Tuna are large fish (some can weigh more than 600 kilos), very greedy, excellent swimmers and predators that can reach speeds up to 90 km/h, but not always, when are eggs these are floating and are at the mercy of the currents of the Mediterranean, where they find the best conditions for their development, grow rapidly, its size ranges between 1 and 1.2 mm, these larvae hatch within 2 days growing up to 4 mm in size, almost in two weeks are shaped like little fish with all the well-developed structures and in a month they reach metamorphosis, after which they become a little tuna adults. The maturation lasts at least a whole year, until young of 1 to 3 years migrate out of the Mediterranean, then the juveniles remain in the Atlantic where they can best meet their energy needs. From that time to join adults takes about 5 years to reach sexual maturity (15-30kg / 1m), with the purpose of contributing in breeding migration.
Every year the tuna fulfil their biological and migratory cycle characterized by a constant offset, influenced by seasonal changes, from the cold waters of the Norwegian coast to the Mediterranean warm as they do not support large temperature changes and seek water with temperatures above 10 °C. When they arrive to Spain, looking clean or low salinity and the influence of the full moon waters, they are driven by the need to spawning, with the wild instinct to preserve their species.
During this trip they go together leading head by the older one, which reach up to 3 meters long and more than 700 kilograms, and after them come younger and smaller individuals. These predators need to eat a lot in their inter-player and a fish of 2 meters need to eat 60 kg daily, for which if necessary travel 100 km. to satisfy their appetite.
The "right tuna" coming in May and June, is a full and glorious tuna, tight and tasty meat summarizing vigorous blood fat and juicy. During this stage, dominated by the instinct of reproduction, tuna they have no other concern than reaching its place to start the spawn. However, there are some other fish that when they start their migration process, they stop feeding themselves completely, it is not the case of the tuna, which continues to eat but does not deviate from its path and eat only what is in its given route. It is what is known as "right emigration".
The "back tuna," lean and weary from the start, is dried meat and unappreciated gastronomically, are out in July looking for westward Atlantic waters. In this short period of time they stop feeding absolutely, losing up to 35% of their original weight.
Those are some of the reasons why the tuna has a fluctuation hefty price during the different migrant seasons, these are between 15 and 45 € per kg depending, of course, is also well known that when the tuna returns with low quality after spawning with no fat and no flavor, we just have to wait for the season, these are price factor that affects the binding.
There are several record sales, mostly in Japan, the latter figure in the biggest fish market of the world, the Tsukiji market, was for 9.75 million yen (about 70,000 euros), the second highest price recorded in Japan. The piece of 325 kilograms was caught in Tsugaru, between Honshu and Hokkaido Islands, and came from the port of Oma, in Aomori province.
Parts of the tuna:
Many sushi lovers are use to call Maguro to the tuna for sushi and they mention it in a sushi bar like "experts" in the field, however, is rather a generic word, some Japanese or specialized suppliers use although the term akami, which literally means red meat and may be heard much better if they want to show off.
If we take a look closely at a good Itamae or master sushi chef, we can realizing that he cuts the tuna in a whole different way than other fish, he cuts it into five parts, which in Japanese is known as oroshi gomai, is done, mostly because of its large size and with the clear purpose of being able to get a higher productivity, that's a custom and a method or procedure that has passed from generation to generation over time.
These parts has specific names, the upper back is divided into three, se-kami (front), se-naka (central part) se-shimo (back), as for the lower splits ha-rakami in ha-ranaka and hara-shimi. The word SE, means back, and the word Hara, belly. These cuts are also divided, but is better to leave it here.
The corresponding fillets to the sides are cut longitudinally into two halves, the top we all know as back and the bottom or belly meat which is the famous part called Toro, stands out as a curiosity, long time ago this part of the tuna was considered cat food, the time has gained ground, especially in the Western world due to its high fat content makes it more palatable to our taste, hence it is very much in demand and consequently unpayable for many sushi lovers, which it can be divided into otoro the most part pale fat and less oily chutoro more pink.
Now every specific cut is used for a certain type of sushi, it is advisable to use the back for cuts of sashimi and hosomaki, which in this case would be tekkamaki.
You could visit the National Ocean Tuna Fishery Association (Japan) for more information...