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Alarming Cod Population Decline Linked to Warm Water in Gulf of Maine

As early English explorers set sail across the Atlantic to New England's shores in pursuit of harvesting fragrant sassafras, the abundance of cod in the region's coastal waters led one man to dub the tip of the landmass known today as "Cape Cod."

As a result of this discovery, fishing outposts and commercial fishing operations sprouted in the centuries that followed, making cod an important resource of the colonial era. Today, the once abundant groundfish population has been deteriorating at an alarming rate.

"Since [2007], the biomass has declined quite precipitously," NOAA Research Biologist Michael C. Palmer said.

According to a recent report from NOAA's Northeast Fisheries Science Center, which was penned by Palmer, the species' spawning biomass is nearly five percent less than what is needed to sustain the stock.

Overfishing is a major component in the population decline, but there are a range of other factors that could be contributing causes as well, Palmer said, citing the uncertainty facing scientists now despite their best efforts to reduce how many fish are being harvested.

"We failed to constrain fishing mortality," Palmer said, adding that the current fishing restrictions have not been effective, and that the models overestimated spawning stock size and underestimated fishing mortality.

A recent study published in Science, also attributes warming waters in the Gulf of Maine as a significant factor in the reduction of the region's Atlantic cod population.

The study illustrates that rapid warming of the Gulf of Maine's waters, which is "occurring 99 percent faster than anywhere else on the planet, and has reduced the capacity of cod to rebound from fishing, leading to collapse."

"Managers kept reducing quotas, but the cod population kept declining," Andrew Pershing said.

Pershing is the 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 cod, and the management response was too slow to keep up with the changes," he said.

According to Pershing's study, which was funded by the Lenfest Ocean Program and the U.S. National Science Foundation, the rapid warming in the Gulf of Maine is linked to changes in the position of the Gulf Stream and to climate oscillations in the Atlantic and Pacific.

"We used surface temperature from satellites operated by NOAA and NASA," Pershing said. "From 2004-2013, the Gulf warmed at a rate of 0.23 C (32.415 F) per year, meaning that it warmed faster than 99.9 percent of the other areas in the ocean over this period."

Both the long-term, and recent warming rates are related to changes in the ocean and the atmosphere, he added.

"In the ocean, we found a connection between the position of the Gulf Stream, a river of warm water to the south of the Gulf of Maine," he said. "When the Gulf Stream shifts to the north, more warm water and less cold water from Canada comes into the Gulf. Warmer atmospheric conditions have also contributed to the temperature changes in the Gulf."

"All fish species, whether or not they're groundfish, are impacted by the environment," Palmer said, adding that more research still needs to be conducted to determine if rising temperatures are directly impacting the cod population.

"Our work suggests that in warmer years, fewer new cod are produced and these cod have a lower probability of survival," Pershing said. "We have not looked at other fish, but we would expect that some of the cold water species will show a similar pattern. Warming waters were likely an important factor in the decline of the shrimp fishery in the Gulf of Maine."

Northern shrimp are another subpolar species that, like cod, are at the southern end of their range in the Gulf of Maine, he added.

It's difficult for scientists to untangle the many factors that could contribute to the decline including overfishing, a decreased food supply and other environmental changes like Pershing's research related to warming waters, Palmer said.

According to Pershing, the role of water temperature may be directly impacting the fish population, or possibly the food supply.

"We don't know, but likely both could be operating," Pershing said. "Studies have shown a decline in the food supply for larval cod in recent years. We also know that warm water directly impacts metabolism in fish, meaning that cod would require more food to grow and reproduce during warm years. We also think that warm waters could extend the period when migratory predators are active in the Gulf of Maine."

Another major component of Palmer's report is the recruitment, or number of young fish introduced into the population.

"Declining spawning stock biomass and truncation of the age-structure could compromise the future recruitment success of this stock," Palmer wrote in his report.

"Recruitment over the last five years [2009-2013] has been well below the long-term recruitment levels. If recent weak recruitment of Gulf of Maine cod continues, productivity and rebuilding of the stock will be less than projected."

While cod has become less desirable to commercial fishing when compared to other fish species like haddock, pollock and redfish since 2000, it is still in demand commercially and in recreational fishing, Palmer said.

While cod remains in drastic decline in the region's waters, there has been a population increase in the other commercially targeted species in the region, he added.

As the new year begins, Palmer said many of the fishing restrictions will be revised and more research needs to be conducted to understand the issue.

"They will be revising quotas [for all species listed in the report]," Palmer said.

"The quotas for Gulf of Maine cod are now set about as low as they can get," Pershing said. "This should allow the stock to rebuild, but how quickly it rebuilds will depend on environmental conditions and on keeping fishing pressure low."