Snow crab (Chionoecetes opilio) numbers in the eastern Bering Sea declined sharply following the marine heatwave of 2018 and 2019.
More than 10 billion snow crabs have disappeared from the Bering Sea in recent years, and we now understand why: new research suggests they were victims of one of the largest mass extinctions in history, caused by a marine heatwave.
A deadly heatwave that swept across the polar waters between Alaska and Siberia in 2018 and lasted for two years resulted in record-breaking ocean temperatures and historic declines in sea ice. These “unprecedented” conditions dealt a severe blow to the sizable population of snow crabs (Chionoecetes opilio) that live in the eastern Bering Sea, according to a new study published Thursday (Oct. 19) in the journal Science.
“The collapse of the snow crab population was a dramatic response to the marine heat wave,” the researchers wrote in their study. However, it appears that the crabs did not die directly from the high ocean temperatures, but suffered from a lack of food.
Snow crabs are small, round-shelled crustaceans that can live up to 20 years on soft seafloors shallower than 650 feet (200 meters), according to the National Oceanic and Atmospheric Administration (NOAA). The species is closely monitored and controlled in the eastern Bering Sea because of its commercial value as a seafood product.
Scientists first noticed the dramatic decline in snow crab numbers during a 2021 study that “recorded the lowest number of snow crabs on the eastern Bering Sea shelf since records began in 1975,” the researchers wrote in their paper. The study was not conducted in 2020 due to the coronavirus pandemic, so scientists didn’t discover the population decline until the following year. But until now, the cause of the decline has remained a mystery.
It turns out that the warm water temperatures caused by the heat wave likely affected the crabs’ metabolism and increased their calorie needs, according to the study. Previous lab studies have shown that the snow crabs’ energy needs doubled when water temperatures rose from 32 to 37.4 degrees Fahrenheit (0 to 3 degrees Celsius). That jump in temperature is equivalent to the change experienced by juvenile snow crabs from 2017 to 2018, which live in cold waters known as the “cold pool” and migrate to warmer zones as they mature, according to the study.
According to the study, the snow crabs' increased caloric requirements were reflected in their changing size between 2017 and 2018, with smaller crabs caught after the heat wave began in the study.
Snow crabs also faced bad timing. As a heat wave hit, the crab population in the eastern Bering Sea also exploded, according to the study. The combination of increased crab numbers and higher calorie requirements proved fatal.
Other factors, such as predation by Pacific cod (Gadus macrocephalus), cannibalism by small crabs, fishing and disease, also likely contributed to the population decline, but “temperature and population density were key variables in the recent collapse,” they added.
The researchers note in their study that the impacts of rapidly rising ocean temperatures and more frequent heat waves caused by climate change are difficult to predict, but the snow crab die-off is “a clear example of how quickly the population outlook can change.”
And while the future of snow crabs in the eastern Bering Sea is currently “uncertain” because they have not recovered from the die-off, the population may eventually find refuge in colder waters to the north. How the mass die-off might impact the broader ecosystem remains unclear.
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