Zach Whitener, a research associate at the Gulf of Maine Research Institute, holds a cod while collecting samples for later study. | Gulf of Maine Research Institute
Rapid warming in the Gulf of Maine explains why New England's cod stocks are on the verge of collapse despite cuts to fishing, according to a study published online in the 29 October issue of Science.
Over the last decade, "the Gulf of Maine experienced a rate of warming that few large marine ecosystems have ever encountered," said lead author Andrew Pershing, chief scientific officer at the Gulf of Maine Research Institute (GMRI) in Portland. "The rapid changes outpaced our ability to recognize and react to what was happening in the water."
Because models used to set the quotas for cod over the last decade did not always account for the impact of rising temperatures on cod survival, the new study concludes, the number of new fish available in a given season was often overestimated.
For centuries, Atlantic cod were pillars of New England's fisheries, carefully managed by programs designed to reduce harvesting levels in response to periods when the numbers of cod old enough to spawn were greatly diminished. In 2010, when cod stocks were already low, fisheries managers placed a series of restrictions on harvesting this species — but even strict quota limits on fishermen failed to help cod rebound.
Pershing and his colleagues wanted to understand if ocean warming was contributing to cod decline, just as it had for other species in the Gulf of Maine, like the American lobster.
First, they used sea surface temperature data dating back to 1982 to characterize temperature trends in the Gulf of Maine. Comparing changes in the region with global trends, they found that the ocean in the Gulf of Maine has warmed very rapidly — indeed, 99% faster than anywhere else on the planet between 2004 and 2013 — in part due to changes in the position of the Gulf Stream. The Gulf Stream is a warm Atlantic ocean current that originates at the tip of Florida and follows the eastern coastline of the United States. Recent shifts in its position have brought smaller amounts of cold water to the Gulf of Maine.
Next, the team analyzed Gulf of Maine cod population estimates, finding that the recent warming has clearly impacted the species. "Our analysis suggests that in warm years, each female cod produces fewer fish that survive to one year and that these young fish are less likely to reach adulthood," Pershing said.
The ocean in the Gulf of Maine has warmed very rapidly — indeed, 99% faster than anywhere else on the planet between 2004 and 2013.
"Managers tried to set fishing quotas at levels they expected to be sustainable given the information scientists could provide about the condition of the stock," said co-author Katherine Mills, an associate research scientist at GMRI, "but as new information came in about the stock, it became apparent it was not performing at expected levels, meaning that the previous quotas had actually been too high."
Mills, Pershing, and their colleagues used their data to project the rebuilding potential of the Atlantic cod stock under three future temperature scenarios: a hot scenario reflecting warming at 0.07 degrees Celsius per year (the rate observed in the summer in the Gulf of Maine since 1982), a warm scenario reflecting warming at a rate of 0.03 degrees Celsius per year, and a cool scenario reflecting warming at 0.02 degrees Celsius annually.
"We project that under a modest amount of warming, it will take this population 11 years to rebuild to a sustainable level," Pershing said. He emphasized that the rate of rebuilding depends on both favorable temperatures and more adaptive, responsive management by fisheries.
"The relationships between temperature and cod recruitment…have never been brought into the models that support management of the species," Mills said. "The Gulf of Maine cod case is a wake-up call that we need to bridge the [chasm] that currently exists between oceanography, fisheries ecology, and stock assessment science."
Mills, Pershing and co-author Janet Nye, a quantitative fisheries ecologist at the School of Marine and Atmospheric Sciences at Stony Brook University in New York, highlighted their research on 29 October, at a press briefing that was part of a daylong symposium on climate-change science. Reporters from the Associated Press, The New York Times, The Boston Globe, and other outlets were in attendance for the briefing, or tuned in via a live webcast.
The climate-change science symposium commemorates the 50th anniversary of the first official climate-change warning to a U.S. President (Lyndon B. Johnson). Organized by AAAS and Carnegie Science, the event was held at the Carnegie Institution for Science in Washington, D.C. A free webstream of the event is available at http://www.aaas.org/climate50