You need to go to a good fishmonger and request 'sushi grade' tuna. It will cost more than tuna you buy for cooking. Unless you have been trained on identifying sushi grade fish and know what parasites and imperfections to look for, leave it to the professionals. Another good source is to ask your local sushi shop where they buy their fish from.
Tuna Sushi
http://www.sushifaq.com/sushi-grade-fish.htm I did a little quick googling. According to the site above, sushi grade tuna has been frozen at a particular temperature and for a particular period of time. This apparently kills any microbes.
The best answer I've found so far, was from when I worked in restaurants. The sushi we served was always fresh, real, and caught by local fisherman from around the world. Alas, all sushi is not made the same, I was told that the "block" sushi I saw at lower grade sushi establishments was made from the loose bits of tuna that are pasturized and molded into blocks. By the looks of it, this is what I suppose saku tuna is Edit: As far as I know Saku Tuna is yellowfined tuna that is labeled with Maguro, most often only used for labeling bluefin tuna. So you can call the tuna Maguro Tuna-Saku. I suppose it is a way to sell the cheaper yellowfin tuna to a higher price.
Many countries go fishing for tuna. For example, Japan fishes lots of tuna for use in sushi, which is a popular type of food there.
I like tuna sushi with cutting usa zebras I like tuna sushi with cutting usa zebras
It's something in every restaurant called SEAFOOD! more specific, sushi, fish, crab,lobster and TUNA!!!! I LOVE TUNA!!!!!!!!!!!!!
Sushi...... I love sushi ...lov u ray ray
Because there are basically two types of what we westerners think of as 'tuna' in sushi, the lean cut of the fish, called 'akami' and the fattier part of the fish, cut from the belly region, called 'toro.' Tori is also graded up by the degree of fattiness, and can be referred to as 'toro,' 'chu-toro,' or 'o-toro.' More on this can be found at The Sushi FAQ at http://www.sushifaq.com/sushi-items/sushi-items-tuna-maguro.htm"
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Even though MahiMahi are native to both the Atlantic and Pacific Oceans, I have yet seen them as a choice in any Japanese restaurants. A certified Sushi Chef (in Japan, a sushi chef must be certified) would have to deem them edible as raw fish before they can become used 'officially' as Sushi or Sashimi.