Events & Media

Kyle Meng

Steve Gaines

July 27, 2016

Cod and Climate
Using the North Atlantic Oscillation as a predictive tool for managing
an iconic fishery

In recent decades, it has been hard to miss the alarming front-page stories detailing the plight of the Atlantic cod off the coast of New England, one of the most important and iconic fisheries in North America.

Since the 1980s in particular, the once-seemingly inexhaustible stocks of Gadus morhua have declined dramatically. In 2008, a formal stock assessment forecasted that stocks would rebound, but by 2012, they were once again on the verge of collapse, and in 2014 the National Oceanic and Atmospheric Administration instituted an unprecedented six-month closure of the entire Gulf of Maine cod fishery to allow stocks to recover.

Overfishing has been one culprit, but a new study co-authored by researchers at the UC Santa Barbara and Columbia University and published today in the journal PLOS ONE finds that the climatological phenomenon known as the North Atlantic Oscillation (NAO) also contributes ― and in a predictable way that may enable fishery managers to protect this cod fishery from future collapse.

This figure shows the biomass for Gulf of Maine adult cod from 1980-2013. The black line shows actual surveyed biomass of adult cod. The orange line shows what the adult cod stock would be if the NAO did not exist. The green line shows the stock with the NAO. The message is this: NAO conditions explain 17% of the long-term decline in adult cod from 1980-2013.

The NAO is a periodic climatic phenomenon that, like El Niño, causes changes in water temperatures, though the mechanism is different and the NAO affects the North Atlantic rather than the Pacific. And like El Niño, the NAO may also be affected in terms of both strength and frequency by climate change. The researchers ― Kyle Meng (UC Santa Barbara’s Bren School of Environmental Science & Management), Kimberly Oremus (Columbia University’s School of International and Public Affairs) and Steve Gaines (the Bren School) ― found that since 1980, NAO conditions have accounted for up to 17 percent of the decline in New England cod stocks.

“The Atlantic cod fishery has been the poster child of fishery science and the challenges in the field,” says Oremus.

The study arose out of an opportunity presented by a climatological anomaly. “In the 1980s, the North Atlantic was stuck in a positive phase of NAO,” explains Meng, an economist and lead author of the paper. “We show not only that positive NAO conditions diminish a few consecutive cohorts of cod larvae, but also that this effect follows a cohort as it matures.”

“This study does something new in that we followed the effect of climate variability on cod throughout their life cycle,” Oremus adds. “We also find evidence suggesting how, through fishing, human actions might be exacerbating the effect.”

As Gaines, a fish ecologist and the dean of the Bren School explains, “Fishery managers face big challenges in predicting how many new fish will come into the fishery each year. They use models to predict the average, but actual values vary wildly. Climate variation is one of the big challenges, especially if the recruitment forecasts turn out to be repeatedly too high, as we saw following NAO events. Then the mistakes compound, and yields can be compromised for a long time.”

Because the cod stock is well defined over a large but specific area and has been studied extensively for over a century, the researchers had access to abundant data that enabled them to establish that warmer NAO conditions reduced cod larval recruitment by 17 percent, resulting in fewer young fish. The authors write that while the NAO-induced population decrease persists until the fish are six years old, it affects cod catch for up to two decades.

That empirical link means that NAO can be used to predict the future size of the stock, which would allow for improved management. “It would provide us with an early warning, before the declines appear,” Meng says. “So we now know to expect that 17-percent drop in adult fish during a positive phase of NAO, giving management enough time to adjust practices.”

“We’re not just saying that the climate is part of the problem; we’re showing how it can be used to forecast and respond in an appropriate and cheap way,” says Gaines. “Many papers show that cod are in bad shape and identify climate as part of the problem,” he continues “But what they don’t do is give us a management solution.”

Fishery closures, like the one in 2014, are controversial and hugely disruptive to fishing communities and local economies. “If we know the state of the NAO, managers can respond by reducing catch appropriately in the short term to avoid long-term closures” Gaines notes.

Gaines praised his co-authors’ expertise in econometrics, saying, “That is what really solved this. People haven’t applied these kinds of approaches to this problem previously. Climate change may have costs for fisheries, but you don’t have to make bad choices that accentuate it for decades.”