How In Trouble Are Bluefin Tuna, Really? Controversial Study Makes Waves

Enlarge this image toggle caption Pablo Blazquez Dominguez/Getty Images Pablo Blazquez Dominguez/Getty Images

Bluefin tuna have been severely depleted by fishermen, and the fish have become a globally recognized poster child for the impacts of overfishing. Many chefs refuse to serve its rich, buttery flesh; many retailers no longer carry it; and consumers have become increasingly aware of the environmental costs associated with the bluefin fishery.

But a group of scientists is now making the case that Atlantic bluefin may be more resilient to fishing than commonly thought — and perhaps better able to rebound from the species' depleted state. In a paper published Monday in the journal Proceedings of the National Academy of Sciences, the researchers suggest that fishery managers reassess the western Atlantic bluefin's population, which could ultimately allow more of the fish to be caught.

The 10 co-authors, most of whom are scientists with the National Oceanic and Atmospheric Administration, say they've all but confirmed that bluefin tuna spawn in an area of the Atlantic Ocean previously suspected but not known to be a breeding ground. Not only that; the tuna spawning in this area off the Atlantic Coast are much younger and smaller than the age and size at which it was previously believed the fish become sexually mature, according to the scientists.

This, their paper claims, would make the western Atlantic bluefin tuna "less vulnerable to overexploitation and extinction than is currently estimated."

But the study is controversial. Several tuna researchers we spoke with warned that the results are preliminary, and it's much too soon to use them to guide how fisheries are managed.

"New science and new information is good. What one has to be careful of is attempting to manage the Atlantic bluefin population from a single study. The situation is always complex," says Amanda Nickson, director of global tuna conservation at the Pew Charitable Trusts.

Ecologist, author and former tuna fisherman Carl Safina cautions that the research doesn't change how fishing has already impacted the Atlantic bluefin, which is listed as endangered by the International Union for Conservation of Nature Red List. The U.S. federal government considers the species overfished.

"If this is in fact true, that they're spawning in this area [in addition to the Gulf of Mexico] and it wasn't just a one-year occurrence, it's good to know that the potential for recovery is brighter than we would have thought. But it certainly doesn't mean they were less depleted than they've been," Safina tells The Salt.

And Duke University research scientist Andre Boustany, a bluefin expert who was not involved in the study, says the findings should be used cautiously.

If the current population of Atlantic bluefin of reproductive age is indeed larger than once believed, that would mean that, historically, there were a lot more spawning bluefin than once thought, Boustany says. And that means that goals for recovery of the Atlantic bluefin would have to be set at a higher level, he says.

"If we're trying to rebuild the population within a certain time frame, then we might need to actually reduce the amount of fish we're catching now," Boustany explains.

To produce their results, the researchers behind the PNAS paper dragged a fine-meshed plankton net through a portion of the coastal North Atlantic known as the Slope Sea in 2013. They captured dozens of bluefin tuna larvae no more than 5 or 6 days old. The site is far from known spawning areas in the Gulf of Mexico. That great distance, coupled with the slow speed of the ocean currents, meant only one thing, explains David Richardson, the study's lead author: These young tuna had been born in the immediate vicinity.

"It was very clear these fish had not come from the Gulf of Mexico," Richardson, a larval fish biologist with the NOAA's Northeast Fisheries Science Center, tells us. "It's just too far a distance."

Richardson and his colleagues have also analyzed the movements of adult bluefin tuna tagged with electronic transmitters by the Large Pelagics Research Center at the University of Massachusetts, Boston. The tracking data provided evidence that western Atlantic bluefin appear to begin spawning at a much younger age than previously believed, when they're about 5 years old, instead of 14 or 15.

"What we found in our tagging data is that really big bluefin swim through the Slope Sea really fast, in three or four days, whereas the smaller bluefin — around 100 pounds to 400 or 500 pounds — are staying in the Slope Sea for about a 20-day duration," he says. The suspicion, he elaborates, is that younger, smaller tuna are spawning in the Slope Sea. Then, when they become older and bigger, they begin spawning in the Gulf of Mexico.

This is significant, because species that don't reach sexual maturity until they're older are considered especially vulnerable to overfishing. That's because such fish may easily be caught years before they've spawned even once.

But if the western Atlantic bluefin are actually spawning much younger than once believed, that should be factored into population assessments, the study authors argue.

"Lowering the age at maturity will increase estimates of spawning stock biomass and will likely lead to higher estimates" of how much bluefin can be fished sustainably, their paper says.

Barbara Block, a marine biologist with the Stanford Woods Institute for the Environment, says the study is "interesting." But she says much more evidence is needed — like actually seeing these sexually mature tuna in the Slope Sea — before it can be concluded that Atlantic bluefin are spawning at a younger age and in a new region than believed before.

The researchers' conclusions — that perhaps more bluefin can be caught — also have Safina highly skeptical of their study, which he says looks like a ploy by fishery-friendly scientists to create a higher catch allowance for the Atlantic tuna fleet.

In an email exchange with The Salt, Safina writes, "[T]heir main concern is not recovery, not conservation, but how their findings can allow additional exploitation and more stress to be inflicted on a very beleaguered creature."

But Molly Lutcavage, a researcher at the University of Massachusetts, Boston and a co-author of the PNAS study, dismissed Safina as an "enviro bully" and an ideologue who ignores the science.

"You can't do good conservation without good science," she told us, responding to Safina's comments. In a post published Tuesday on Medium, she offers a vehement defense of her research.