Salmon is endemic to the North Atlantic Ocean, where it is dominant in the subarctic surface zone, but is not distributed in the Southern Hemisphere. Since the same or closely related species of Bonito, Albacore, and other surface fishes and squids are distributed in the North and South Pacific, one wonders why salmon is not distributed there.

Although still in the research stage, Slender tuna (Allothunnus fallai Serventy, 1948), which has a similar diet to Pink salmon, Chum salmon, and Sockeye salmon in the North Atlantic and is analogous to the huge stocks of these species, seems to occupy the same ecological role as the plankton-eating salmon.

Slender tuna is a species of tuna, the only species in the genus Allothunnus, found around the world in the southern oceans between latitudes 20°and 50° South. It is a close relative of genus Sarda, whose Japanese names are Arotsunasu and Hosogatsuo.

It has small second dorsal and anal fins resembling a small albacore, but the slender tuna lacks the long sweeping pectoral fins characteristic of albacores. As the name implies, it’s more slender and elongated than other tuna types. It has a blue-black back and silver-to-gray sides. The pectoral and pelvic fins are purple on their distal portions and black near their bases. Its length is up to 1 meter and it can weigh up to 12 kilograms.

It is a species of minor commercial importance, taken mainly as a bycatch by fisheries for other tuna species. It is richer in fat than bonito, and its dark flesh is not suitable for sashimi. It tastes better when cooked, but is rarely used as fresh fish, and is instead used for canning.

It can pack 3700 milligrams of omega-3 polyunsaturated fatty acids per 100 grams. Next highest in omega-3s are the more readily available farmed fish, such as the striped perch, and Atlantic salmon.

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Revision date: February 24, 2023

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What is Longtail tuna (Koshinaga maguro)?

Longtail tuna inhabits continental shell and ocean waters in warm temperate and tropical regions of the Ind-west pacific. The dark blue-backed fish is recognized by their short pectoral fins and slender body. Its tail is long compared to other tuna. It is also distinguished by the presence of elongated, colorless spots on the underside and belly, between the pectoral and anal fins. As the name suggests, it is characterized by a rather long tail from the tail fins to the tail.

It reaches a maximum length of 1.5 m and up to 32 kg in weight. It is caught by longline fishing in Southeast Asia and Australia.

The Japanese name is Koshinaga maguro (腰長鮪). It is caught in small numbers in Kagoshima, Nagasaki, and Okinawa prefectures, but its numbers are small and it is the least caught species of the tuna genus, so it is traded only in its place of origin and rarely appears on the market. In addition, juvenile tuna look similar to bluefin tuna, yellowfin tuna, and albacore. Therefore, they are sometimes confused in the market.

What does Longtail tuna (Koshinaga maguro) sushi taste like?

In northern Kyushu and Sanin regions where bonito are not caught, however, it is an autumn treat. Most of its flesh is red meat, and its taste is refreshing so it is eaten as sashimi. However its fat is not sweet and has little acidity, so it is not suitable for nigiri sushi. In Australia and Southeast Asia, it is eaten as steak or sauteed.

Its scientific name is Thunnus tonggol (Bleeker, 1851).

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Revision date: February 2, 2023

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What is Blackfin tuna (Taiseiyou maguro)?

Blackfin tuna is one of the smallest members of the species of Tuna belonging to the genus called Thunnus and family of Scombridae. The English name blackfin tuna comes from its black pectoral fins and other fins. It is known by various interesting names like Blackfinned Albacore, Bermuda and football due to its typical football like appearance.

The Japanese name is Taiseiyou maguro (大西洋鮪). There is also a species called Taiseiyou Kuro-maguro (Atlantic bluefin tuna), which is very confusing. It is not distributed in the Japanese market.

The range of distribution of these tuna is restricted as they can only be spotted in the West Atlantic Ocean, encompassing the Gulf of Mexico, southern Brazil and the Caribbean Sea. This is unique for a species of Tuna to have a limited range but it probably has something to do with the Blackfin Tuna’s preference to migrate to temperate waters above 20℃. It is especially common in the Gulf of Mexico, where it is a favorite target of sports fishermen.

It can be identified by a dark-blue to black stripe across its back usually with a golden hue under it and a silver belly. It is the smallest in the Tuna family reaching lengths of about 1 m and weight of 20 kg. It is at its peak during autumn, winter, and spring in Florida Keys. Only 0.4 tons per year of blackfin tuna is caught in the world.

What does Blackfin tuna (Taiseiyou maguro) sushi taste like?

Though it doesn’t quite measure up to world-class bluefin, yellowfin or bigeye tuna in food value, it is very good in its own right. The top preparation method with fresh blackfin is undoubtedly sushi or sashimi, as it yields an incredible flavor and texture when raw, but it also excels on the grill or is seared over high heat. It also makes a delectable tuna salad.

Its scientific name is Thunnus atlanticus (Lesson, 1831).

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Revision date: February 2, 2023

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They are decidedly, color, fragrance and texture.

First of all, color does not simply refer to the color of the cross-section when the fish is cut with a butcher knife.

It refers to how long the fish maintains its “characteristic coloring”. After all, tuna darkens as time passes, eventually turning a burnt-brown color. The true, visual charm of Kuromaguro is the eye-awakening red color that catches your attention when the sushi is placed on a counter or plate. This wonderful coloring is a characteristic of tuna caught through longline fishing or single‐hook fishing. On the other hand, it is said that those caught in round haul nets don’t hold their color and don’t last long. When frozen tuna is thawed, it turns brown within a day. This focus on color is based more on Japanese restaurants that serve sashimi at tables that are located farther away from where the food is being prepared, than on sushi. Incidentally, Mebachi, despite also being tuna, holds its color longer than Kuromaguro does and is suitable for take-out sushi. Minamimaguro is a darker red color than Kuromaguro, but it also loses its color quicker. Kihada maintains its color best.

Next, where does Kuromaguro’s appealing fragrance come from?

The truth is that the source of fish flavor components is still being researched and there are a lot of unknowns. What we do know is that this fragrance is made up of many volatile compounds, but since there are only trace amounts of each one, they are difficult to analyze.

Yet, in the case of expensive Kuromaguro, every measure possible is taken to make sure this fragrance is maintained. Kuromaguro is a migratory fish that gets around by swimming at high speeds in the surface layer of the sea. It needs strong muscles to swim this fast. it also needs to circulate blood throughout its entire body in order to vigorously move those muscles. When a tuna violently struggles to resist and twists its body to avoid being caught, the proteins in its muscles (myosin and myoglobin) rapidly react with oxygen and start to degenerate. This causes Yake and the oxidized odor of tuna. It is important to catch the tuna while causing it as little stress as possible, quickly remove the organs and then use ice to rapidly cool the entire body. If this process is delayed then the pleasant fragrance will transform into an odor. In other words, the scent of Kuromaguro all depends on how it is processed after being caught.

The meat of Kuromaguro that is properly processed emits a unique aroma with a slightly acidic taste when you put it in your mouth. That fragrance lingers for a long time and it combines beautifully with the acetic acid of the sushi rice to go straight to your nose. Then, the moment the fish has disappeared down your throat, the scent of the iron and a subtle acidic taste linger very nicely. This experience is only possible with the exceptional Kuromaguro.

As for the texture, this is determined by the fat distribution of the tuna.

Especially in winter, the Harakami cross-section of Kuromaguro is marbled, much like the Ribulose of Wagyu beef. The melting point for the fat of high-quality tuna is low, and it starts to dissolve even at human skin temperature. This is why sushi chefs who are particular about the sense of unity between shari (sushi rice) and tuna, say, “Shari should be skin temperature.”

There are many chefs who say that the umami of Kuromaguro is in the fat. In the first place, Kuromaguro is one of the fattier fish, and especially between autumn and winter, the Kuromaguro that fed on Surumeika and Sanma has exceptional Harakami. The organs of both Surumeika and Sanma are rich, full-bodied and delicious, even when grilled and eaten by humans. In other words, it is the Surumeika and Sanma that the tuna feeds on that determine the quality of meat. As an example of the meat quality of tuna being affected by what that tuna fed on, Kuromaguro in the Atlantic ocean that has fed on Nishin may exhibit the scent of Nishin when made into sashimi.

However, having a high fat content is not the most important factor. For example, farmed Kuromaguro is fed a high-protein diet so that it will be fatty regardless of the season. Yet, the fat is tougher than that of the wild fish and it’s not something that leaves you wanting a lot more. In comparison, the fat of wild Kuromaguro has a fine texture throughout, immediately melts in your mouth and is digested easily. As sushi chefs know, the fresh fish seems to suction to their hands when making the nigiri sushi. In the same way, it has a smoothness that seems to suction to your tongue when you put it in your mouth. Once you’ve swallowed, it leaves you craving more.

This is the depth of wild Kuromaguro.

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When this tuna is cut into pieces, the meat is whitish, as if grilled in a fire, so it is called “Yake (焼け)” or “Yake-maguro (焼け鮪)” as “Yake” means “burnt” in this case. The meat can also be a grayish-brown. In the industry it’s known as “White tuna” due to its color. It loses its characteristic red color, the texture of the meat when eating it is flakey and it doesn’t taste good.

Even if the fish body is cooled with ice water after catching, once the meat has been “burnt”, it won’t turn back to red. This renders even the premium Pacific bluefin tuna and southern bluefin tuna worthless, with zero commercial value as food. However, the fishermen don’t just toss the dead fish back into the sea without a second thought. This is mostly processed into pet food. Also, since it can’t be determined externally whether the meat is “burnt” or not, no one can tell until the fish has been cut open. It runs a big risk to the broker and can be a cause of its fishing port brand losing credibility.

It is currently believed that burnt tuna occurs from high temperature and low pH in the muscles of the fish after death, and that there is a complicated, indirect causal relationship between the conditions such as the environmental temperature, fishing method, handling of the catch, length of processing time, type of fish and size of fish. However, the mechanism that causes this phenomenon is not clearly understood.

Related contents: TYPES OF TUNA

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There have been changes in the Pacific bluefin tuna as well. In the late 1990s, the southern bluefin tuna fish fattening method started to be used in the seas throughout the world, and this led to the fattening of the Pacific bluefin tuna, which became all the rage. Most of these are exported to Japan, and account for about half of the consumption of southern bluefin tuna and Pacific bluefin tuna in Japan.

As a bonus, tuna that has been fattened in this way has such a high volume of fat that it is said to be “all toro”, and it’s taken the Japan high-grade toro market by storm. Also, both southern bluefin tuna and Pacific bluefin tuna are served at kaiten-zushi (conveyor belt sushi) restaurants.

The fattening of Pacific bluefin tuna started on the east coast of Canada in the mid-seventies. In the summer, large volumes of Pacific bluefin tuna are caught in the fixed shore nets on the Atlantic seaboard, and since they have already spawned, they have slimmed down and aren’t worth much commercially. These fish are fed and fattened, and a new route to Japan has been developed, giving the fish new commercial value.

Both fish fattening and fish farming (養殖) mean to hold fish in fish preserves, but the purpose differs between the two. The purpose of fish fattening is an adjustment for fishing while the purpose of fish farming is to grow fish to a certain size. Therefore, in fish fattening, they are not fed food to promote growth, but they are in fish farming. However, even in fish fattening, if the period of time they are held for shipping adjustment stretches out too long, they are fed in order to prevent a decrease in meat quality and cannibalism. This can blur the line between fish fattening and fish farming quite a bit.

The Pacific bluefin tuna fish fattening started in Canada is similarly vague. In that case, the purpose was fattening, so it may be fair to call it fish farming. However, shipping adjustment was also one of the major objectives. What about the fish fattening of southern bluefin tuna and Pacific bluefin tuna in Australia and the Mediterranean Sea? These are in place clearly for the purpose of growing small fish, so we can call them fish farming.

In the Mediterranean, where resources are already declining, methods are used to catch natural tuna alive. The only mobile fishing gear that can catch tuna alive is purse seine nets.

The period when tuna are transported to the fattening farms is generally from May or June to July or September, but this varies depending on the country. They are then transferred to cages, where they are fattened for a relatively short period to meet Japanese market standards. The fattening period is usually six to seven months, because demand in the Japanese market peaks at the end of the year. In Croatia, the size of tuna transferred to cages is small, so the fattening period can be as long as 20 months.

Mediterranean coastal countries involved in tuna fattening include Algeria, Croatia, Cyprus, Greece, Libya, Malta, Spain, Tunisia, and Turkey. French tuna purse seine fishing fleets are the largest suppliers of live tuna to fattening farms in Mediterranean coastal countries.

The Kindai University Aquaculture Research Institute has succeeded in a complete farming process of taking eggs from spawning Pacific bluefin tuna, incubating them and raising adult fish. Research for a complete farming process began around 1970, so it has taken 30 years for success. These fully farmed tuna are also raised in fish preserves and fed small fish like sardines as well as artificial feed for fattening, resulting in all-toro tuna. However, even though the Kindai University tuna have been featured by the media, you never see them in the supermarket. Why is that? Even though the university has succeeded in a mass-production method, the absolute quantity is extremely low even though raising the fish takes a lot of time and effort, so the price is extremely high.

Whether they are fattened, farmed, or fished in the wild, consumers always welcome delicious, high-quality toro at a reasonable price.

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Revision date: May 28, 2025

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What exactly is drip anyway?

I’m sure you’ve seen it before in any type of thawing frozen meat, but there is a red liquid that comes out of the tuna when thawing. This is called ‘drip’. This liquid includes the tuna’s umami, and when the fish loses a lot, it naturally detracts from the flavor.

First of all, we will explain the worst thawing methods. Never thaw naturally at room temperature or in the microwave. These are common methods at home, but they are out of the question.

Next we will explain the general method of thawing.

1. Mix 30 g of salt with 1000 cc of warm water at 40℃ to create a saltwater mixture.
2. Place the frozen block of tuna in this 40℃ saltwater, submerge for one to two minutes and then drain the water.
3. Wash any remaining salt off the surface of the block of fish with fresh water and remove moisture from the surface with paper towels.
4. Wrap the fish in clean paper towels, wrap with plastic over that and leave it in the refrigerator for about a day to thaw naturally.
5. Cut from the block directly before consuming.

Now for the thawing method used by professionals, such as sushi chefs.

The first three steps are the same as the general thawing method above.

4. Place block in an air-tight plastic bag. Push out as much air as possible before sealing the bag.
5. Prepare ice water in a bowl or container and submerge the plastic bag in the water for one hour. Normally the bag will float, so it must be weighed down with something.
6. Remove the thawed tuna from the plastic bag and remove moisture with paper towels.
7. Wrap the fish in clean paper towels, wrap with plastic over that and leave it in the refrigerator for about a day to mature.

After thawing, the meat of the tuna may have shrunk. This is called ‘chijire’. The reason for this is, after the tuna is caught, it is frozen before rigor mortis begins, so the rigor mortis process starts once the fish is thawed. Therefore, this is proof of freshness. The meat of tuna for which ‘chijire’ has begun is tough and isn’t yet matured. However, amateurs can’t tell if ‘chijire’ is happening or not. That’s why it’s better to let the fish mature in the refrigerator for half a day to one day. Please use these explanations for your own reference.

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Revision date: October 2, 2020

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Why is that, you ask?

The most commonly used ingredient at kaiten-zushi is tuna. At kaiten-zushi, the tuna is imported and frozen nearly 100% of the time. The most famous is the Southern blue-fin tun, but you’ll also find Boston bluefin tuna, Canadian tuna farmed in fish preserves, inland sea tuna from Turkey and Spain, New Zealand offshore tuna, Atlantic tuna, etc. Also, the season of each type of tuna and the timing of high-volume catch differ, which makes the prices fluctuate greatly.

Therefore, the biggest task of a kaiten-zushi chain buyer is to decide where to import tuna from. Looking for high cost-performance, they watch fluctuations in the market every single day without fail, check the flavor in detail and constantly change the locality.

In other words, the chain restaurant purchaser’s efforts are concentrated on tuna. Restaurants that serve tuna that has lost its fat, is watery or rubbery, clearly either have a purchaser with a poor eye, or poor thawing skills. Therefore, if you are disappointed with that first tuna plate, then reign in your expectations for other toppings. On the other hand, if you enjoy the tuna then you’ll have a lot of other toppings to look forward to.

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Revision date: August 17, 2020

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Revision date: April 16, 2019

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Most people probably think of “Negitoro Gunkan” or “Negitoro Roll” served at sushi restaurants.

But the real “negitoro” does not have leeks!

The original name for negitoro was negitoru. It translates to chip away in Japanese. The name eventually evolved to negitoro.

Here we come to the point.

The original negitoro is made from medium fatty tuna or nakaochi* chopped up finely with a knife and then mixed with chopped green onions on top. But the tuna may be switched out with filler, leading to a variety in quality of the negitoro available.

First of all, the lowest in the ranking are the offcuts of tuna that can’t be made into sashimi (mainly Yellowfin or Albacore tuna) and this is mixed with vegetable oil and minced. The type of onion used is normally green onions. You can pick this type out because it will be whitish in color. This version is normally served at conveyor belt sushi.

The medium quality uses the nakaochi of cheap Albacore tuna or Swordfish.

High quality negitoro uses the nakaochi of Pacific bluefin tuna or Southern bluefin tuna. Sometimes the green onion shoots are then rolled up inside. If you have a chance to try negitoro in Japan, we recommend you try the top quality options without a doubt. One piece will probably cost around $15 USD. But that’s the price for the real thing!

Finally, let me introduce some negitoro trivia. There is a lot of flesh on the middle bone (spine) and the surrounding area for tuna and the like. This is called “nakaochi*”. Scraping the meat from this area surrounding the spine is known as “negitoru”, which is where the word “negitoro” comes from. In other words, the name “negitoro” is not actually from the words onion (negi) and tuna belly (toro).

What is negitoro like at Sushi restaurants?

Originally, the meat from the middle cut or in between the bones of the tuna was used for negitoro. In order to get meat from these parts, the chef would have to purchase an entire tuna, or buy the cut that includes the mid-ribs. However, both of these purchases are difficult for a single sushi restaurant, so now the chef chops the meat from the body with a butcher knife until it forms a paste that is sticky and smooth from the fat in the tuna. This paste is used for negitoro.

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Revision date: July 30, 2022

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