Rapid warming in the Gulf of Maine contributed to the collapse of cod fishing in New England, and might help explain why the cod population has failed to recover, even though fishing has largely ceased, according to a new study.  A team of marine scientists found that rising temperatures in the gulf decreased reproduction and increased mortality among the once-plentiful Atlantic cod, adding to the toll of many decades of overfishing.  Fisheries managers have tried to reverse the cod’s decline in the gulf by imposing increasingly severe limits on fishing since 2010, reducing quotas to the point that recreational cod fishing has been effectively closed and few commercial fishermen now set out intending to catch cod.  But the quotas, the study’s authors say, were based on population estimates that did not take into account the temperature changes and therefore were set too high. Even when fishermen stayed within the quotas, they were in effect overfishing, the researchers write in their report, which appears in the Oct. 30 issue of the journal Science.  “The failure to consider temperature impacts on Gulf of Maine cod recruitment created unrealistic expectations for how large this stock can be and how quickly it can rebuild,” the researchers write.  The study uses data about water surface temperatures to look at warming trends in the gulf since 1982 and compares the rate of increase to ocean waters in other parts of the world. From 2004 to 2013, the scientists found, temperatures rose faster in the Gulf of Maine than in 99.9 percent of the global ocean.  “It was a magnitude of temperature change that few ocean ecosystems have ever experienced,” said Andrew J. Pershing, the chief scientific officer at the Gulf of Maine Research Institute and the lead author of the study.  The jump in temperatures was probably caused by a combination of atmospheric warming and warm water carried into the gulf by a northward shift of the powerful Atlantic current known as the gulf stream, according to the researchers, who include scientists at the University of Maine, the National Oceanic and Atmospheric Administration and other institutions.  Marine experts have known that the gulf is warming more quickly than other places and that the increasing heat is probably affecting the dwindling cod population. But the new study calculated the surprising speed of the temperature increases and found a relationship between warming and increased fish mortality.  When fisheries collapse, “we often wonder if it’s fishing or climate, but it’s both,” said Janet Nye, a scientist at the Stony Brook University School of Marine and Atmospheric Sciences, at a news conference in Washington on Thursday.  The researchers also project how long it will take for the cod population to rebuild if rising temperatures are taken into account: from two to eight years longer than the upper limit of 10 years required by the federal Magnuson-Stevens Fishery Conservation and Management Act for the rebuilding of overfished populations  Dr. Pershing and his colleagues speculate that the warmer waters might result in young cod starving from a lack of prey or dying from increased exposure to predators before they reached maturity. The cod, they say, might move from shallow to deeper waters where more predators lurk, and earlier seasons might extend predation. The researchers also report a link between temperatures and mortality in adult fish, though some other scientists questioned that finding.  Some marine experts praised the study for highlighting the challenges that climate change poses for the management of fisheries, and for focusing attention on the interaction between environmental shifts and overfishing.  “We know the ecosystem is fundamentally changing and that the cod stock is not recovering and management has not been as effective as we hoped,” said Jake Kritzer, an ocean and fisheries expert at the Environmental Defense Fund and the chairman of the scientific and statistical committee of the New England Fishery Management Council. He called the report “an important step toward reconciling the science we use for management and the reality of a changing ecosystem.”  Other scientists said they were “delving into the details” of the study and needed to understand more about how the researchers reached their conclusions before accepting them as valid. Some experts noted that there were competing theories about why cod had failed to rebound: for example, the building of dams that reduced the availability of prey and the disappearance of traditional spawning sites from overfishing in those areas.  Michael Fogarty, the chief of the ecosystem assessment program at the Northeast Fisheries Science Center, who has studied the potential effect of climate change on cod, said one reason that the warming waters in the Gulf of Maine affected cod adversely was that the region was at the southernmost edge of the fish’s range, where the water is already warmest, even without temperature rise.  “As temperatures increase, those population that are already at the margins are more stressed,” Dr. Fogarty said. “For coldblooded animals like cod, temperature changes affect virtually every part of their life, their physiology, their reproduction, their eating habits.”  What everyone agrees on is that the warming waters seem to be having a variety of effects, hurting some species, helping others and bringing new species, like black sea bass, into the Gulf of Maine.  Dr. Robert Steneck, a professor in the school of marine sciences at the University of Maine, noted that a heat wave in 2012, when surface temperatures rose higher than they had been in 150 years, brought an explosion of green crabs, a nonnative species. The green crabs, in turn, “caused almost a complete collapse of edible mussels and a decline in soft-shell clams,” he said.  But the warming trend has been beneficial for lobsters, Dr. Steneck said, creating a “sweet spot” that provides perfect conditions for larvae to make their way to the seabed.  Developing management strategies that can adequately cope with such rapid changes is not easy. Thomas A. Nies, the executive director of the New England Fishery Management Council, said, “I don’t think there’s any doubt in our minds that climate change is very real and it’s affecting our fisheries in ways that we’re just beginning to understand.”  But Mr. Nies added that it was much easier to recognize the effects of temperature increases in hindsight than it was at the time they were occurring.  “We have spent a lot of time to figure out how we can set quotas to get the results we want,” he said. “It’s very difficult to know how to do that.” The council has reduced cod catch quotas by 90 percent over the last three years in an attempt to rebuild the species, according to Dr. Kritzer.  Jackie Odell, the director of the Northeast Seafood Coalition, which represents commercial fishermen, said coalition members found it frustrating that fishing laws did not take environmental changes into account.  “I think that’s going to have to take place,” she said. “You can’t say climate change is occurring on the one hand. You have to somehow take that and incorporate it into the rebuilding targets.”  A version of this article appears in print on October 30, 2015, on page A20 of the New York edition with the headline: Cod’s Failure to Recover Is Linked to Warming Gulf of Maine. Order Reprints| Today's Paper|Subscribe
A new study finds that drastic cuts in fishing quotas have not helped cod recover because rising temperatures have also decreased reproduction and increased mortality.