All tuna are not alike. The canned tuna fish in sandwiches and salads comes from either skipjack, a meter-long species that is caught in prodigious quantities around the world and served as “light meat tuna,” or albacore, another small fish that is marketed as “white meat tuna.” The yellowfin and the bigeye tuna are larger species that are also heavily fished, but neither makes for particularly wonderful sushi, and they are usually served grilled. But the bluefin tuna, a giant among fishes, is the premier choice for sushi and sashimi and has become the most desirable food fish in the world. As such, it has vaulted to the top of another, more insidious list: it is probably the most endangered of all large fish species. Heedless overfishing is steadily pushing the bluefin toward extinction, and the species may soon disappear unless entrepreneurial fish farmers can learn how to breed the tuna in captivity.