There are seven species of oceanic fish within the genus Thunnus: Northern bluefin, albacore, yellowfin, southern bluefin, bigeye, blackfin and longtail. They range from moderate to very large in size. Northern bluefin, which can live up to 40 years, are the largest tuna, up to 15 feet in length and 1,800 pounds in weight. Built for speed, bluefins are shaped like torpedos with retractable fins and eyes set flush with their bodies. These streamlined predators can dive more than 4,000 feet, swim 25 miles per hour and cross the ocean in 21 days.
Bluefins are classified as an endangered species by the World Wildlife Foundation. Overfishing and illegal fishing have caused bluefin populations to decline severely in recent decades, due largely to the demand at high end Japanese sushi markets where a single giant Atlantic bluefin can sell for $100,000 or more. The situation is worsened by the fact that bluefins are slow to mature and many fisheries are catching young fish that have not reproduced. While scientists and environmental organizations have called for a moratorium on the harvesting of this species, to date no ban has been implemented. And because the fish are highly migratory, an effective management plan would have to have multinational cooperation (including Japan, Mexico and the United States) in order to succeed. As a result, The Monterey Bay Aquarium’s Seafood Watch advises cooks to avoid bluefin tuna. They counsel that albacore or skipjack tuna (“maguro” in sushi) caught in the Pacific Ocean are the best choices.
Whatever tuna you select must be absolutely fresh (often labeled ahi, or sushi-quality at the fish market) for the following recipes because, while the fish is seared on the outside, it remains essentially raw inside. The slightest fishy taste or smell, acceptable (or at least less noticeable) in cooked fish, will be offensive in fish eaten raw. That is the reason Japanese restaurants pay twice what other restaurants pay for fish – it must be utterly fresh. You also will notice that several of these recipes suggest a rub with a small amount of sugar before the steaks go into the pan. The sugar caramelizes quickly and will create grill marks despite the short cooking time, allowing you to have your exterior sear while maintaining your interior sashimi.