After a decade of efforts to revive Atlantic bluefin tuna populations decimated by overfishing, conservationists were eagerly awaiting a study expected to report good news about the fortunes of a fish that can grow to the size of a car and fetch hundreds of thousands of dollars on the sushi market.

They were disappointed.

The International Commission for the Conservation of Atlantic Tuna (ICCAT), the organization that tracks and manages Atlantic bluefin, did not find that populations had recovered when it released the results of its annual stock assessment last month.

Yet the same scientists who reached that conclusion also recommended allowing catches of eastern Atlantic bluefin to increase by 52 percent. Western Atlantic bluefin catches could grow by 25 percent. By 2020, annual quotas for eastern Atlantic bluefin could rise to 36,000 metric tons a year – a record high, according to conservationists. Quotas for western Atlantic bluefin could increase to 2,500 metric tons. (The final quotas for 2018 will be set at ICCAT’s meeting that begins on November 14 in Marrakech, Morocco.)

How did that happen?

The new report shows that stocks of the beleaguered bluefin are likely to rebound in the future, but that the recovery cannot be confirmed. The root of the problem is uncertainty, which has pushed ICCAT to adopt a new basis for its quota recommendations. That, in turn, has led to an apparent disconnect between the report’s recovery findings and its catch recommendations.

“This was supposed to be a very important year because we were all expecting the stock to be assessed as recovered and were ready to celebrate that,” said Alessandro Buzzi, marine projects manager at WWF Mediterranean. “But unfortunately it was quite a surprise, and also a disappointment, because it appeared that the situation wasn’t so clear.”

Buzzi was in Madrid when ICCAT announced its findings that the eastern Atlantic and Mediterranean bluefin stocks could be considered recovered under only two of three scenarios of “stock recruitment,” a measure of when fish are large enough to be caught under ICCAT regulations.

That was the same result as three years ago, the last time scientists conducted a full stock assessment.

“Under two of three scenarios in 2014, it was considered recovered, and under one overfished,” said Rachel Hopkins, senior officer at the Pew Charitable Trusts Global Tuna Conservation program. “We were expecting that it would be recovered under all three.”

One of the problems in 2014, she said, was there was too much uncertainty and poor-quality data in the models. But, after several years of working to reduce uncertainties in the models, the problems were the same this year.

“We were expecting recovery of the eastern stock to be confirmed, which did not happen, unfortunately,” Hopkins said.

The ICCAT report does not state whether the western Atlantic bluefin tuna, which spawns mainly in the Gulf of Mexico and migrates along the western Atlantic Ocean, was recovered or overfished, due to numerous uncertainties about the size of that population.

The scientists’ recommendation to increase catch quotas for the eastern and western bluefin tuna left Hopkins and other environmentalists perplexed. “The management advice was pretty strange advice,” Hopkins said. “It’s understandable that the scientific community would come to the managers and say, ‘We can’t say what the status of the stock is.’ What doesn’t make sense is why they would then recommend increasing the quotas.”

Buzzi agreed. “I have a feeling that the stock is progressing well,” he said. “But even if the stock wouldn’t be at risk of collapsing now, still what is the point of increasing the quota, and then maybe finding in three years’ time the stock is decreasing again and you have to decrease quotas again?”

When there’s uncertainty, as there inevitably is in the assessment of fish stocks – even those of extensively studied species such as bluefin – managers should err on the side of caution, Buzzi said.

Craig Brown is the chief of the highly migratory species branch of the National Oceanic and Atmospheric Administration’s Sustainable Fisheries Division and a member of the U.S. delegation to ICCAT. “There’s a lot of different points of view and you eventually have to arrive at a consensus,” he said. “So that was the advice that got approved to go forward.

“It’s clear that the (eastern) population has been rebounding and is continuing to grow, but there’s a lot of uncertainty,” Brown added.

That uncertainty was largely the result of not being able to determine the relationship between stock recruitment and the biomass of the stock, he said. As the biomass – the aggregate weight of all the fish in the eastern bluefin population – increases with the recovery of the stock, more and more fish could reach maturity and enter the fishery. But that’s not necessarily the case. The number of fish entering the fishery could level off, for instance, as competition for food and resources increases in the growing population.

Since the researchers couldn’t establish what that relationship was, Brown said, they couldn’t determine what the biomass would need to be to produce a sustainable level of catches, which is the standard that must be met for ICCAT to declare the population recovered.

So the scientists included projections of the biomass based on the three different recruitment scenarios, taken from periods in the past when recruitment was particularly high, low or in between. “The problem,” Brown said, “is there is no way of knowing which of those is right and which is more likely to be right.”

In any case, he said, those projections were essentially included as ancillary information and were not part of the data the quota recommendations were based on this time around. Between the last full assessment in 2014, and this year’s report, the ICCAT secretariat had decided that it wanted “advice that is more useful,” Brown said. Advice that said the stock was rebuilt under one scenario and far from it in another wasn’t very helpful.

So this year’s quota recommendations were based on a model that doesn’t consider at all the unclear relationship between recruitment and biomass. Instead, the model takes the average recruitment over the most recent six years for which there is quality data and projects it forward, five years for the eastern stock and three for the western.

“That’s why there’s a bit of disconnect” between the recommendations and the assessments under the three different models, Brown said. If the stock is managed using the new strategy then catches can increase. “But it also says in the projections that if you go up then the stock size is going to go down.”

In its eastern bluefin recommendations, the ICCAT report acknowledged “some important sources of uncertainties that currently remain unquantified.” It said,“This points to the need to be cautious,” before warning against increasing catches as high as 38,000 metric tons by 2020 and settling on the 36,000 number.

But that’s still too high, according to Hopkins. “If you were to adopt a quota of 36,000 it would be the highest quota there’s ever been for eastern bluefin,” she said.

The recommendations have put conservationists in an awkward position. Back when the Atlantic bluefin recovery plan was implemented around a decade ago, the plea of conservation advocates was to not let politics get in the way and to follow the scientific advice. “Our focus has always been on following the scientific advice,” Hopkins said. “But what do you do when the scientific advice is not precautionary?”