Russian And American Scientists Say Warming Water Is Pushing Bering Sea Pollock Into New Territory

Nov 18, 2020

With the loss of the cold water barrier in 2017-2019, historically present in the Bering Sea in summer months, pollock are spreading out and moving north.
Credit NOAA FishWatch

In a new study, scientists have linked warming Arctic temperatures, changing wind patterns, and shifting currents to movement of commercially valuable Alaska pollock in the Bering Sea.

The Bering Sea has seen the loss of a summer cold water barrier in recent years, which used to keep pollock from spreading out and moving north.

But while scientists are seeing drastic shifts in pollock movement patterns, further research needs to be conducted to know what the changes mean for communities like Unalaska/Dutch Harbor and the billion-dollar pollock industry.

"This research is really critical because pollock are a key ecological component of the Bering Sea shelf food web supporting the largest commercial fishery in the U.S. by biomass," said Robert Foy, NOAA's Alaska Fisheries Science Center director. "To get an accurate assessment of pollock abundance so that resource managers can set sustainable catch limits, we have to be able to understand pollock distribution, which certainly looks different under a warm water regime."

While the implications of changing pollock distributions in the Bering Sea are not yet known, this study marks the first time American and Russian scientists have been able to work together to look at why the groundfish species has shown up in new places in recent years.

By looking at historical and recent data, they've been able to confirm both a northward shift of the species, as well as a long-suspected movement of fish between U.S. and Russian waters.

"We were trying to compare what was driving those changes," said Lisa Eisner, a NOAA Fisheries oceanographer and lead author of the study. "And also if it was possible for some of the fish from the eastern shelf to mix with the fish on the western side of the Bering Sea."

While scientists have been surveying Bering Sea fisheries for nearly four decades, Eisner said this specific study was born out of the unusual warming events they've seen in recent years, and it also drew on historical data sets from both the U.S. and Russia. 

According to Stan Kotwicki, program manager for NOAA's Groundfish Assessment Program, pollock generally have a north-south migration. And typically, as ice comes down from the Arctic over the course of the winter, it pushes fish south to feed in warmer areas.

"And, of course, then during the spring, summer and fall, when the ice is melting, pollock move back north," he said.

But as winters warm, and sea ice melts, Kotwicki said the pollock are able to migrate much further north and stay there for longer. That's in part because of a shrinking cold pool — an area of frigid water left behind by melted ice that fish don't like to swim through. According to the study, with declines in the cold pool, there appears to be more intensive mixing between the Russian stock as it moves north and eastward, and the U.S. stock as it moves north and westward.

Lyle Britt leads a team of NOAA Fisheries scientists who do yearly surveys of the eastern Bering Sea shelf and northern Bering Sea to track fish stocks.

Britt said studies like this one often worry people in communities like Unalaska/Dutch Harbor, where the economy is dependent on the commercial fishing industry. He said people can interpret these studies as saying that all pollock are moving north and to Russia. 

But, he said, that's not the message here. It's much more about understanding pollock movements and behavior than it is an alarm bell that all the pollock are swimming out of reach of fishing boats.

"We are now just starting to fully understand really what their migration pattern is, and how they interact with going into Russian waters, or staying in U.S. waters, being constrained by a cold pool or less constrained when there's a limited or even no cold pool," Britt said.

The change in temperatures and shifting sea life has happened very rapidly in recent years throughout the eastern Bering Sea ecosystem, according to Britt. And for him and fellow scientists, the big question is how these environmental changes will affect pollock over the long term.

"Science for us is only as good as the number of observations we have," he said. "And in this case of unprecedented warming, and we really only have a couple of years [of data], it's really hard to draw really large scale conclusions at this point."

But he said scientists are planning an array of studies on how whole ecosystems are changing. Not just on pollock movements, but on how they follow prey like plankton or smaller fish, and on how they interact with bird and marine mammal migrations.

"All of these are questions that are ramping up within our research community," he said. "And we realize it has to ramp up very quickly because of the amount of change we're seeing."