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.
Reaching a maximum known weight close to three quarters of a ton and a length of four meters, the bluefin is a massive hunk of superheated muscle that cleaves the water by flicking its scimitar-shaped tail. Whereas most of the approximately 20,000 fish species are cold-blooded, possessing a body temperature the same as that of the water in which they swim, the bluefin is one of the few warm-blooded fishes. During a dive to one kilometer below the surface, where the ambient water temperature can be five degrees Celsius (41 degrees Fahrenheit), the bluefin can maintain a body temperature of 27 degrees C (81 degrees F), close to that of a mammal. The bluefin is also among the fastest of all fishes, capable of speeds up to 80 kilometers per hour and able to migrate across entire oceans. It is such a marvelous swimmer that when scientists in the 1990s endeavored to build a mechanical fish, they used the species as a model, designing a robot with a tapered, bullet-shaped body and a rigid, quarter-moon tail fin [see “An Efficient Swimming Machine,” by Michael S. Triantafyllou and George S. Triantafyllou; Scientific American, March 1995]. The researchers found that the tail’s efficiency lay in the interaction of the vortices created by its rapid flexing, but the hydrodynamics of their electronic models did not even come close to that of a true bluefin. “The more sophisticated our robotic-tuna designs become,” the Triantafyllou brothers wrote, “the more admiration we have for the flesh-and-blood model.”