Last winter, something unprecedented happened in Alaska. For the first time on record, there was no sea ice in the northern Bering Sea, and biologists are now scrambling to figure out how that will affect scores of area fisheries – from crab to salmon to rockfish to various pelagic stocks – in the coming years.
Because there are few fisheries in the northern Bering Sea, historically it has not been subject to as much surveying as the southeastern Bering Sea and the Gulf of Alaska. But the boundary between the north and south – set at around 60 degrees north – is for research purposes, and stocks migrate freely over that boundary.
According to Diana Stram, a fisheries analyst and management plan coordinator at the North Pacific Fishery Management Council for the past 16 years, warming waters have highlighted the increasing connectivity between the two sides of the boundary.
“We’ve had the warm blob in the Gulf of Alaska, which caused the huge Gulf cod decline, in addition to the extremely warm waters this year in the Bering Sea, and that has caused a lot of species to move north. So we’re seeing these warm water masses pushing fish north and meanwhile increasing metabolic demands in fish and causing higher mortality,” Stram told SeafoodSource.
When fish like cod and pollock head north, it’s the sea ice that pushes them back down, Stram said. Without that ice, biologists are left in the dark, unsure where stocks end up.
Elizabeth Siddon is a research fisheries biologist for NOAA who heads up an ecosystem status report that looks at conditions in the Bering Sea, and how those conditions might figure into managing Alaska’s fisheries. She is investigating the possibility that pollock and cod wintered over in the northern Bering Sea last year and never made it back to fishing grounds in the Aleutians and the Gulf.
Siddon points to a concerning sea bird die-off in 2018 that lasted longer, had a wider geographical range, and affected more species than normal, coupled with a decimated biomass, especially in forage fish such as herring and year-zero cod.
Siddon hypothesizes that if the cod and pollock did stay up north over the winter, they may have been responsible for decimating the biomass – eating the herring, for example – and causing the sea bird die-off. If this is the case, it would signify an unprecedented shift in the region’s food web brought on by the warmer water temperatures – a shift that casts uncertainty over scores of stocks in the region.
“The northern Bering Sea has always had sea ice to some extent, until 2018. There was a weird trifecta of [weather conditions] that resulted in there not being sea ice last winter and that has really shifted what’s happening,” Siddon said.
Beyond guiding different species in their migrations, sea ice functions as a structural organizing force in the Bering Sea. When it retreats in the spring, it leaves behind a thumbprint of bottom water known as the cold pool. That mass of colder, fresh water sits at the bottom of the sea and helps structure the ecosystem; some species avoid the pool and others prefer it.
Miranda Westphal, an area manager for the Alaska Department of Fish and Game in Dutch Harbor, told SeafoodSource the absence of a cold pool makes it even harder to forecast crab stocks, which are already unpredictable and volatile.
“No one really knows how these stocks will respond, with the unanticipated super warm year and no cold pool,” Westphal said. “We’re seeing unusual things, like female red king crab in very deep water.”
Stram echoed Westphal’s concerns.
“Really warm waters this year, no cold pool. That’s had a huge effect on crab availability, as well as groundfish availability. Over the past seven years, we’ve just been seeing a general decline, particularly in the king crab stocks,” Stram said.
The temperature and salinity from ice melt also act as a stabilizing band in the water. Without that stability, the water column mixes and churns more easily, an action that kills the spring bloom of phytoplankton and decimates the primary production that seeds the ecosystem with energy.
“That means that up the food chain – the zooplankton, things like copepods and krill – were in pretty low abundance,” said Siddon, while pointing out that last spring saw a weak bloom in both the north and the south.
And while Siddon said the conditions that prevented sea ice from forming in the northern Bering Sea last winter are unlikely to be repeated any time soon, biologists are getting a glimpse of the potential instability that could be unleashed as warmer waters push north.
“It’s difficult without the sea ice pushing animals around,” Westphal said. “We don’t have the water temperature structure to define where the stocks typically reside, so we’re just sorting of waiting to see.”
Photo courtesy of National Park Service/Stacia Backensto