That cold pool is a body of water below 2 degrees celsius (35.6 degrees fahrenheit) left on the eastern Bering Sea bottom after sea ice retreats. It has a strong influence on distribution of walleye Pollock and most flatfish, and Pacific cod avoid it.
As ocean temperatures have warmed and sea ice diminished, the cold pool has shrunk and last year, for the first time in recorded history, the pool was gone.
“Until 2017, cod would usually avoid the cold pool – they’d bump into it and go no farther north,” Spies said. “Then in 2018, the cold pool was gone. There was nothing stopping fish from going north.”
That prompted more questions about where those cod were headed.
Biologists don’t know if the fish whole life cycle has shifted northward or whether they will return to their typical southeastern Bering Sea spawning areas for the winter and then undertake long feeding migrations north during the summer.
“This emphasizes the need for continued northern and southeastern Bering Sea surveys and for tagging studies,” Spies said. “This is probably not the only species we will see changing. We really need to monitor the northern Bering Sea with surveys every single year until things stabilize,” she added.
The report indicates that until recently Pacific cod were rarely encountered in the northern Bering Sea. In the 1970s, fishery surveys reported “trace amounts” of cod. A 2010 survey, by the Alaska Fisheries Science Center, estimated that the entire northern population of Pacific cod amounted to approximately 3 percent of the large southeastern Bering Sea stock supporting the commercial fishery.
The 2017 summer survey however, recorded dramatically higher abundances in the north, a 900-fold increase since 2010. At the same time, southeastern Bering Sea abundances were down by 37 percent from 2016. Researchers noted that the increase in the north nearly matched the decrease in the southeastern Bering Sea. In 2018, survey results revealed more cod in the northern than southeastern Bering Sea.
Because cod show natal homing and spawning fidelity, they return to where they were spawned to spawn, the spawning population is considered representative of a population.
Spies’ team compared genetic markers of the northern cod with spawning fish from the three other stocks.
“We found that the northern fish were indistinguishable from the southeastern Bering Sea population,” she said. “That meant the fish were moving north from their historical southeastern Bering Sea habitat.”