For centuries, cod were the backbone of New England's fisheries and a key species in the Gulf of Maine ecosystem. Today, cod stocks in the gulf are on the verge of collapse, hovering at 3-4 percent of sustainable levels. Even setting tighter limits on fishing has failed to slow this rapid decline. Now a new report in Science concludes that rapid warming of Gulf of Maine waters—warming in the last decade faster than in 99 percent of the global ocean—has reduced the capacity of cod to rebound from overfishing, leading to collapse.

"Several processes contributed to the rapid warming off the coast of New England including natural and manmade (anthropogenic) changes in the ocean and atmosphere," said Michael Alexander, a research meteorologist at NOAA ESRL's Physical Sciences Laboratory (PSL) and co-author of the new analysis.

This rapid warming is linked to decadal climate oscillations in the Atlantic and the Pacific and a northward shift in the Gulf Stream, which brings warmer waters into the region, according to Alexander and his co-authors, who include James Scott, a scientist at the Cooperative Institute for Research in Environmental Sciences (CIRES) at the University of Colorado Boulder working at PSL. These factors accelerate the steady pace of ocean warming caused by global climate change.

In the face of already depleted cod stocks, fisheries managers have placed a series of restrictions on harvesting this key Gulf of Maine species. When strict quota limits on fishermen failed to help cod rebound, fisheries managers tightened catch limits even further. "The cod population kept declining," said Andrew Pershing, Chief Scientific Officer of the Gulf of Maine Research Institute (GMRI) and lead author of the study. "It turns out that warming waters were making the Gulf of Maine less hospitable for the fish."

Pershing and colleagues from GMRI, NOAA ESRL, CIRES, the University of Maine, Stony Brook University, and the Bigelow Laboratory for Ocean Sciences, found that increasing water temperatures reduced the number of cod produced by spawning. Their study also suggests that warming waters led to fewer young fish surviving to adulthood.

Those conclusions square with the understanding of regional fisheries managers. "The current Gulf of Maine cod assessment acknowledges that natural mortality may well have increased in recent years as suggested by this paper," said Bill Karp, director of the NOAA Fisheries Northeast Fisheries Science Center. "We are devoting significant resources to linking climate models and fish population models to better understand current status and better estimate what the stock will look like in the future—both of which are important to setting quotas that promote sustainable fishing," he said.

According to the new study, recovery of Gulf of Maine cod depends on both sound fishery management and on future temperatures. Cod are a coldwater species, and the Gulf of Maine is near the edge of their geographic range. As the ocean warms, the capacity of the Gulf of Maine to support cod will decline, leading to a smaller population and a smaller fishery.

Andrew Pershing and two other authors will take part in a press conference today during a AAAS-hosted conference commemorating the 50th anniversary of the first time a US president was warned about CO2 and climate change.

This study was funded by the Lenfest Ocean Program and the National Science Foundation.

Posted: October 29, 2015

CIRES is a research partnership of NOAA and CU-Boulder

Authors of " Slow Adaptation in the Face of Rapid Warming Leads to Collapse of the Gulf of Maine Cod Fishery" are Andrew Pershing (GMRI), Michael Alexander (ESRL PSL), Christina Hernandez (GMRI), Lisa Kerr (GMRI), Arnault LeBris (GMRI), Katherine Mills (GMRI), Janet Nye (Stony Brook University), Nicholas Record (Bigelow Laboratory for Ocean Sciences), Hillary Scannell (GMRI and U Maine), James Scott (CIRES and ESRL PSL), Graham Sherwood (GMRI), and Andrew Thomas (University of Maine).