Second snow crab season canceled as researchers pinpoint cause of decline

Rewind, for just a second, to 2018 and imagine a series of nets trawling the depths of the east Bering Sea.

Most every year, scientists with the National Oceanic and Atmospheric Administration look for snow crabs. They drop nets for half an hour across 400 different spots in the sea. They haul in and weigh their catch and then calculate a rough number of snow crabs in the area.

For that particular year, those scientists estimated 11 billion crabs were living, crawling, eating and reproducing in the frigid waters below, said Cody Szuwalski, a fishery biologist at NOAA. They had never seen such high numbers.

But by 2021, more than 90% of them would vanish.

That year’s survey showed only about 1 billion crabs remaining. If the crustaceans were few, so were the theories. “Were they overfished?” Szuwalski asked, considering the options. Did they die of disease? Were they eaten? Did they eat each other?

“It was a very precipitous drop,” he said.

Szuwalski and a few other NOAA scientists set out to find an answer and last week published a report indicating that marine heat waves collapsed the snow crab population.

Those findings fit into a long line of data points showing the devastating effects of climate change caused by humans burning fossil fuels. Greenhouse gases are warming the Earth, and as ocean waters absorb that heat, marine temperatures also rise.

[In the aftermath of the Bering Sea snow crab collapse, a ‘cultural, social, and economic emergency’]

Remaining crab populations will take years to rebound, Szuwalski said. That spells pain for fishing fleets who suffered a first-ever canceled snow crab season in 2022-23. Officials in Alaska decided earlier this month to cancel yet another snow crab season.

These swings are likely the new normal, scientists and wildlife experts say because marine heat waves are predicted to become more common and severe as climate change worsens.

“There are going to be winners and losers with climate change,” Szuwalski said.

Whether those winners are species that humans like to eat remains to be seen, he said.

Compounding heat waves

While the ocean captures about a third of the planet’s greenhouse gas emissions, it also absorbs about 90% of its heat, and over the past half-century, marine heat waves have been increasingly common.

Perhaps the most prominent example in recent memory is a widespread marine heat wave that spanned from 2013-16. Now known as “The Blob,” those years of warmer temperatures killed hundreds of thousands of seabirds and led to increased toxic algal blooms.

Marine temperatures then spiked in 2018 and 2019 and again in 2022 and 2023, said Andrew Leising, a research scientist with NOAA.

[Bering Sea crab surveys show populations still low a year after marquee Alaska harvests closed]

Of those three spikes, the 2018 and 2019 marine heat waves resulted in a mass die-off of snow crabs for a few reasons, Leising and Szuwalski said.

First, Szuwalski said high numbers of snow crabs — like the record peak in 2018 — put the whole population at greater risk, particularly because there were greater numbers of mature crabs.

Then, warming temperatures increased their metabolism, meaning the crabs needed more food than normal to survive, Szuwalski said.

So the snow crabs starved, he said. Billions of them.

The depth and location of the marine heat waves also play a role, Leising said. While The Blob’s warm temperatures reached into the Bering Sea and penetrated some 160 meters down, snow crab populations weren’t as high so the entire group wasn’t as much at risk, he said.

The 2018 and 2019 marine heat wave reached into the sea as well, Leising said. While it warmed waters half as deep, the high crab populations and repeated warm waters in recent years compounded and led to the mass die-off.

More recent heat waves have neither reached as far north nor as deep and likely haven’t cut into crab populations nearly as much, Leising said.

No two marine heat waves are exactly alike, Leising said, so their influence on the ecosystems they hit will differ.

Crustaceans are particularly sensitive to these types of water temperature fluctuations, Leising added, so as marine heat waves become more common and severe, the damage they cause will add up.

“The long-term underlying trend is really going to zap them,” Leising said. “Twenty years down the line, it’s probably going to be really tough for them.”

Another season lost

Scientists canceled the 2020 snow crab survey because of the pandemic. Some hoped the population would rebound, but the 2021 data showed the situation had worsened.

“We knew we were in deep doo doo,” said Benjamin Daly, a research coordinator with Alaska’s Department of Fish and Game. “We were in the midst of a population collapse.”

State officials decided to allow snow crab fisheries to keep operating for the 2021-2022 season, which runs from fall to spring, trying to “eke out” even the smallest harvests but returns were dismal, Daly said. The 2022 survey showed snow crab numbers were still shrinking.

So, for the first time, officials with Alaska Fish and Game canceled the snow crab season.

The economic impacts were in the hundreds of millions of dollars, Daly said. A fleet of about 60 vessels run, based out of Alaska, Washington and Oregon, typically with a crew of six or seven people.

This year’s survey remained below the state’s threshold, Daly said, and earlier this month, the department canceled the 2023-24 season.

Snow crab numbers will likely take between three and four years to recover, Szuwalski said.

Marine conditions are expected to be cooler in the months ahead than in recent years, Leising said, which might offer the crustaceans a much-needed break.

Even so, Daly said conditions might sink lower before they improve. Officials in Alaska won’t decide until late next year whether to open the 2024-2025 snow crab season, and even if they do, harvests will likely be modest.

These closures and recovery periods are a sign of things to come, Daly said. Climate change is making for unpredictable conditions. At one point, the snow crab might suffer. Another year, Bristol Bay red crab might be on deck — state officials closed those fisheries in 2021 and 2022. At other points, an entirely different species might fall prey to the changes, he said.

The marine heat wave that killed billions of snow crabs amounts to a lightning strike, Daly said. It was the combination of several factors that led to catastrophe.

“The problem is we’re probably going to have more lightning strikes in the future,” he said.