there’s also a beloved variation called “Shio Tataki” (salt tataki). Instead of ponzu, the seared bonito is simply seasoned with salt—but what truly defines this style is the generous use of raw garlic slices. Typically, two or three slices are placed on each piece of fish. The bold combination of salt and garlic enhances the natural flavor of the bonito, making it incredibly delicious. Garlic also provides antibacterial benefits, which adds a functional layer to its use as a condiment.
The Meaning of “Tataki”
Many people assume that Tataki refers only to fish that’s been seared with straw, but the term originally comes from the act of “tataku” (to pound or slap). In earlier times, when seasonings like soy sauce and salt were expensive, fishermen’s families would rub a small amount into their hands and pound it into the fish to make the flavor penetrate more deeply. That technique—literally pounding the seasoning in—is where the name Tataki comes from.
Even today, after searing and chilling the fish, it’s firm and tightly textured, making it difficult for sauces to soak in. To solve this, the fish is gently pounded with a knife or by hand to help the seasoning absorb. This step is essential and preserves the original spirit of tataki preparation.
Why Straw-Firing Matters
Why is straw preferred over wood for searing? Straw contains natural oils, which produce an intense flame when burned. This allows the fish’s surface to be seared in just seconds, keeping the inside rare—perfect for tataki. Moreover, the fragrant smoke from the straw infuses the fish with a uniquely appealing aroma. The high heat also helps remove excess moisture, concentrating the flavor and improving the texture.
In addition to flavor, searing the surface plays a practical role: it reduces the risk of food poisoning or parasites, making the dish safer to eat.
Have you noticed an increase in plastic bottles washing up on the beach?
Marine Plastic Pollution
Plastic waste is one of the most concerning types of marine debris due to its harmful impact on marine life. Although plastic can physically break down into smaller fragments in the environment, it is rarely decomposed by organisms. Its degradation through processes like thermal oxidation is extremely slow, allowing it to persist in the environment for hundreds or even thousands of years.
Plastic waste in the ocean causes direct harm to marine animals. Seabirds and sea turtles may ingest plastic, mistaking it for food, and marine animals often become entangled in abandoned or lost fishing lines and nets, leading to injury or death. Additionally, discarded fishing gear such as crab traps can continue to catch marine creatures even after being lost—a phenomenon known as “ghost fishing.” In these cases, trapped animals die and become bait, attracting more victims and creating an ongoing cycle of capture and death.
Indirectly, the toxic chemicals contained in plastics pose another threat. When marine organisms ingest plastic, these chemicals can accumulate in their bodies and potentially enter the human food chain, raising concerns about impacts on human health.
Why are Microplastics a problem?
The appearance of Microplastics
In addition to the harm caused by larger plastic waste, microplastics—plastic particles smaller than 5 mm—have become a growing concern due to their potential impacts on wildlife, ecosystems, and human health.
Microplastics can be divided into two types. Primary microplastics include microbeads used in products like facial cleansers and toothpaste, as well as resin pellets (the raw materials for plastic production) that are accidentally released into the environment. In recent years, many countries have taken steps to address these risks by banning the use of microbeads in cosmetics and tightening regulations on resin pellets.
However, a more serious long-term concern is posed by secondary microplastics, which are formed when larger plastic items degrade in the environment. Plastic waste that enters rivers, coastal areas, or the ocean gradually breaks down due to factors such as temperature fluctuations (thermal oxidation) and sunlight (photo-oxidation). This process causes the plastic to fragment into tiny particles, which are then dispersed throughout the marine environment by tides and waves. These microplastics pose several major problems.
A study conducted in 1997 in the northeastern Pacific Ocean, between Hawaii and the west coast of the United States, found approximately 330,000 microplastic particles (5 mm or smaller) per square kilometer. This was reported to be more than the amount of plankton collected in the same area, suggesting that fish feeding on plankton may actually be ingesting more plastic than food.
Plastics often contain petroleum-based additives—such as antioxidants, antistatic agents, plasticizers, and flame retardants—to preserve or enhance their properties. As plastics degrade, microplastics can release these additives into the marine environment.
For example, nonylphenol, a chemical derived from antioxidants, has been detected in plastics. Nonylphenol is known as an endocrine disruptor—or environmental hormone—and is linked to potential health risks such as breast cancer, endometriosis, and decreased fertility.
Another additive, brominated diphenyl ether, is used as a flame retardant. It is highly lipophilic and bioaccumulative. While its acute toxicity is considered low, certain high-concentration anti-fogging experiments suggest it may disrupt thyroid hormone function and affect sperm production.
Moreover, persistent organic pollutants (POPs)—such as PCBs, DDT, and dioxins—previously released into the ocean continue to circulate through the marine environment. These substances are resistant to degradation and are known to cause serious health problems, including cancer.
Since plastics are petroleum-based, microplastics in the ocean can adsorb fat-soluble or chemically reactive pollutants like PCBs from seawater or the seafloor. This has been confirmed by actual chemical analyses.
Additionally, while microplastics are commonly collected using plankton nets with mesh sizes of around 0.33 mm, researchers believe that a much larger quantity of smaller, nano-sized microplastics passes through these nets undetected.
These tiny particles pose serious problems in the ocean because their small size makes them virtually impossible to recover and highly resistant to further breakdown. Once released into the marine environment, there is currently no effective way to stop marine organisms from ingesting them or to mitigate the harmful effects of the chemicals they contain. This makes nano-sized microplastics one of the most urgent environmental concerns.
The Impact of Microplastics Ingested by Fish on Human Health
Concerns have been raised about the impact of microplastics on fish—the first marine organisms to ingest them—and how this may, in turn, affect human health through seafood consumption.
Regarding fish, a report published in the Marine Pollution Bulletin in November 2022 found that additives in microplastics can transfer to the muscles and livers of fish through the food chain. However, the specific adverse effects on fish health remain unclear. In contrast, orcas—top predators in the marine food web—have shown more concerning results. In some individuals from the western North Atlantic population, which mainly preys on other marine mammals, levels of persistent organic pollutants (POPs) in their blubber were found to be more than twice the threshold associated with a high risk of reproductive disorders.
Although the direct effects of ingesting microplastics are still uncertain, these findings suggest that POPs may bioaccumulate through prey species. There are also concerns about further bioaccumulation in orca calves via breast milk and the potential for reproductive effects. However, there is currently no conclusive evidence linking POPs to declining orca reproduction rates. Given the many unknowns surrounding the biological impacts of these chemicals, a precautionary approach is warranted.
As for human health, Professor Hideshige Takada of Tokyo University of Agriculture and Technology, a leading expert on microplastic pollution, notes that microplastics themselves are not retained in the human body but are excreted. He also points out that we are exposed to greater amounts of harmful substances through drinking water and food than through microplastics, suggesting that there is no immediate cause for concern.
Nevertheless, research on the effects of microplastics on fish populations and human health remains limited. Meanwhile, the amount of plastic waste entering the ocean is projected to continue rising globally, making the outlook far from optimistic. Future studies—including in-house biological administration tests and marine surveys to evaluate how microplastics accumulate in organisms—will be essential for assessing potential health risks.
For instance, similar to current guidelines from Japan’s Ministry of Health, Labour and Welfare advising pregnant women to limit tuna consumption due to methylmercury, we may one day see recommendations to limit consumption of seafood likely to contain microplastics—such as a weekly cap on salted mackerel intake. Of course, we hope such measures will never become necessary.
Policy Initiatives
1. Status of Negotiations on International Treaties
Following a resolution adopted at the United Nations Environment Assembly (UNEA) in 2022, efforts are underway to establish a legally binding international treaty to address plastic pollution.
Although the initial goal was to finalize an agreement by the end of 2024, no consensus was reached at the fifth session of the Intergovernmental Negotiating Committee (INC-5), held in Busan, South Korea, from November 25 to December 1, 2024. The main point of contention was whether to include targets for reducing plastic production.
Notably, the High Ambition Coalition (HAC)—a group of over 100 countries, including the European Union (EU) and various island nations—advocated for international regulations that address plastic production. However, this proposal faced strong opposition from some countries, particularly oil-producing nations.
As a result, negotiations will continue. The next session, INC-5.2, is scheduled to take place in Geneva, Switzerland, from August 5 to 14, 2025.
2. Efforts by the European Union (EU)
In 2019, the EU adopted the Single-Use Plastics Directive, which bans certain plastic products and imposes strict controls on the intentional use of microplastics.
Further action was taken in October 2023 with the adoption of EU Regulation 2023/2055, under the REACH Regulation framework. This regulation restricts the sale of products containing intentionally added microplastics.
Starting October 17, 2025, suppliers of microplastics for industrial use will be required to provide specific instructions for their use and disposal.
To support compliance, the EU published an implementation guide for the REACH microplastics restrictions in April 2025. This guide provides detailed explanations to assist businesses, regulators, and the general public in understanding and applying the new requirements.
3. Initiatives in Japan In Japan, efforts to address plastic pollution are guided by the Plastic Resource Circulation Strategy, established in 2019. This strategy promotes the “3R + Renewable” concept—adding the use of renewable resources to the traditional principles of Reduce, Reuse, and Recycle.
Based on this strategy, the Plastic Resource Circulation Promotion Act was enacted in 2022. It requires businesses and local governments to sort, collect, and recycle plastic waste. As of May 2025, certifications of business plans for resource recycling and voluntary plastic collection continue under this law, strengthening efforts to build a circular plastic economy.
Additionally, the Ministry of the Environment released the 2024 edition of “Good Practices for Reducing Microplastics”, highlighting leading initiatives by Japanese companies in areas such as microplastic reduction, leakage prevention, alternative materials, and collection technology development.
These policy efforts reflect concrete progress toward the goals outlined in the Osaka Blue Ocean Vision, presented at the 2019 G20 Osaka Summit, and the G7 Hiroshima Summit’s 2023 commitment to achieve zero additional plastic pollution by 2040.
Summary
Once microplastics enter the ocean, they are extremely difficult to remove and degrade very slowly. Therefore, it is essential to focus on preventing their generation in the first place. Practical measures include properly managing plastic products in our daily lives, regularly cleaning out fiber debris from washing machine filters, and collecting plastic waste from rivers and coastlines before it breaks down into microplastics.
In addition, more fundamental solutions are needed, such as reducing overall plastic use, promoting biodegradable plastics, and developing alternative materials and products to replace conventional plastics. By implementing these comprehensive strategies, we can significantly reduce the environmental impact of microplastics.
