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In November 1981, a fleet of briefcase-toting lobbyists, scientists, and political negotiators gathered in sunny Tenerife, Spain, to decide the fate of Atlantic bluefin tuna. Representing more than a dozen countries, including Canada, the United States, Spain, and Italy, the besuited men knew crisis loomed. Since the early 1970s, rising global demand for bluefin flesh had spurred fishing fleets—hailing from ports on both sides of the Atlantic Ocean—to kill untold thousands of the wide-ranging predator every year. Under this heavy fishing pressure, primarily driven by the Japanese appetite for sushi-grade tuna, the species careened toward collapse.
During the meeting in Tenerife, the American delegation to the International Commission for the Conservation of Atlantic Tunas proposed a disarmingly simple solution: they would draw a line down the middle of the Atlantic Ocean and split the bluefin into two separate stocks. The Europeans could only fish east of the line, while the Canadians, Americans, and Japanese would fish west of it, limiting their catches to let the population recover.
The proposal passed and, eventually, for a variety of reasons, Atlantic bluefin tuna did bounce back. For more than four decades, that proposal has shaped how the fish are managed and understood. The only problem is that, as one former delegate put it, the two-stock idea may have only ever been a “convenient fiction.”
Since the 1950s, scientists have broadly accepted that Atlantic bluefin tuna live in two general populations: an eastern stock, which spawns in the Mediterranean Sea, and a western stock, which spawns in the Gulf of Mexico. But a growing body of evidence, including one study published in February 2023, now threatens to upend that binary theory. This developing research points to the existence of a third spawning site in a patch of ocean off North Carolina called the Slope Sea.
In 2016, fisheries biologist David Richardson, with the US National Oceanic and Atmospheric Administration, coauthored a study containing images of scrawny bluefin larvae that, they concluded, had been born in the Slope Sea. In 2022, biological oceanographer Christina Hernández, then at the Massachusetts Institute of Technology, confirmed Richardson’s findings after painstakingly analyzing the ear bones of tuna larvae collected in ocean currents flowing out of the Slope Sea.
Now, in the newest paper to cast doubt on the two-stock theory, Emil Aalto, a fisheries ecologist at Stanford University in California, has used an entirely different method. Combing through decades of tagging data, Aalto found and analyzed 24 fish that had made deep-plunging “spike dives” in the Slope Sea—dramatic ascents and descents that Atlantic bluefin display during mating.
“It’s really exciting that [Aalto’s team] found evidence of spawning behaviors in the Slope Sea,” says Hernández. The dives identified in the Aalto study, she adds, occurred when ocean conditions and larval evidence suggest that tuna should have been spawning. “We’re just adding a piece of evidence to a gradually developing story,” Aalto agrees, “that the Slope Sea is definitely seeing bluefin spawn.”
However, it’s still enormously unclear how many tuna—or which population of tuna—are breeding in the Slope Sea, Aalto explains. They could be from the western or eastern stocks, or members of some other yet-to-be-determined category, he says. But if large numbers of fish spawn there, he and his coauthors point out in their paper, the mathematical models currently being used to manage tuna would need a total overhaul. And changing those models could affect how much of the tasty fish makes it onto our sushi platters.
To get a sense of how much upheaval this change would cause, consider that late in 2022 the International Commission for the Conservation of Atlantic Tunas introduced a new mathematical modeling approach for bluefin tuna. The models are based on complex algorithms that have been crafted and negotiated by hundreds of delegates over thousands of hours. The process took years. Attempting to add a third spawning ground to the equation could increase catches, but it could also possibly reduce them or restrict where or when fishermen can target tuna in the vicinity of the Slope Sea. Part of the challenge, says Aalto, is that there’s still so much uncertainty.
For instance, Aalto and his coauthors suggest that eastern and western tuna may actually be interbreeding in the Slope Sea. If that’s true, it could introduce even more complexity into the models, which currently assume that Mediterranean and Gulf of Mexico spawners do not mix. And it’s unclear, Aalto says, if tuna born in the Slope Sea would return there to spawn or migrate to one of the other two areas. “Many labs are working on it urgently, including ours,” he adds.
Some scientists, fishermen, and even tuna conservationists have demanded more evidence that the two-stock theory needs to be canned, though. For instance, prominent environmentalist Carl Safina argued in a 2016 blog post, published in reaction to Richardson’s study, that a third spawning zone would likely be leveraged by commercial fishermen to catch more fish.
Still, if it is eventually proved that bluefin are spawning in the Slope Sea in significant numbers, this could be good news for the species. Climate change is heating the Gulf of Mexico roughly twice as fast as other oceans around the world; once the water temperature consistently surpasses 30 °C, bluefin will no longer be able to spawn there. It could be “critical to the health of the western stock,” Aalto and his coauthors write, if these tuna are able to move north to the cooler Slope Sea.
Even with his team’s discovery, there is so much yet to learn about Atlantic bluefin tuna, Aalto says: their recent paper is simply a stepping stone. “People can run with the ball, as we say, from there.”