What made billions of snow crabs disappear from the Bering Sea?

A snow crab (Chionoecetes opilio) pictured underwater in Canada. There were once billions of snow crabs in the Bering Sea, but the population has dramatically declined in recent years.

(Image credit: LaSalle-Photo via Getty Images)

Snow crabs in the Bering Sea once numbered in the billions. But after a recent and massive population crash the crabs have all but vanished from these waters — and they may not be coming back anytime soon.

In 2018, about 3 billion mature snow crabs (Chionoecetes opilio) inhabited the Bering Sea along with roughly five billion immature crabs, the Seattle Times reported. But by late 2021, those numbers hovered around 2.5 million and 6.5 million, respectively — a loss of nearly eight billion crabs in just three years. In February, the National Marine Fishing Service issued an official overfishing notice for the population, and in early October, officials at the Alaska Department of Fish and Game (ADFG) made the difficult decision to cancel the season's snow crab harvest for fear of wiping out the crustaceans altogether.

Latest Videos From

Watch full video here:

Snow crabs thrive in the cold northern waters of the Bering Sea floor. For these crabs, water temperature isn’t just a matter of comfort; it plays a critical role in their lifecycle. As seawater cools, it becomes less salty and less buoyant, causing it to sink to the bottom of the ocean. Marine biologists refer to this chilly layer of water as the "cold pool, according to the National Oceanic and Atmospheric Administration (NOAA). Many fish and other types of marine life avoid the cold pool, but for juvenile snow crabs, it’s a sanctuary. With virtually no predators willing to venture into this layer's frigid waters, young crabs can grow up in peace.

But lately that protection has waned. Record heat waves in 2016, 2018 and 2019 stunted cold pool formation in the Bering Sea, leaving baby crabs vulnerable to predators, according to a Sept. 2 report published by NOAA. What's more, Westphal said, the warmer waters likely sped up the adult crabs' metabolism, causing them to starve. As anthropogenic climate change progresses over the next few decades, these types of heatwaves are projected to become more common, according to the report.

In addition to climate change, some commercial fishing practices may have contributed to the sharp decline in crab numbers. Trawling vessels targeting other marine species in the Bering Sea frequently encounter, catch and discard unwanted snow crabs as "bycatch" ” And when snow crab fishers haul a catch aboard, they toss out crabs that are deemed too small, too young, or whose shells are discolored or marred in some way.

Sometimes the crabs survive the shock of being suddenly transported to the surface and then thrown back into the water — but often they do not. In 2020, the ADFG estimated that over 30% of all snow crabs that were captured and tossed back into the Bering Sea died as a result, the Seattle Times reported in April. NOAA’s 2021 assessment for Bering Sea snow crabs corroborated these grim findings, with snow crab mortality rising that year and populations plummeting.

Joanna Thompson is a science journalist and runner based in New York. She holds a B.S. in Zoology and a B.A. in Creative Writing from North Carolina State University, as well as a Master's in Science Journalism from NYU's Science, Health and Environmental Reporting Program. Find more of her work in Scientific American, The Daily Beast, Atlas Obscura or Audubon Magazine.