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Editor's Note: This story was originally printed in the March 2008 issue of Scientific American.
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.”
Like wolves, bluefins often hunt in packs, forming a high-speed parabola that concentrates the prey, making it easier for the hunters to close in. Tuna are metabolically adapted for high-speed chases, but as opportunistic (and by necessity, compulsive) feeders, they will eat whatever presents itself, whether it is fast-swimming mackerel, bottom-dwelling flounder or sedentary sponge. A study of the stomach contents of New England bluefins by Bradford Chase of the Massachusetts Division of Marine Fisheries revealed that the predominant food item, by weight, was Atlantic herring, followed by sand lance, bluefish and miscellaneous squid. (Other prey included butterfish, silver hake, windowpane flounder, winter flounder, menhaden, sea horses, cod, plaice, pollack, filefish, halfbeak, sculpin, spiny dogfish, skate, octopus, shrimp, lobster, crab, salp and sponges.) Tuna will eat anything they can catch, and they can catch almost anything that swims (or floats, crawls or just sits on the bottom). By and large, they hunt by vision.
From Horse Mackerel to Sushi
The bluefin was not always considered a delicacy. In the early 1900s the fish was known as “horse mackerel,” and its red, strong-flavored flesh was considered suitable fare only for dogs and cats. Nevertheless, big-game fishers off New Jersey and Nova Scotia targeted the bluefin because these powerful fish were considered worthy opponents. Zane Grey, the popular author of Western novels such as Riders of the Purple Sage, invested most of his not inconsiderable royalties (his books sold more than 13 million copies) on fishing gear, boats and travel to exotic locales in search of tuna, swordfish and marlin. Although swordfish were certainly considered edible, tuna and marlin were thought of as strictly objects of the hunt. The bluefin did not become valuable as a food fish until the latter half of the 20th century, when sushi began to appear on menus around the globe.