Science

Why Did 11 Billion Alaskan Snow Crabs Suddenly Disappear?

Scientists are still trying to understand the mystery that has shut down a major fishing industry—but they have theories.

Photo illustration of a "missing" sign with an Alaskan snow crab
Photo illustration by Slate. Photos by Getty Images Plus.

Earlier this month, Alaska announced that it had canceled the entire snow crab harvest for the year. The reason? Nearly 11 billion crabs had suddenly disappeared from the Bering Sea.

The news heralded a catastrophic population collapse for the animals, in which nine out of ten died out between 2018 to 2021. It’s a terrible development for those who make a living harvesting the crabs in a region of the world that’s warming unusually fast because of its proximity to the North Pole. (Alaska officials also canceled the fall Bristol Bay red king crab harvest for a second year in a row.) This isn’t a small industry; Alaska’s crab fishing is worth more than $200 million a year. The sudden shutdown has left the state, well, shell-shocked.

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Experts widely agree that global warming likely played a major role in the crabs vanishment—but they are still trying to figure out the details of how. To get a sense of what may have caused this mysterious mass die-off and why the fishing and scientific communities had been taken by surprise, Slate spoke with Dr. Mike Litzow, the shellfish assessment program manager at the Alaska Fisheries Science Center for the National Oceanic and Atmospheric Administration. Litzow’s team conducts the survey that measures snow crab populations in the Bering Sea each year, which in turn helps populate the models that the state uses to set its fishing quotas.

This interview has been condensed and edited for clarity.

Slate: This seems like it would have been a dramatic thing to watch unfold. When did you realize the scope of this population decline?

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Litzow: Dramatic is definitely the right word. In 2018, we observed the highest abundance of snow crab that we’ve ever seen. Our estimate was that there were 11.7 billion snow crab in the southeast Bering Sea—largely smaller animals that were about to grow large enough to harvest. So in 2018, things were looking better than they’ve ever looked for snow crab.

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In 2019, we went out and did the survey. And while it was still good, suddenly, the abundance of small animals was half as good as it had been. We were concerned but not alarmed—just sort of wondering what was going on.

In 2020, of course, COVID hit, so we didn’t go out. And then 2021, we went out, and there were 10 billion fewer animals than there had been in 2018. So the abundance went from 11.7 billion in 2018 to 940 million in 2021. So there’s just a wholesale collapse in the population. And it was very sudden, and was not predicted.

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We went out this year, in 2022, and we got results that confirm the 2021 survey—that the crab are really gone. It’s not like they moved somewhere else. It really looks like a mass mortality event.

What can we know with certainty as to what happened here?

Our data show that in 2018, 2019, and 2020, the Bering Sea was just far warmer than anything we’ve seen before. That [change] actually began in 2016. And we know that you can’t get the Bering Sea that warm without human-caused global warming.

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If we look at the core area of the snow crab range on our survey, the average temperature from 1982 to 2012 was 1.3 degrees. In 2018, we had 3.5 degrees. All available data really suggest this is a climate change story.

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Have we ever seen anything like this die-off before?

No, the temperatures we’ve seen in the Bering Sea since 2016 have absolutely no precedent. It’s been uniquely warm.

How exactly does a 2-degree temperature increase lead to the deaths of billions of crabs? 

It’s hard to disentangle, but there’s a few leading hypotheses. The first thing to know is that snow crab are an arctic animal. In the Bering Sea, we only find them in areas that have winter ice cover, with bottom temperatures below 2 degrees C. In this ecosystem, two [additional] degrees really takes you over an ecological threshold.

If your bottom temperatures in the system are colder than 2 degrees C, it’s an arctic system. Most of the predatory ground fish that are more abundant to the south, like Pacific cod, don’t tolerate those temperatures. But then if you warm that up to 3, 3.5 degrees, it’s like flipping a switch. There are more subarctic ground fish that can come into that area. So one hypothesis is that warming waters allowed predatory ground fish into the snow crab range, and that there was a massive predation event.

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Another hypothesis is that there were some diseases that affect this population—that when you warm them up, you get an increased incidence of those diseases.

And then another hypothesis is that because you had this high abundance of crab [from the 2018 population boom] in a very small area [because they clustered in pockets of cold bottom water] with very high metabolic demand, they outstripped their available food, and there was starvation. It’s hard to disentangle those different explanations. They’re all plausible. But it appears some combination of those is the actual link between the warming and the mortality event.

How exactly did you figure out that all those crabs disappeared? How did this survey work?

We have 375 standardized stations across the southeast Bering Sea, which is sort of the breadbasket for all these incredibly productive fisheries. Every year, we go out and we sample these 375 spots, and we set a bottom trawl from a commercial fishing boat, which we run for half an hour. We then process the catch: identify all the fish, the crabs, sex them, measure them, collect all kinds of different biological data. And that gives us an estimate of the abundance of fish and crabs. It gives us a comparison through time of how the populations are changing.

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So we take the survey data, we take data from the fishery, we take data from bycatch from other fisheries that capture snow crab. You take all these datasets, and they all tell a different story. So the best way to understand what’s going on is to use a mathematical model of the population that incorporates all these different datasets.

How do you know the crabs didn’t simply move outside the range of your stations?

When the 2021 survey had such an extreme result, maybe it just warmed up and the crabs moved was an explanation to examine. But we also have a survey in the northern Bering Sea, and so we can confirm that the crab did not move north, as you might expect. And there was a possibility discussed about the crowd moving deeper in the Bering Sea, deeper than the area that we sample. But when you look at the available habitat in deeper waters, there’s just not enough room for 10 billion crab to be there.

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Then there was interest in whether the stock moved to Russian waters. We don’t have data from the Russian side to test that hypothesis. But the available information from the Russian side doesn’t really support the idea.

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Are snow crabs uniquely vulnerable? Are there other animals we should expect to see die off as well?

Snow crab are unique in that they’re by far the most economically valuable arctic species. Most Bering Sea fisheries operate on subarctic or boreal species: things like cod and pollock, or red king crab or red salmon. [Editor’s note: The “subarctic” refers to the slightly warmer region just south of the arctic.] And these species, you wouldn’t expect them to have such immediate negative consequences from a warming event. Some of these species have been seen in huge numbers moving up north and occupying this formally arctic habitat. So they’re not collapsing in the way snow crab have.

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Did we know the degree to which snow crabs were susceptible to warming temperatures? Why did this die-off take people by surprise?

People have been saying for decades that if the Bering Sea loses its sea ice, we should expect snow crab to be one of the stocks that’s most strongly affected. And we have known for decades, big picture, that the arctic is warming up more quickly than the planet as a whole. So we’ve known that big changes were coming, in sort of a general way. But there is all this sort of natural climate variability, and it’s very hard to predict what the temperature is going to be next year or the year after. We just know that in 50 years, it’s going to be much warmer than it is now. But for a population like snow crab, what really matters is this year or next year.

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And we have seen in the Bering Sea really abrupt, really strong warming, that in some cases was a lot more rapid than what we had expected. Until recently, the oceanographers were telling us that the northern Bering Sea would be ice-covered in the winter for our lifetimes. And then in 2018 and 2019, the northern Bering Sea lost its ice cover. And so we’re seeing this switch to ice-free conditions. It just happened more quickly than the scientific community was expecting.

And we’ve never seen the Bering Sea this warm before. So it’s hard to predict the exact consequences. But now that we have this observation of the system in a really warm state, it’s easier to make predictions about what we should see next time. So the next time we see such warm conditions and very low sea ice cover in the preceding winter, we know that that’s going to set up the bottom temperatures to be much warmer than normal. We’ll definitely have more of an ability to predict the response.

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So what do we know about the future of the snow crab population?

This year, the Bering sea temperatures have reverted to something that’s more similar to average conditions over the last 30 years. And at the same time, we’re seeing a lot of small snow crab appear in the survey this year. So there’s hope that if we get some run of colder years, and if the small snow crab are able to survive up to commercial size over the next four to five years, there might be some hope on the horizon for this fishery.

For the fishery, unfortunately, it doesn’t look promising for the next four or five years. For those small crabs that appeared in the survey this year to live long enough to get big enough to support the fishery, that would be four or five years. I mean, it’s bad. And there’s real hardship involved. But it doesn’t mean it’s going to be this way forever. So we’re certainly hoping for that run of colder years. And we’re hoping for the smaller crab to revitalize the population.

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And there’s another side. There are a lot of snow crab up in the northern Bering Sea. Typically, they don’t grow large enough to be fished. We think that’s because the northern Bering Sea is so cold. So as the northern Bering Sea continues to warm, there’s certainly the potential for snow crab to grow to fishable size in that region. So over a slightly longer term, it might be that there’s room for the fishery to move north, as we see animals getting larger from the north. So we’ll just have to see how that plays out.

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