Pacific Rim nations met in Busan, South Korea, in early September and reached the first-ever agreement to restore severely depleted populations of Pacific bluefin tuna. Stocks of the large, fast and tasty fish have plunged more than 97 percent in recent decades, due largely to overfishing to supply voracious demand in Japan.

China, Japan, South Korea, the United States and other members of the Western and Central Pacific Fisheries Commission and the Inter-American Tropical Tuna Commission agreed to take action to rebuild Pacific bluefin tuna populations to 20 percent of historic levels by 2034.

Now new research indicates that if those targets are to be achieved, the way tuna is managed may have to significantly change.

Bluefin tuna, which can weigh up to 1,000lb (450kg) and grow to 9ft (2.7m) in length, migrate 6,000 miles (10,000km) between the eastern and western Pacific. A recent analysis of tuna migration patterns published in Science magazine found that in some years a majority of spawning-age tuna in the western Pacific near Japan had migrated from the eastern Pacific off the coasts of Mexico and California. Scientists analyzed chemical traces in tuna tissue to track their movements and determined that far more young fish aged 1–2 years in the western Pacific migrate to the eastern Pacific than previously thought. The tuna remained in the eastern Pacific until they reached sexual maturity around 7 years of age and then made the return journey to the western Pacific to spawn.

That upends the conventional thinking that few bluefin tuna migrate to the eastern Pacific. As a result, scientists said the findings show that fishing restrictions must be imposed across the tuna’s range, and not just in one region of the Pacific, to ensure a recovery of the species.

“Understanding when tuna mature and spawn, how many young are produced by adult fish of each age class, and how many of those young survive into adulthood is critical for knowing how much fishing the population can handle,” said study coauthor Andre Boustany, a research scientist at Duke University and a principal fisheries investigator for the Monterey Bay Aquarium.

“The research is helping to answer questions on the biology and ecology of Pacific bluefin tuna that will inform the stock assessment process and make the stock assessments more accurate,” Boustany added, noting that not much is known about the fish’s reproduction dynamics.

To gather data on tuna migration, Boustany and scientists from Monterey Bay Aquarium, Stanford and Harvard universities and the National Museum of Natural History, as well as from Japan, spend long days on boats, sometimes in rough conditions, to tag tuna and collect tissue samples for testing.

An equally big challenge is persuading fisheries to revamp tuna management practices.

“Any time you are talking about changing how fish are managed, it usually means affecting people’s livelihoods,” Boustany said. “And that means you can face a lot of resistance to change, especially if those changes to management mean that fishers should be catching fewer fish.

“This makes for complicated and intense fisheries management meetings with strong vested interests by most of the participants, and navigating this process can take more effort and time than the original science that fed into the process,” he added.

To achieve the targets agreed to in South Korea this month, nations must monitor the tuna population’s recovery and more closely track Pacific fisheries to ensure illegally caught tuna don’t make it to market.

“This is a significant and encouraging departure from the previous stance of this Commission,” Margaret Spring, chief conservation officer at the Monterey Bay Aquarium, said in a statement. “If it is implemented appropriately, it will recover the population.

“This will require all Pacific nations to follow through on their responsibilities – especially to comply with and enforce fishing quotas, monitor their catches and invest in additional research to advance our understanding of the basic biology and migration routes of Pacific bluefin tuna,” she added.

Bluefin tuna conservation expert Carl Safina, founder of the nonprofit Safina Center in New York, said the Commission’s decision was a step in the right direction, but it is nowhere near ambitious enough to ensure recovery of tuna populations.

He said the goal for Pacific bluefin tuna should be like that for most other fish, what fishing managers call a “full recovery.” That’s the number necessary to reach the “maximum sustainable yield” – taking as many tuna as possible without threatening the survival of the species. That target typically is around 60 percent of a fish’s historic population before industrial fishing began.

Safina added that it’s difficult to enforce international catch limits. He points to Japan, which has been accused of underreporting its fishing vessels’ hauls of southern bluefin tuna, a closely related species. And earlier this year, Japan exceeded an international catch quota on Pacific bluefin.

While science is important, Safina said that knowing more about tuna’s life histories alone won’t help conserve them.

“The problem is with us, not them,” he said. “What is unknown is whether they can really rebuild at this point. And how fast. Other than that, we know what’s needed: There is too much fishing, and there needs to be a lot less fishing.”

Disclosure: The writer previously received a conservation fellowship from the Safina Center.