The Atlantic cod, a fish that came to symbolize bounty to America’s colonial settlers, is on the brink of disappearing, despite years of fishing limits aimed at rebuilding stocks. A new study reveals why: Cod spawning and survival has been hampered by rapid, extraordinary ocean warming in the Gulf of Maine, where sea surface temperatures rose faster than anywhere else on the planet between 2003 and 2014.
“Over this same 10-year period, fishery managers set quotas that they felt were informed by high-quality science, and the stock just kept going down, down, down,” says study leader Andrew Pershing, the chief scientist for the Gulf of Maine Research Institute in Portland, Maine. When his team looked closely at factors behind the trends, he says, “We found the fingerprints of temperature throughout the data.”
The scientists used satellite data to track the daily sea surface temperature trend in the Gulf of Maine. From 1982 until 2004, they found, temperatures rose by 0.03°C per year, or three times the global mean rate. That warming accelerated sevenfold beginning in 2004, peaking in 2012 with a large “ocean heat wave” that persisted for 18 months, according to the study.
To see what drove the trend, the researchers correlated quarterly Gulf of Maine temperatures over that time with large-scale climate factors. They found that a northward shift of the warm ocean current known as the Gulf Stream hit the New England coast at the same time that two decade-long ocean climate cycles, the Atlantic Multidecadal Oscillation and the Pacific Decadal Oscillation, entered a warmer phase, turning the heat up even further. Previous research has linked the Gulf Stream’s gradual northward shift over the 20th century to increasing greenhouse gases in the atmosphere. Those combined effects packed a punch, the authors say: “The Gulf of Maine experienced decadal warming that few marine ecosystems have encountered,” they write online today in Science. Since 1900, the chance of any similar segment of the ocean warming so fast was less than 0.3%, they found.
Using recent Gulf of Maine cod stock assessments, the researchers then tested a number of models for predicting the factors that affected cod reproduction. Warming was the best predictor, they reported: When summer temperatures went up, the number of fish reaching maturity went down. “The number of new cod for each year that appear in the population is strongly related to temperature,” Pershing says. “And that’s ultimately what you need to rebuild a population and sustain a fishery: new fish coming in.”
The study did not explore exactly how warming hurts cod reproduction, but it points to previous research showing that heat may diminish the populations of tiny sea creatures that larval cod rely on for food. Warmer temperatures also may be causing the larval cod to move out of their typical shallow habitat into deeper, cooler waters, where they’re at greater risk from predation, the authors say. Higher temperatures also could be increasing the metabolic energy that the young have to spend to stay alive. The average weights of cod have been below the long-term mean since 2002, the authors note, and these poorly conditioned fish will likely have a lower chance of survival.
Because they didn’t account for the impact of climate change, New England’s fishery managers set catch quotas too high, particularly during the 2012 ocean heat wave, Pershing says. They were trying to help fishing communities, but it may have hurt the cod—and the fishers who rely on it—in the long run. “We have the advantage at the top of the hill in 2015 of looking back and being able to say, ‘How crazy was that?’” Pershing says. “But in the thick of it, when you’re having to make those decisions, it’s much harder to put those pieces together.”
The research should serve as impetus for a long-needed overhaul in how fish are managed in the region, says biologist Jud Crawford of The Pew Charitable Trusts’ northeast oceans program in Boston, who was not involved in the study. “Hopefully, this is enough evidence to finally push New England to really seriously embrace models that include temperature as a variable,” as well as habitat for cod larvae.
Still, Gib Brogan, fishery campaign manager for the nonprofit advocacy group Oceana in Boston worries that the findings will have the opposite effect. “It’s my concern that climate change will be seen as an overwhelming factor, and [fishery managers] will move from recovery mode to liquidation mode,” he says. “In other words, ‘the cod are on their way out. Let’s catch all we can now.’ And that’s a terrifying possibility.”
On the other hand, Brogan says that the study shows the need for a new caution in fisheries beyond just the Atlantic cod. “Science is telling them to put a buffer in there for the uncertainty that comes with climate change,” he says. “That should be factored into long-term management for every fishery.”