Just off Alaska’s coast, the east Bering Sea usually teems with snow crabs, their spindly legs scuttling across an almost frozen ocean realm. Those legs, prized by seafood lovers, underpin a crabbing industry in the state worth $160m (£143m) annually.
But this year, federal fisheries managers have closed the Alaska snow crab season for the first time, because of record population declines of more than 80% since 2018.
Beyond the unknown ecosystem effects of this loss, the closure has alarmed the fishers dependent on this industry, who will lose millions. Yet the Alaska Department of Fish and Game, which has imposed the limits, says that with the snow crab population in such a dire condition, they are left with no choice.
What can explain the decline? With so much at stake, scientists are investigating potential causes behind the crab collapse.
Origins of crisis
The first thing to understand is that it wasn’t a sudden decline, says Erin Fedewa, a fisheries research biologist for the National Oceanic and Atmospheric Administration (Noaa). “The snow crab story has to start back in 2018,” she says. That year, an unusually high snow crab population count coincided with one of the warmest years, and periods of lowest sea ice extent, on record in the Bering Sea.
This event – which was fallout from the north-east Pacific marine heatwave – was an anomaly attributable to the climate crisis, and associated with die-offs in a number of species including seals and seabirds.
In 2019 (another year of record-breaking temperatures), Noaa’s annual trawl survey in the east Bering Sea – designed to give fisheries managers and fishers an indication of the health of crab stocks – revealed steep declines in the numbers of juvenile crabs.
It is thought the warmer seas brought a unique challenge for these younglings, because they mature in cold-water pools on the ocean floor that are sustained by melting sea ice. Accelerated melting, coupled with warmer waters, likely shrunk the available habitat by pushing this chilled nursery above the 2C maximum that the juveniles need.
But by 2021, the survey revealed that crabs of all ages, not just juveniles, had decreased, Fedewa says. “I just remember being out on the boat and knowing that something was wrong, because at stations that we normally sample several thousand snow crabs, we were catching maybe a couple of hundred.” The situation continued into 2022, when the survey revealed that the snow crab population had dropped from an estimated 11.7 billion in 2018, to 1.9 billion.
Dwindling crab numbers also means a missing food source for predators “that will have to be picked up somewhere else in the ecosystem”, says Darrell Mullowney, a research scientist with the Department of Fisheries and Oceans in Canada who studies snow crabs in the waters of eastern Canada.
A shrinking refuge
The picture that Noaa scientists and others describe is of a dramatically shrinking cold-water habitat on the east Bering seafloor, which constrained snow crabs, and forced more animals into a smaller domain with fewer resources. That is a recipe for starvation – especially as warmer water increases crab metabolism, Fedewa says.
A related theory is that as the snow crabs’ cold-water territory has shrunk, they have been left exposed to predators.
Previously, the frigid waters preferred by snow crabs created what Fedewa calls a “refuge” – a reliable buffer against species less adapted to the cold, such as Pacific cod, that prey on crabs. But, she says, “one assumption as the Bering Sea continues to warm, is that cod suddenly have access to these cold-water grounds that are now no longer cold, and they have access to snow crab”. And yet it can’t be the “smoking gun”, Fedewa adds, because cod aren’t able to consume the larger adult crabs, which doesn’t explain the wider population decline.
Another theory is that these cramped conditions could accelerate the spread of diseases such as bitter crab syndrome, caused by a parasite. These are all active lines of inquiry for Noaa researchers, Fedewa says.
What all these potential drivers have in common, however, is a warming ocean. “All these mechanisms are explained by the dramatic changes in temperature that we saw in 2018 and 2019,” Fedewa adds.
The impacts of fishing
The decline in crab populations has also led to speculation that fishing might be partly responsible.
Crabbers have raised concerns, specifically about commercial groundfish trawlers whose nets indiscriminately rake the ocean floor. In a situation where high-value commercial species such as cod are pushing north into warmer seas – paired with shrinking sea ice that make previously unreachable grounds more accessible – there’s a concern that trawlers are following them, and raking up crab as bycatch. As well as scooping them up in nets, trawlers may also be damaging crab in ways that influence survival down the line, known as “unobserved mortality”, Fedewa says.
“In any fishery there’s always some sort of bycatch. In the springtime when the crab are moulting, they are potentially very vulnerable to interactions with gear.”
When we dig further into the vessel logs, a striking pattern emerges: scores of ships trawling for hundreds of hours across the Northern reaches of the Eastern Bering Sea floor during critical breeding months in areas where snow crabs once could take shelter beneath the ice. pic.twitter.com/uVPfhzn6qD
— spencer 🦀 (@Unpop_Science) October 16, 2022
More research is needed to understand the contribution of fishing to the crabs’ overall decline. Fedewa says agencies such as the North Pacific Fishery Management Council (NPFMC) are taking this seriously, with calls for more research into the impact of trawl gear on crab mortality. “Everyone acknowledges that [fisheries’ impact] is a real possibility. It just needs to be studied further,” she says.
Mullowney echoes the concerns about over fishing, but his research on the impact of trawlers in Canadian snow crab populations has shown that any effect is frequently overshadowed by the effects of the climate crisis. The wide geographical scope of the declines in the east Bering Sea, and the fact that the entire population was affected, suggests a more fundamental driver – such as a change in temperature – is at work, he says.
What happens next?
Since the announcement of the crab fishery closure, the NPFMC has launched plans for a stock rebuilding assessment. Fishers are also hoping to receive federal financial support for the season’s loss.
Meanwhile, Fedewa currently has a number of snow crabs in her laboratory, where she will be exploring the effects of disease, and the influence of temperature on their metabolism, among other factors. It cannot replicate the complexity of the Bering Sea ecosystem, but hopefully it can bring her closer to an answer as to what lies behind the crab collapse – which, she believes, has more than one cause.
“If the Bering Sea has taught us anything, it’s that it’s an entire ecosystem. There’s rarely one process that is the sole driver.”
