TUNA
Tuna (Thunnus thynnus) is a relative to mackerel, although much bigger, and it has a body that is similarly shaped, suited for fast swimming. The head is small, the body is slender, and the fins can even be deflected so to reduce the water resistance. The color of the tuna is characteristic, it has a blue and black back and silver belly and sides, with some species being colored all around the body. Tuna fish can be very big, it can weigh up to a ton (that's 1000 kg) and can be very long, up to 5 meters/16 feet. Nowadays though, such a big tuna is a rare catch, although 1 meter/3 feet catches are not uncommon. Tuna is a ferocious predator, it eats other fish and whatever comes in its way.
See a video of tuna fish swimming in water:
http://www.youtube.com/watch?v=f_4rhfFO69k
There are several variants of tuna and they are all eaten around the world. In Japan, the Bluefin tuna (southern or northern) is praised and used for sushi and sashimi. Albacore and Skipjack tuna are often cooked and canned in oil or water. Other tuna species include Yellowfin tuna, Bigeye, Bonito, and Tongol.
The typical behavior of the tuna is that it has to travel long distances, and it does so together with its own brethrens. It tends to swim looking for food, and approaches the coasts during the summers. Tuna is one of the few fish species that is able to keep a constant body temperature, and it can spend lots of time in colder waters, although it seems to prefer warmer conditions. It is in the heat of the summer months that new tuna fish are spawning, and they grow pretty quickly in size. One unique feature of the tuna, different from most other fish, is that its flesh is red and it tastes a little like a fine beef due to its tenderness. The red color comes from high levels of oxygen capturing molecule called myoglobin, which is important for supplying the fish with oxygen at deeper levels of water. Therefore it also has a good amount of B vitamins, and it also carries lots of omega-3 oil that is good for your health.
There is a common say that tuna should be not eaten because the dolphins are caught in the nets along with them. The fact is true - tuna fish often swim along with the dolphins, although some tunas like albacore and skipjack do NOT. The reason that tuna like dolphins so much is that they offer them more protection from sharks. Anyway, the catching procedure that uses nets will therefore kill some dolphins as well as tunas, and some people are concerned about it. Lately, some companies have developed a different catching method that is more laborious but dolphin safe. However, even if the label on the fish packet says 'dolphin safe' one has to know that the catching procedure has probably not been inspected. Obviously I could also put such a label on my tuna and who would tell whether it's genuine?
In cooking, tuna is best when it's fresh, canned tuna is... well, not really haute cuisine, but you can naturally eat it if you like it. Tuna is perfect for grilling, but it can also be cooked using other methods. It can also be eaten raw. A common way to cook tuna is to brown it on the outside and leave the inner part red.
Tuna is good with olive and sesame oil, as well as sesame seeds and soy sauce - this is a common way to go. You can serve it with tomatoes, onions, lemon, garlic, vinaigrette and vinegar, or leeks. Common add-ons include mayonnaise, aioli, or avocado. From the herbs and seasoning you can try mint, thyme, saffron, black pepper, parsley, chives, dill, and anchovies or sardines. Experiment and enjoy!