Prized for sushi, the fast and powerful Atlantic bluefin tuna is being relentlessly overfished.

This story appears in the March 2014 issue of National Geographic magazine.

One moment the undersea is featureless blue, an empty cathedral, the sun an undulating hot spot in the vault of waves overhead, its beams radiating down as if from stained glass. The next moment the ocean is full of giant, bomb-shaped bluefin tuna, the largest measuring 14 feet long and weighing three-quarters of a ton. In the sea’s refracted sunlight, their pale flanks flare and scintillate like polished shields. Their fixed fins—the long, curved anal fin and the second dorsal—flash like sabers. Their quick-sculling tail fins drive the formation forward at ten knots, with sprints to 25, a ceaseless, staccato beat. And just as suddenly they are gone. The ocean is empty again. Here and there a small galaxy of scales marks where a bluefin swallowed a herring. The victim’s scales swirl in the turbulence of the departed tuna, now bearing off at high speed. Then each vortex slows and stops. The sinking scales gleam like diamonds from a spilled necklace. Then they dim. Finally they wink out with depth. The true tunas, genus Thunnus, are supercharged fish, streamlined to perfection and jammed with state-of-the-art biological gear. The characteristics that distinguish the true tunas include great size, great range, efficient swimming stroke, warm bodies, large gills, finesse at thermoregulation, rapid oxygen uptake, high hemoglobin concentration, and clever physiology of the heart. All of these reach their apogee in the bluefin.

The three species of bluefin—the Atlantic, Pacific, and southern—have divided the world’s oceans among themselves, and they roam all planetary seas except the polar. The bluefin is a modern fish, yet its relationship with humanity is ancient. Japanese fishermen have caught Pacific bluefin for more than 5,000 years. The Haida of the Pacific Northwest have hunted the same species for at least as long, based on the evidence of bluefin bones in their middens. Stone Age artists painted Atlantic bluefin tuna on the walls of Sicilian caves. Iron Age fishermen—Phoenician, Carthaginian, Greek, Roman, Moroccan, Turkish—watched from promontories for the arrival of bluefin schools at their Mediterranean spawning grounds.

“Bluefin helped build Western civilization,” Stanford University professor Barbara Block, a preeminent scholar of this fish, told me. “Across all the Mediterranean, everybody netted giant tuna. The bluefin have annual migrations in through the Strait of Gibraltar, and everyone knew when they came. In the Bosporus there were 30 different words for bluefin. Everyone put out net pens that had different names in the different countries. Penning created cash. Bluefin were traded. The coins of Greece and Celtic coins, they had giant bluefin on them.”

“The king of all fish,” Ernest Hemingway reported in the Toronto Star Weekly in 1922, after seeing Atlantic bluefin off Spain. Carl Linnaeus, the father of modern scientific classification, named the Atlantic bluefin in 1758. Linnaeus often resorted to repetition in flagging superlative animals. Gulo gulo he named the wolverine, king of the weasels. Bison bison he named the bison, king of the prairie. Thunnus thynnus he named the Atlantic bluefin: tuna of tunas.

The day dawned red-orange over Cape Breton, Nova Scotia. It was cold in Port Hood, down on the village dock, but the eastern sky was encouraging, a long horizon of warm color. We cast off, and Dennis Cameron, captain of the Bay Queen IV, steered north toward the Gulf of St. Lawrence. Along the back wall of the boat’s cabin, fishing rods were racked like rifles in an armory. In the open waters ahead, fishermen haul in the biggest bluefin tuna in the world.

To starboard passed the big island of Cape Breton. To port passed a small outlier, Port Hood Island, low and green, with a scattering of white clapboard houses. Cameron grew up on Port Hood Island in one of those houses. He remembers squirrel hunting in the woods, and beachcombing for old buoys and gaffs, and collecting stranded squid as bait for his father—a vanished way of life. The big lobster cannery on the islet closed long ago. The waterfront, crowded with fishing dories in the 1920s, a forest of masts, is now deserted. Twenty-odd families of fishermen and farmers survived through the 1950s but steadily thinned, and the island now has just one full-time resident.

And so it goes in fishing communities everywhere. The oceans are dying. The collapse of fisheries marks the decline, a steady funereal drumbeat: cod in the Maritime Provinces of Canada, anchovies off Peru, salmon off the Pacific Northwest, Patagonian toothfish in Antarctic waters, sharks in all the oceans.

Bluefin tuna are among the most overfished species on Earth. The stock that spawns on the western side of the Atlantic has been reduced by 64 percent since 1970. The tonnara of Sicily—the mazes of net pens in which, for millennia, Sicilians have collected giant bluefin to kill in the ritualized climax called mattanza—have been folding one after another for decades, as have similar mazes, by different names, throughout the rest of the Mediterranean.

Cameron, like any son of a Canadian fishing family, is familiar with the vogues and vicissitudes of his profession. “We didn’t fish tuna,” he says of his father’s generation. “Tuna fishing was more of a sport. Years ago they used to call it ‘horse mackerel.’ It was cat food back then, or fertilizer.”

In January 2013 a single bluefin tuna sold in Tokyo for $1.76 million. The outrageous price was part publicity stunt, part Japanese ritual: The first tuna on the auction market each year is subject to a bidding war that’s over the top, even by Japanese standards. Yet even the normal price of one medium-size bluefin—between $10,000 and $20,000, depending on quality—is a startling measure of how much 21st-century Japanese have come to treasure maguro, bluefin sushi. It is a measure, too, of what the bluefin tuna is up against if more than a handful are to see the 22nd century.

While Cameron steered toward deep water, Steve Wilson, a Stanford University researcher who works with the Tuna Research and Conservation Center (TRCC) in Monterey, California, checked the satellite tags he hoped to implant that day. Robbie Schallert, of the bluefin conservation group Tag-a-Giant and Wilson’s colleague at TRCC, unrolled a blue padded mat just forward of the “tuna door” in the transom at the stern. The mat did not read “Welcome,” but that was the idea. We had come to tag and measure bluefin, not to kill them.

Eight miles offshore, drifting with three lines out baited with mackerel, we had a strike. Sheldon Gillis, Captain Cameron’s assistant, fought the fish. There was a taut twang each time the bluefin took out line. Twenty minutes later, a good distance off the stern, the fish made its first appearance. Gillis judged it to be about 700 pounds. He reeled in furiously each time the tuna gave him the chance, and he was sweating now despite the cool of the morning. After another 20 minutes came the loud, slapping bang of tail fin against the stern. Hoisted aboard through the tuna door, the fish lay on its side, perfectly still and enormous on the mat. Out of water, it looked like some kind of wonderful machine, biologically inspired and poured of living metal.

Wilson and his tagging team worked efficiently and fast, like a crew swarming an underwater racing machine at a pit stop. A wet black cloth went over the eyes as blindfold. A green hose went in the mouth and began pumping seawater past the gills. A roll of measuring tape flew over the fish, tossed from one man to another. The tape was laid flush against the body between the tip of the nose and the point where the tail fin forked. This measurement, the curved fork length, or CFL, in this fish was 300 centimeters, just short of ten feet. CFL is an accurate predictor of a tuna’s weight: 1,226 pounds in this case, nearly twice Gillis’s original estimate. It was the third biggest bluefin ever tagged by the team in nearly 20 years of work.

Straddling the rear of the fish, Wilson drove in a titanium dart to anchor a satellite tag just forward of the second dorsal fin. Four team members took up positions at the corners of the blue mat and lifted. Clearing the deck, the mat became a hammock. Straining under the burden of the fish, the four men walked a semicircle of tiny mincing steps, rotating the fish 180 degrees to bring it around to face the tuna door. From the scimitar of the anal fin, Schallert snipped a sliver for DNA analysis. Then the two men at the tail hoisted their end of the mat. The tuna plunged through the doorway and back into the gulf, raising a splash like a horse diving off a pier. Two flicks of its tail fin and it was gone.

On his laptop the night before, Wilson had programmed the satellite tag on this fish to pop off on June 1 of the next year. Nine months and two weeks from this day, in whatever time zone the bluefin happened to be, the tag would send an electric current through the metal pin attaching it to the leader and dart in the fish. The electrolyzed pin would begin to corrode. Within a few hours it would sever. A bulb on top of the tag is made of foam that’s incompressible and therefore buoyant at any depth. The tag would rise through the cathedral rays of the ocean toward the brightness of the vault. On breaking the surface, it would begin uploading the encoded secrets of this bluefin—its travels, its seasons, its dive patterns—to a small constellation of Argos satellites orbiting overhead.

Block runs TRCC out of Hopkins Marine Station on Cannery Row, in collaboration with the Monterey Bay Aquarium next door. After the tag pops off at its programmed time, the satellite data rises from the Atlantic, jumps the continent to California, and comes home here to Hopkins Station for interpretation. Thirty years ago science was in the dark about the movements of tunas. Since then the mysteries of their migration, one after another, have been solved by tagging technology pioneered by Block and others.

The interior of Block’s lab makes a sort of gallery. The walls and cabinet doors, plastered with charts, maps, and illustrations from scientific journals, amount to an exhibit. If it had a title, it might be called “State of the Bluefin.”

The state of the bluefin is not good. One poster, “Estimated Spawning Stock Atlantic Bluefin Tuna (1950-2008),” shows a graph of the spawning biomass of Gulf of Mexico breeders atop a similar graph for the Mediterranean breeders. Both populations are represented by lines in the shape of eels, and both eels are diving toward the bottom of their graphs. They have plunged past the dotted line representing sustainable yield and are headed for a spot where the kilotons of spawning biomass read zero.

The art on the maps is a kind of pointillism. The locations of bluefin, as reported by the many electronic tags deployed by the lab over the years, are represented as a proliferation of small circles in many colors. The maps of most interest to Block show the distribution of bluefin in relation to something called the ICCAT line.

The fisheries for the Atlantic bluefin tuna are managed by the International Commission for the Conservation of Atlantic Tunas. ICCAT stock assessment models make use of a dotted line dividing the North Atlantic vertically. Drawn in 1981, this demarcation follows the meridian at 45° west longitude and divides the western stock of Atlantic bluefin from the eastern. The lab’s pointillist maps show a curious thing. The positions of electronically tagged western bluefin, represented as reddish-orange circles, pack the Gulf of Mexico, the spawning grounds for this stock, and from there spill eastward into the Atlantic. They cross the ICCAT line with impunity and spread all the way to Portugal and Spain. The positions of tagged eastern-breeding bluefin, represented as white circles, pack the Mediterranean, spawning grounds for this stock, and from there spill westward, crossing the ICCAT line to fill the coastal waters of the United States and Canada.

The ICCAT line, the maps testify, is a fiction. Scientists once believed that each stock stayed on its own side of the ocean, but no one believes that now. Everywhere in the Atlantic, all across the feeding grounds of this species, the eastern and western stocks mix. It seems that only in their respective spawning grounds are they separate.

The fact of this mixing was well established by Block, other taggers, and DNA researchers more than a decade ago. It has yet to be incorporated in ICCAT models. The best estimates today are that around half of the bluefin caught off the eastern shores of North America originated in the Mediterranean, yet any of these tuna, if caught in the west, are still counted as fish of western origin. The ICCAT line is not just a dull management tool—it is no tool at all. The ICCAT model fails, as well, to make any allowance for illegal fishing, though studies indicate that the illegal take is large.

For most of its history, ICCAT has ignored the advice of its scientific panel, the Standing Committee on Research and Statistics (SCRS). For the eastern stock that breeds in the Mediterranean, the much larger population, ICCAT has routinely set quotas far higher than science recommends. In 2008 the SCRS issued its most alarming assessment yet on the status of the eastern stock. The actual catch, the scientists reported, was likely more than double the 28,500 metric tons of the allowable catch, and more than quadruple a level that was sustainable. They recommended closing the fishery through the main spawning period and reducing the allowable catch to 15,000 metric tons or less. ICCAT, as usual, ignored this plea.

The same year, ICCAT commissioned an independent review of its policies. The review panel, composed of eminent fisheries managers and fisheries scientists from around the world, was not gentle. It found that ICCAT stewardship of the eastern stock was an “international disgrace” and a “travesty of fisheries management.” The U.S. National Oceanic and Atmospheric Administration and the International Union for Conservation of Nature have chimed in with similar sentiments. Environmentalists joke that ICCAT stands for International Conspiracy to Catch All Tuna, but it’s no joke. It describes reality better than the official version.

The good news is that a faction of fisheries biologists believe that Atlantic bluefin populations, if allowed to rebound, could grow to five times their present size and with wise management could yield healthy quotas forever.

In 2009 Monaco, in response to the decades of mismanagement, proposed that the Atlantic bluefin be listed on Appendix I of CITES, the Convention on International Trade in Endangered Species. Such a listing would have meant an international ban on trade in bluefin, and CITES delegates from fishing nations rallied to defeat the proposal. But ICCAT had its wake-up call. That same year, for the first time, it followed scientific advice in setting quotas for eastern bluefin. In 2011, to address illegal fishing, it began testing a system that would electronically track caught fish from ocean to market, and early in 2014 the system is to be fully implemented. In 2015 ICCAT is committed to revising its antiquated stock-assessment protocols.

This is movement in the right direction, but ICCAT’s structure and governance remain unchanged, vulnerable to political pressure from fishing interests in its member states. Tuna science, always politicized, has recently become much more so. As it is no longer possible for ICCAT to simply ignore scientific advice, there is now an effort to massage the science. “There are inherent uncertainties about these stock assessments,” Amanda Nickson, director of global tuna conservation at the Pew Charitable Trusts, told me. “We’re seeing a mining of the areas of uncertainty to justify increases in quota.”

Industry-funded biologists propose that there might be undiscovered spawning grounds for Atlantic bluefin. It is possible, of course, but there is no real evidence for the proposition. The idea seems awfully convenient for an agenda favoring business as usual.

Hopkins Station, established by Stanford University in 1892, was the first marine lab on the West Coast. Its weathered buildings, like the abandoned canneries immediately eastward, are relics of the age of sardines, a boom time that busted 60 years ago. The place is full of ghosts. Chief among them is Ed Ricketts, the inspiration for “Doc” in John Steinbeck’s Cannery Row. At night this maverick ecologist would leave Pacific Biological Laboratories, his rickety one-man enterprise at 800 Cannery Row, and sneak into the Hopkins library to do research. Ricketts and the sardine fishery went out together. The man was killed in 1948 at a Monterey train crossing, and the last canneries, hit by the locomotive of overfishing, closed a few years later. By the 1980s sardines had shown a fragile recovery, but now the stock is crashing again.

A short stretch of beach and rocky shore separates Hopkins Station and the Monterey Bay Aquarium. Ricketts must have walked a nearby span of beach in his nighttime raids on the library. At the border between Hopkins and the aquarium, in an annex the two institutions operated jointly, are three large, waist-deep tanks stocked with young Pacific bluefin. Block and her colleagues worked out their tag-implantation techniques with the predecessors of these fish.

The bluefin tuna, if it is to have a future, will need wise management informed by good science. Here in Monterey the consequences of the opposite are hard to miss. Directly below the tanks of endlessly circling bluefin are rows of cement pilings, the ruins of cannery piers that reach out into the bay for wide, silvery rivers of sardines that are no longer there.