When you throw a net into the ocean, you never know what you’ll pull out.
That was the case for researchers cruising the freezing Arctic waters off Greenland in August 2012 in search of mackerel to see if there were enough of the fish to support a commercial fishery. In one haul, three endangered Atlantic bluefin tuna, each weighing roughly 220 pounds, were pulled onto the ship’s deck amid six metric tons of mackerel.
“It was a bit surprising,” said Brian MacKenzie, a marine ecologist at the National Institute for Aquatic Resources at the Technical University of Denmark. The research ship was sailing in the Denmark Strait, between Greenland and Iceland, where water temperatures have historically been too cold for bluefin tuna.
More bluefin tuna have been caught off eastern Greenland since then. From June to the end of August of this year, Greenland fishing vessels caught 21 tuna—in addition to 65,000 metric tons of mackerel, according to Greenland Today.
The ever warmer Arctic waters could have profound impacts on how fisheries and food webs are managed and conserved in the future as tropical and Mediterranean species migrate into what were once colder waters.
With Arctic waters warming and attracting bluefin tuna, Iceland and Norway in 2014 implemented commercial quotas for the prized fish. “It’s small, only 30 [metric] tons each,” said MacKenzie. “But it indicates that the distribution is really changing.”
“Climate change is really challenging political and diplomatic relationships,” said Nick Dulvy, a professor of marine biodiversity and conservation at Simon Fraser University in Burnaby, British Columbia. “Species names will change, and if your quotas are tied to a species name, that’s a problem for the fishery,”
In 2009, after mackerel had spread to the coastlines of Iceland and the Faroe Islands, Iceland set itself a mackerel quota of 112,000 metric tons. That angered the European Union, and conservationists worried that stocks of the humble fish would suffer.
MacKenzie and his colleagues analyzed the water temperatures east of Greenland using satellite imagery, oceanographic buoys, and measurements from ships. They found warm water had spread from the Southeast Atlantic to Eastern Greenland. August temperatures in 2010 and 2012 were warmer than at any other time since 1870. They recently published their findings in the journal Global Change Biology.
Between 1985 and 1994 and 2007 and 2012, waters with temperatures greater than 11 degrees Celsius in the Denmark Strait and Irminger Sea increased by 278,000 square miles—an area larger than Texas. “It’s only in the past two to three years that we can see that the temperatures of the waters east of Greenland have gotten above 10 degrees Celsius in the summertime,” MacKenzie said.
Not only can bluefin tuna tolerate warming Arctic waters more easily, but their prey can too.
Mackerel have been increasing their reach since the mid-2000s, according to MacKenzie, moving from the European continental shelf out toward the Faroe Islands and on to Iceland.
The oily fish is a preferred prey for tuna, which usually only search in waters where the minimum surface temperature is above 11 degrees Celsius, said MacKenzie. That the tuna were brought in with a load of mackerel in 2012 suggests there was a school of tuna hunting the smaller fish, he said.
Finding bluefin tuna off Greenland is more evidence that climate change is shuffling the species swimming about the world’s oceans. Fish generally found in warmer waters are being spotted in regions formerly filled by cold-tolerant species or are expanding their range. Mackerel have moved into the waters south of Iceland, and anchovy now swim the North Sea.
“Around Denmark, we’re seeing species that 15 to 20 years ago would have been extremely rare, such as anchovy and red mullet,” said MacKenzie.