Cod, haddock, and other Atlantic fish, which used to be confined to the southwest part of the Barents Sea, are now moving rapidly northward. Meanwhile, Arctic fish like sculpin and snailfish are retreating to the northernmost parts of the sea, according to a new paper published in Nature Climate Change.
The authors call the northward migration a “take-over” by more southerly species.
“I don’t think we expected to see such a major signal and such large movements,” said Maria Fossheim, a researcher at Norway’s Institute of Marine Research and the paper’s lead author. Fossheim collected data from 3,800 stations in the Barents Sea over the last decade, along with colleagues at the University of Tromsø and the Knipovich Polar Research Institute of Marine Fisheries and Oceanography in Murmansk.
Fossheim said last year’s Intergovernmental Panel on Climate Change report predicted that fish would migrate at an average rate of 40 kilometres per decade in response to climate change. But her research shows that fish communities in the Barents Sea are moving at speeds up to four times faster than that.
The temperature of the Barents Sea has increased by 0.8 degrees Celsius since the 1980s. Fossheim said that warming, and the fact that the sea is now almost free of ice during the summer, has allowed cod and other Atlantic species to push further north.
Fish numbers growing
But it’s not only the fish species that are changing – the sheer number of fish has also increased dramatically.
“What we found in the northern Barents Sea is that the total abundance has increased by four times,” Fossheim said. “So that’s a very big increase.”
And it’s the Atlantic fish – cod and haddock, for example – that are becoming more abundant. Meanwhile, some Arctic species are dwindling in numbers as they’re pushed further north.
Fossheim said it’s unclear why the Atlantic species are doing so much better than their northern counterparts. Large predators, like cod, may be feeding directly on smaller Arctic fish and outcompeting Arctic predators.
It may also be getting harder for Arctic fish to find food. Many Arctic species feed on bottom-dwelling invertebrates that are nourished by algae as it falls out of the sea ice. As the ice retreats, that algae may be getting scarcer.
Good news for fisheries?
For now, these findings may be good news for the fishing industry in the Barents Sea. Cod is the major commercial fishery in the region, and during the study, researchers found that cod numbers reached a record high not seen since the 1950s.
But Fossheim stressed that these changes are unpredictable.
“It’s not expected that biodiversity will benefit from such big changes,” she said. “So if we see Arctic species or smaller species disappearing from this area, it can affect the stability of the system.”
If cod numbers do keep increasing, that could be a problem for other major predators in the Barents Sea.
Another recent study from the Institute of Marine Research found that while cod stocks have improved over the last decade, the number and body condition of harp seals have declined.
“There’s a cost of having a big cod population, I think,” said Ulf Lindstrøm, one of the study’s authors. “Because they compete for food.”
Lindstrøm doesn’t believe the cod stocks will increase indefinitely. But he said the “take-over” of the Atlantic species is a striking example of the effects of climate change, seen in real time.
“It points out that things can go really fast when you start heating up the system.”