Scientists have discovered acoustic evidence of a small but unexpected trove of small fish and zooplankton hundreds of meters below the surface of the icy Central Arctic Ocean, an area that will become more exposed due to climate change in the coming decades.

The findings indicate that whatever fish dwell beneath the ice aren't plentiful enough to drive sustainable fisheries. However, they're probably a crucial source of nourishment for larger predators in this remote environment, the researchers reported on March 19 in Progress in Oceanography. 

"If the ice wasn't melting so fast in the Central Arctic Ocean this question would not have come up," said Pauline Snoeijs-Leijonmalm, a professor of marine ecology at Stockholm University and first author of the study. "But I feel that it's important ... that we do this research and find out how the ecosystem functions and the role of these fish in this ecosystem so that we can give it the proper protection." 

The Central Arctic Ocean, a 3.3 million-square kilometer area around the North Pole, is permanently covered in ice and can only be reached by icebreakers. Although Arctic cod have been spotted at the surface, the meters-thick pack ice has prevented scientists from gathering fish from deeper waters. 

"Nobody has ever looked at fish there,"Snoeijs-Leijonmalm said. "There is a large gap of knowledge concerning the whole ecosystem." 

However, global warming is rapidly altering this region, and the ice cover is becoming thinner and less extensive. 

"In about 30 years, the models say that the North Pole will be ice-free in summer," Snoeijs-Leijonmalm said. "When you get seasonal ice, you get a much more dynamic ecosystem." 

Much of the Central Arctic Ocean lies in international waters, and as the North Pole loses its summer ice it will become more accessible to fishing vessels. However, in 2018, nine nations and the European Union agreed not to engage in commercial fishing for 16 years after the Agreement to Prevent Unregulated High Seas Fisheries in the Central Arctic Ocean entered force. 

To better understand this vulnerable wilderness and how it might be affected by human activities, Snoeijs-Leijonmalm and her colleagues examined data collected during a 2016 expedition of the Swedish icebreaker Oden. Researchers gathered oceanographic and acoustic measurements from 13 areas across the Central Arctic Ocean, 12 of which fell within the high seas. 

The swim bladders of fish contain a small bubble of gas that sonar pings bounce off of, Snoeijs-Leijonmalm says. Based on how the signal is scattered on its way back to the receiver, researchers can detect the presence of fish. 

Researchers have detected a so-called deep scattering layer across the world's oceans at depths of 200 to 1,000 meters. This layer is filled with fish and zooplankton that migrate up toward the surface every night to feed before retreating to the safety of deeper waters during the day.  

To their surprise, the researchers found that such a layer could be detected in the open waters of the Central Arctic Ocean, as well. 

"We didn't believe it at first," Snoeijs-Leijonmalm said. "Everybody thought there would be no layer there." 

She and her team discovered the "living layer" at depths of 300 to 600 meters. In this region, a layer of relatively warmer and saltier water is carried in from the Atlantic Ocean, potentially offering fish and zooplankton a haven in an otherwise inhospitable habitat. 

That said, the fish that the researchers could detect seemed to be small and scarce. The team calculated that there were perhaps 50 kilograms of fish, or around 2,000 individuals, per square kilometer around the North Pole. 

"That's very, very little; it's almost negligible," Snoeijs-Leijonmalm said. However, these fish likely play a vital role as food for whales and seals, which are in turn preyed upon by polar bears.

"You can find seals at the North Pole, and everybody has wondered, how can they survive?" Snoeijs-Leijonmalm said. "And it is probably because there are fish and the seals can dive a few hundred meters." 

Compared with other areas, the Central Arctic Ocean is low in nutrients and will probably remain so in the future. 

"The Central Arctic Ocean will never be an area where you will get lots of fish," Snoeijs-Leijonmalm said. "In Antarctica you have large protected areas, and in the same way we need to protect the area around the North Pole from exploitation." 

She and her colleagues weren't directly able to observe any fish from the deep scattering layer. However, this summer they will return to the Central Arctic Ocean with more sophisticated acoustic instruments, fishing equipment and cameras in hopes of capturing some of the elusive fish. 

In addition to potentially confirming that there are fish deep within the arctic waters and establishing their identities, Snoeijs-Leijonmalm said, "If we get fish samples, we can look in their stomachs [and] we can know what they live off — how they can survive in this extreme environment." 

The study, "A deep scattering layer under the North Pole pack ice," published March 19 in Progress in Oceanography, was authored by Pauline Snoeijs-Leijonmalm, Christian Stranne and Martin Jakobsson, Stockholm University; Harald Gjøsæter, Randi B. Ingvaldsen, Tor Knutsen, Rolf Korneliussen, Egil Ona and Hein Rune Skjoldal, Institute of Marine Research; Larry Mayer, University of New Hampshire; and Katarina Gårdfeldt, Chalmers University of Technology.