“I’m leaving here in a coffin.” That’s what The New York Times quoted the owner of a Massachusetts fishing business as saying at a 2013 meeting of the New England Fishery Management Council. Cod numbers in the Gulf of Maine, once an incredible bounty, had crashed so hard over the decades that a strict quota system for fishing was set up in 2010. With the population continuing to fall, those quotas were slashed by around 75 percent in 2013. It’s not hard to imagine the hardship for those whose livelihoods are hauled in with nets.

The promised trade off was that steep cuts in the short-term catch could ensure the return of larger (and sustainable) catches for the longterm. But even with the sacrifice, the problem is still getting worse. The latest numbers put the fish population at about 4 percent of what it needs to be to produce the largest sustainable catch—the money in the bank that generates the most interest without crushing the bank under its weight. It’s a small fraction of what the population was even 30 years ago.

If overfishing is the problem and you limit the catch accordingly, why wouldn’t the population rebound? In a new study published in Science, a team led by Andrew Pershing of the Gulf of Maine Research Institute provides an answer. Overfishing isn’t the only problem.

That’s certainly not a new idea, but the researchers went about quantifying it in the population models used to manage the fishery. One physical change in the Gulf of Maine that jumps right out at you is warming waters. Surface warming over the last few decades there has exceeded warming just about anywhere else in the ocean, with a change of around 1 degree Celsius since 1980. Due to a couple of cooler years around 2004 and the warmest years coming in 2012 and 2013, the change over that decade has occurred at an even more extreme rate. This recent warming hotspot correlates with a longterm northward shift in the Gulf Stream current in our warming climate, as well as natural oscillations in the Atlantic and Pacific Oceans.

The Gulf of Maine is actually at about the southern edge of the waters Atlantic cod call home. We’re not entirely sure how it is that warmer water impacts them, but we know that it does. It reduces the number of young that survive their first year, possibly through declines in the plankton they eat or by moving them into deeper, cooler water where they are more likely to become snacks themselves. Warming also seems to reduce the number of fish that make it through the biologically stressful transition to sexual maturity, perhaps by taxing their bodies even further.

The researchers incorporated a statistical model of the impact of the warming into a cod population model to get at the ramifications for fishing quotas. They found that this additional factor cut the estimated addition of young to the population to half (or less, in some years) what it would be otherwise. Fishery managers should have been eyeing a smaller bank account, because the warming had significantly limited how big it could get. By using quotas that would have been appropriate in the absence of warming—as strict as they were—we were actually still pulling in too many fish for the population to rebound.

The researchers also ran the population model forward to project how long it would take for the cod to fully recover in the complete absence of fishing. They did this for a few different scenarios of warming, from roughly the average rate of the last 30 years, to a rate a little more than double that (but still much less than the unusual swing of the last decade). In the lower scenarios, the population could recover to its sweet spot—the optimal money in the bank—around 2025. In the scenario with faster warming, the population could hit the target just one year after that, but this is because the amount of money the bank can hold drops substantially with those temperatures. Even a minimal amount of fishing pushes these dates out a few years or more. By law in the US, you shouldn’t be in a position where that recovery process would take more than 10 years.

Even if the populations were allowed to recover to their targets, the sustainable catch (the interest on the bank account) in these scenarios isn’t great. With less warming, the researchers projected sustainable catches in the Gulf of Maine at around 5,000 tons per year—more than the 386 tons being allowed this year but a fraction of what used to be hauled in back in the 1980s and early 1990s. In the higher warming scenario, the sustainable catch was just 1,800 tons per year.

In a press conference, the researchers talked about the need for a more nimble fishery management system that was better equipped to account for changing environmental factors. “The rate of change outpaced the ability of people to make decisions about the ecosystem” in this case, Andrew Pershing said. “We put together this system that works really well for a lot of stock, but it doesn’t work really well for stocks that are on the edge, whether [they are] on the edge due to their distribution or on the edge due to very low abundance, and both of those we have in cod.”

“Our conclusion is that the future of this fishery now depends on both sound management and on favorable temperatures,” Pershing said.

Science, 2015. DOI: 10.1126/science.aac9819 (About DOIs).