Tunas are a group of fish which belong to Genus Thunnus, Family Scombridae, Order Perciforms in terms of taxonomy. The genus name "Thunnus" comes from Latin, but originates from a Greek word meaning "dash". The English word "tuna" comes from "Thunnus". Tuna was already found in the Japanese oldest collection of poems in the 8th century, but it was called "Shibi" or "Hatsu at that time". When the Kamakura period started in 12th century when samurai warriors reigned over Japan, "Shibi" was avoided since the pronunciation of this word was the same as that of "the day of death". The word "maguro" was found in the mid Edo period (18th century). There are several views about the naming: one is because the eyes are black (this is called "meguro" in Japanese, later changed to "maguro"); and another is that the back is very black, which means "makkuro" in Japanese later changed to "maguro". (An explanation by Mr. Kenji Saito modified based on "Science of Tunas" written and edited by Dr. Hideki Nakano.)

Six tuna species are caught internationally by long line fisheries.(Photos are from 2021 Status of International Fisheries Resources published by the Japan Fisheries Research and Education Agency)

Pacific bluefin tuna

Large size fish are used for high-quality sushi and sashimi. Juveniles (0 to 1 year old) are called "Meji" or "Yokowa" and used mainly for sashimi and seeds for farming.

Atlantic bluefin tuna

This species migrates in the Atlantic and Mediterranean. There are two stocks: east and west. Most of them caught in the Mediterranean are transferred to farms for fattening.

Southern bluefin tuna

This species is caught in cold waters off Australia, New Zealand and South Africa. As it contains a high degree of fat, it is frequently served as high-quality tuna at sushi restaurants or Japanese-style luxury restaurants. It is also called "Indo tuna" as the Indian Ocean is one of the main fishing grounds.

Bigeye tuna

This species is caught in vast areas between 35 N and 35 S across the equator. The catch amount is relatively large among tunas. The eyes are larger compared to other tunas and called "Mebachi" meaning "big eyes" in Japanese.

Yellowfin tuna

This species is found almost in the same fishing ground as bigeye tuna. The body is slimmer than other tunas. The meat is red and the taste is simple and plain. As the color of the skin is yellowish, it is called "Kihada" meaning "yellow skin" in Japanese.

Albacore tuna

This species is a small tuna that is found all over the world and migrates greatly. Its distinguishing feature is sword-like pectoral fins. It has been used for canned tuna, but in recent years it is eaten in a sashimi style.

In the case of Pacific bluefin tuna, the length is less than 3 mm when it hatches. It grows up to 60 cm in one year. It is considered that it exceeds 2 m in 10 years (*).
(* Source: The Japan Fisheries Research and Education Agency)

It is bluefin tuna. There are two bluefin tunas: Atlantic bluefin and Pacific bluefin. These are the biggest ones among tunas and regarded as the most delicious ones for the sashimi and sushi markets. There is a record of one Atlantic bluefin measuring 304 cm and weighing 679 kg caught off Nova Scotia in Canada.

Bluefin tuna and southern bluefin tuna start migrating greatly as they grow up but basically bluefin tunas are found in the northern hemisphere while southern bluefin tuna is found in the southern hemisphere. Yellowfin and bigeye are found in the same area, but the former swims near the surface while the latter swims in the mid-depth, thereby live separately. Albacore tuna does not cross the equator. The northern hemisphere stock and the southern hemisphere stock are independent.

There is a scientific report that the swimming speed of Atlantic bluefin tuna reaches 54km per hour.（ The muscle twitch and the maximum swimming speed of giant bluefin tuna, Thunnus thynnus L. - Wardle - 1989 - Journal of Fish Biology - Wiley Online Library ）The top speed of yellowfin tuna after hooked by rod and reel fishing was estimated to be 75 km per hour.（ Measurements of Swimming Speeds of Yellowfin Tuna and Wahoo (calpoly.edu) )

Fish ordinarily absorb oxygen through their branchial respiration system. Tunas cannot do branchial pumping respiration by opening and closing the gill covers and mouth. This requires tunas to continuously swim to let fresh water flow into the mouth. If they stop swimming, they die because they cannot respire. Marlins have similar problems.

It is said that Edomae sushi, which means that a sushi chef quickly makes sushi in front of customers and serves it, started in the early 19th century. One of the popular items used in Edomae sushi was “zuke” (tunas soaked in soy source to avoid deterioration of freshness). It is considered that this is the origin of the dietary habit of eating tunas in a raw style. Only red meat of tuna was used for zuke. People started eating fatty “toro” in the 1920’s before which toro was called what even cats avoid eating. Even now sushi connoisseurs prefer red meat as the mainstream of sashimi. (prepared under the supervision of Mr. Kenji Saito)

Toro is the belly meat surrounding guts. The meat closer to the head has more fat. Bluefin tuna can be divided into toro and red meat while southern bluefin tuna contains fat more evenly over the entire body and toro can be taken from the upper body. The meat of bigeye tuna is softer and contains less fat, giving a flavor of sweetness. (prepared under the supervision of Mr. Kenji Saito)

In old times toro was not eaten as the fattiness did not fit the taste of Japanese people. The popularity of toro started rising in the mid 1960’s after the Tokyo Olympic. The generation who grew up eating fried food became adults and got accustomed to foreign cuisines. As a result, people who used to like less salty and fatty foods started feeling that toro is tasty. Instant ramens (noodles) were becoming very popular during the same period. Toro and instant ramens, which are two extreme opposites, were getting popular under the same circumstances. (prepared under the supervision of Mr. Kenji Saito)

The weight becomes 84% of the original weight when gilled, finned and gutted onboard fishing vessels. After landing, it is headed (dressed) and cut into back and belly blocks. Both blocks are further divided into right and left blocks (loins). It becomes 46% as the process eliminates bones, skins and dark meat. This means that 46 kg out of 100 kg tuna is edible. Out of this figure, 37 kg is sold as sashimi and the remainder is sold for other non-sashimi products. Out of 37 kg, toro occupies 8 to 10 kg. (prepared under the supervision of Mr. Kenji Saito)

Fat is one of the conspicuous ingredients forming the taste of tunas. According to the 8th official analysis of food ingredients, the fat amount contained in the red meat of wild bluefin tuna is 1.4 % and that in toro is 27.5%. During the wintertime, these figures rise up to 10% in red meat and 40% in toro. Since the tuna fat contains much unsaturated fatty acid, the taste is smooth and not so strong. The red meat contains inosinic acid, alanine and taurine, giving good flavor and body. Also, not so much connecting tissue and soft texture make tunas good taste. (prepared under the supervision of Mr. Kenji Saito with updated data)

100 grams of tuna red meat contains more protein than chicken, pork and beef. It also contains more potassium, which eliminates sodium from the system and thereby effectively suppresses high blood pressure, than chicken, pork and beef. Toro contains Vitamin D, which are not contained in pork and beef and helps absorption of calcium as a measure against osteoporosis. Toro also contains unsaturated fatty acids such as DHA and EPA, which purify blood and are considered to be measures against lifestyle-related illnesses.

Nakaochi is the red meat sticking to bones after tuna is cut into several blocks. Shells are used to scrape nakaochi off bones. About 1.5 kg of nakaochi can be taken from a 100kg tuna. The less amount of nakaochi means that the tuna processing technique is excellent.

Juveniles and skinny adults caught mainly by purse seine fishing are transferred to farming cages and fed for fattening. Farming is conducted for Atlantic bluefin tuna in the Mediterranean and Adriatic sea, for Pacific bluefin tuna in Mexico and Japan, and for southern bluefin tuna in Australia. In the Mediterranean, fish caught by traps are also used for farming. In Japan, fish caught by troll are also caged.