The Atlantic cod, Gadus morhua, has been a pillar of New England’s fisheries for centuries. Yet, despite carefully managed harvesting limits, these cod stocks teetering on the verge of collapse have failed to rebound. According to a new Science study, rapid warming in the Gulf of Maine is the culprit. Waters there have warmed 99% faster than anywhere else in the world in the last decade.
Because of chronic overfishing in the Gulf of Maine, a quota-based management system was implemented in 2010, and by 2013, quotas had been cut by 73%. Even with these progressively stricter limits (as well as the closure of recreational fishing), spawning stock biomass continued to drop. "Managers kept reducing quotas, but the cod population kept declining," Andrew Pershing from the Gulf of Maine Research Institute explains in a statement. "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."
To learn more, Pershing’s team studied satellite-derived sea surface temperature data for the region dating back to 1982 and then compared these to global trends. Temperatures in the Gulf of Maine, they found, have risen 99% faster than anywhere else on the planet between 2004 and 2013.
This rapid warming is at least partly due to changes in the position of the Gulf Stream and to climate oscillations in both the Atlantic and Pacific oceans: Together, these exacerbate the impacts of global climate change on cold water species. As a result, the number of new cod produced by spawning females was reduced, and fewer young fish survived to adulthood.
Exactly how this happens has yet to be determined, though it likely involves changes in prey availability and predator risk. The abundance of certain plankton groups enjoyed by larval cod has declined in their habitat. Meanwhile, warmer temperatures might be driving juvenile cod to move away from their preferred shallow habitat into deeper waters, where risks of predation are worse. Not to mention, elevated temperatures increase metabolic costs in cod at the onset of reproduction.
Since the models that were used to set quotas for cod fishing over the last decade didn’t always account for the impact of warming waters, the amount of new fish available may have been overestimated. Even though fishermen stayed within these quotas, these limits were likely too high to begin with, and they took more fish than the population could sustain.
Cod fishing in Maine. Gulf of Maine Research Institute