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article imageAtlantic cod collapse linked to warming ocean waters

By Karen Graham     Oct 30, 2015 in Environment
The Atlantic cod has been the backbone of New England's fisheries for centuries. Yet for the past few years, cod stocks have been decreasing at an alarming rate. Now, because of warming ocean waters, Atlantic cod populations are on the verge of collapse.
Digital Journal reported on the decline of the Northwest Atlantic cod population in August 2014. A survey of the Gulf of Maine cod population by NOAA indicated their numbers were decreasing at an alarming rate. While New England fisheries were skeptical of NOAA's data, facts tell the real story. Catches are extremely low, even with limits on the cod fisheries.
A report released this week links the dramatic decline in cod stocks on rapidly warming ocean waters. The report, published in the journal Science, points out the need to add environmental changes to the management of Atlantic cod populations.
"Here is an explanation for why the Gulf of Maine's cod fishery has not recovered despite significantly reduced fishing," says Mike Sieracki, program director in the Division of Ocean Sciences at the National Science Foundation (NSF). "Management plans will need to incorporate climate change factors to be effective."
Several factors were taken into consideration in coming to the dire conclusion on cod stocks. Historically, the declines because of overfishing, and the recent limits on total catch were noted. But even with moratoriums and limits, the cod population continued on its downward spiral.
Then, based on documented temperature records, it was discovered that the Gulf of Maine, a primary fishing ground, was warming faster than 99 percent of the global ocean. This was linked to changes in the flow of the Gulf stream, as well as climate oscillations in both the Atlantic and Pacific Oceans.
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Andrew J.Pershing, et. al.
"Fishers stayed well within their limits for cod, and yet stocks continued to decline" says Andrew Pershing, chief scientific officer of the Gulf of Maine Research Institute (GMRI) and lead author of the study. "That told us something else was going on--and it turns out that warming was driving the decline."
Cod typically prefer the deep colder waters during the day, and the shallower warmer waters at night. The Gulf of Maine is at their geographical range. The study found that as the Gulf of Maine continues to warm it will not be able to support cod.
One finding was very significant, and that was the reduction in the abundance of some zooplankton taxa that are prey for larval cod. It is thought that juvenile cod are moving into deeper water in search of food, leading to their becoming prey to bigger predators. This, along with poorer quality fish due to increased mobility based on increased water temperatures point to a lower probability of survival, according to the study.
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Andrew J. Pershing, et. al.
In the above illustration:(A) Daily (blue, 15d smoothed) and annual (black dots) SST anomalies from 1982-2013 with the long-term trend (black dashed line) and trend over the last decade (2004-2013) (red solid line). (B) Global SST trends (° yr−1) over the period 2004-2013. The Gulf of Maine is outlined in black. (C) Histogram of global 2004-2013 SST trends with the trend from the Gulf of Maine indicated at the right extreme of distribution.
Without a doubt, overfishing of Northwest Atlantic cod helped in reducing their numbers in the past. With advances in technology, factory trawlers were able to increase their landings dramatically, and by 1968, almost 800,000 metric tons of cod were being landed annually.
By 1970, a decline in the cod population began, and a moratorium was put on cod stocks. With the reopening of the limited cod fisheries in 2006, close to 2,700 metric tons of cod were hauled in. However, in 2007, it was estimated that cod stocks were only one percent of what they were in 1977, a dramatic difference. Now, they are even lower.
The study, "Slow adaptation in the face of rapid warming leads to collapse of the Gulf of Maine cod fishery." was published in the journal Science on October 29, 2015.
More about Atlantic cod, fisheries collapse, warming ocean waters, Climate change, gulf of maine
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