When scientists estimated that more than 10 billion snow crab had disappeared from the Eastern Bering Sea between 2018 and 2021, industry stakeholders and fisheries scientists had several ideas about where they’d gone.
Some thought bycatch, disease, cannibalism, or crab fishing, while others believed it could be predation from other sea animals like Pacific cod.
But now, scientists say they’ve distinguished the most likely cause for the disappearance. The culprit is a marine heatwave between 2018 and 2019, according to a new study authored by a group of scientists with the National Oceanic and Atmospheric Administration.
Mike Litzow is a co-author of the study and the director for NOAA’s Kodiak lab. He said starvation mediated by increased temperatures caused the collapse.
“Really the crab were not able to get the food they needed,” Litzow said. “They were just outstripping the resources that were available to them.”
According to Litzow and his fellow researchers, the crab faced a number of compounding factors: First, higher temperatures meant increased metabolism so they needed more food; on top of that, there was less space for the crab to forage that food; and finally, the crab were just smaller than usual.
Researchers took data from the many possible hypotheses for the disappearance and they examined it alongside the data they have on the collapse. They examined possible mortality from a range of sources, including directed fishing from the snow crab industry as well as bitter crab syndrome — a fatal disease among crustaceans caused by parasites — and trawl bycatch.
“The take-home message is really that none of those other proposed mechanisms explains the collapse with the data we have,” Litzow said.
He said it’s tough to know what the collapse from increased ocean temperatures could mean for other species, but it’s safe to say we’ll probably see more marine heatwaves like this, and they’re likely to be bigger and more frequent, as the world continues warming.
“As we're seeing these big surprising collapses, there is a general awareness that we have to build: we're going to see more of those,” he said. “We need systems that can be resilient to those really outsized, surprising events.”
More carbon dioxide in the atmosphere means warmer temperatures, Litzow said, which is bad news for the cold-loving snow crab. And more greenhouse gasses also mean more acidic oceans, which can also be dangerous for some crab.
“Carbon dioxide that we release through fossil fuels is also taken up by the oceans and has the effect of reducing the pH of the ocean — it makes it more acidic,” Litzow explained. “Because crab use calcium carbonate in their exoskeleton, they're vulnerable to that acidification because calcium carbonate dissolves more and more easily as pH goes down.”
The good news — at least for snow crab — is they’re not as sensitive to ocean acidification as other species.
“In our lab in Kodiak, we've run a bunch of different studies over the years looking at different crab species in different ages — life history stages — in terms of how vulnerable they are to acidification,” he said. “And the good news is it looks like snow crab are one of the more resilient species — like we don't see a strong effect for snow crab the way we do for red king crab or the way we do for Tanner crab.”
Alaska’s snow crab fishery has been closed since 2022 when regulators declared the population overfished.
The snow crab crash in combination with a two-year closure of Bristol Bay red king crab was a devastating blow to Alaska’s lucrative crab fishery, and it left some harvesters and coastal communities, such as St. Paul Island, looking for other sources of income.
Late last year, the Secretary of Commerce announced a disaster declaration for both fisheries to assist communities affected by the closures. That funding has historically taken years to reach fishermen and communities. Some Bering Sea harvesters are still waiting on disaster relief from 2019 requests.
While the bigger picture is still pretty grim — crab have been declining in Alaska since about the early 80s — Litzow said there’s still lots of cold water in Alaska’s seas and with it, hope for the spindly crustaceans.
“Snow crab have bounced all over the place,” he said. “Historically, there have been ups and downs — there have been previous overfished declarations. And we're certainly hopeful as we see small crab showing up in the survey in 2022 and 2023.”
But, Litzow said, the rebound could take some time.
“If conditions stay reasonable for the next, say, four years, we should expect this crab to grow up to the size where they can start to support the fishery,” he said.
The snow crab crash really blindsided Alaska’s industry, and more similar surprises are likely on their way, according to Litzow. He said the more dependent a community or fishermen are on a single fishery, the more vulnerable they will be.
While diversification may be one of the most productive solutions to these kinds of startling crashes, Litzow said it’s also likely one of the most challenging.
“We have this management system where everyone has access — it's allocated in a certain way, or everyone has gear, vessels that's really specialized for just this particular kind of fishery,” he said. “And then when those surprises and those disruptions come along, it's hard in practice for people to have a backup.”
Like other research, Litzow said this study isn’t the final answer, but he said it is an important step.
“It really does a great job of framing out our expectations,” he said. “We should not expect that the crab were just gone somewhere else, down on the Slope, or up in Russia, or anything like that. I think this study really makes it clear that they died, and gives us our best explanation for why that happened.”
