Chesapeake blue crabs had an unusual rebound after years of decline. Scientists don't know why

After half a decade of worryingly low numbers, the Chesapeake Bay's blue crab population appears to be fighting its way out of a slump.

This year's field sampling estimated a total population of 349 million blue crabs in the bay, a 46% increase over last year, according to the Maryland Department of Natural Resources and Virginia Institute of Marine Science's annual winter dredge survey. This raised the population overall to the highest it's been since 2019.

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Still, the growth deserves cautious optimism: "We're kind of coming up from the very bottom," said Mandy Bromilow, blue crab program manager at the Department of Natural Resources. The increase is "from basically the lowest point that we've seen."

More than one-third of the nation's blue crab catch comes from the Chesapeake Bay, bringing in at least $74 million dockside in 2024. The results from the annual dredge survey are used by scientists, fisheries and conservation groups to help manage the population.

While juvenile crabs and adult male crabs increased in numbers this year, female blue crabs decreased. Bromilow and other scientists still have no clear explanation for why.

So what's happening to blue crabs in the Chesapeake Bay and what could these changes mean for the future of the population? Here's what to know about the risks they face and what experts say is at stake for the broader ecosystem.

What the numbers show

Every year since the 1990s, the Blue Crab Winter Dredge Survey has raked blue crabs from their mud habitats at sites across the Chesapeake Bay, giving fishery managers and scientists information to help manage the bay's blue crab population.

Behind this year's overall growth was a headline-grabbing 121% increase in juvenile blue crabs and a 43% increase in adult males. Meanwhile, the adult female population dropped by 25% to 81 million, which is below the fishery management target of 196 million.

The increases are a positive sign, said Thomas Ihde, a research assistant professor at Morgan State University's Patuxent Environmental & Aquatic Research Laboratory who was uninvolved in Maryland's assessment. But "unfortunately it only brings us up to the long-term average counts of juvenile blue crab."

Bromilow said surveys are not "infallible" and are a tool for estimating abundance.

"We're going out to certain sites, as much coverage as we possibly can, randomly throughout the bay," she said. "So we're going to be missing pockets of potentially high-density crabs or we're hitting pockets where it's high density, and we think there are more crabs than there."

Michael Wilberg, a fisheries science professor at the University of Maryland Center for Environmental Science who was also not involved with Maryland's assessment, cautioned that the estimated numbers aren't always reflected out in the water.

"In the past, we've seen peaks in blue crabs in that survey that haven't turned out to show up in the fishery — either weren't there originally or didn't survive to adulthood," Wilberg said.

"I think we'd need to see several years like that to really feel like we're in a good spot recovery-wise," he added.

Why blue crabs are at risk

The Chesapeake Bay, which supports more than 3,600 plant and animal species, is the largest estuary in the United States and spans 200 miles from Maryland to Virginia.

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One of the problems the bay and delicate aquatic species like blue crabs experience happens in the winter, when the water's temperature dramatically drops.

"They don't really like the cold," said Wilberg. "When we get a severe winter like we did just this past winter, it can kill off some of the blue crabs."

When the water temperature drops below roughly 50 degrees Fahrenheit, the crabs' metabolism begins to slow, and they bury themselves.

In some cases, this sudden drop in temperature comes with strong winds that push water completely out of shallow areas, where blue crabs can live among sea grasses. With this protective water layer gone, the mud they're hiding within becomes frozen, and the crabs freeze to death.

Scientists refer to these events as "cold shocks," and they're a part of the Chesapeake Bay's natural seasonal cycle. But climate change has made these cold shocks unpredictable, despite milder and shorter winters. On average, about 4% of the total crab population dies during a typical winter. That rate rose to just more than 5% because the cold snaps were much more extended this year.

For the blue crabs that survived the winter, another problem awaits: predation.

A blue crab makes an intimidating threat display. Blue crabs like this one will eat almost anything they can find. Video provided by Kristen Goodhue/Smithsonian Environmental Research Center

"When they're little, snack-sized, there are a lot of different animals that are eating them," Ihde said.

According to a nearly 40-year Smithsonian Environmental Research Center study of crabs along Maryland's Rhode River, a tributary of the Chesapeake Bay, adult blue crabs were responsible for roughly 97% of juvenile deaths through cannibalism. The second-leading cause of death is the invasive blue catfish that hunt and dig up dormant blue crabs year-round, which is happening more frequently as greenhouse gases cause water temperatures to rise.

"As the waters warm, with our increasing temperatures here, everything is affected," Ihde said. "The more southern populations of predators also are frequenting the bay more often. So they're gonna affect this area and eat more crabs."

In the past 30 years, the surface water temperature of the Chesapeake Bay has increased 1.8 degrees Fahrenheit. Projections from the U.S. Environmental Protection Agency indicate warming of up to 4.5 to 10 degrees by the 2080s.

Hotter estuaries are also causing blue crabs to fail to molt thicker shells due to hormonal imbalances, less oxygen and overloads of their metabolism. Hotter temperatures also cause the death of eelgrass, a foundational plant that the crabs use for protection.

But blue crab mortality isn't solely linked to environmental causes. Human industrialization over shoreland habitats has also made it harder for blue crabs to find crucial protection.

"As we harden our shorelines, as we develop those shoreline areas, there's less habitat for baby crabs to get protection from those predators," Ihde said.

Because there are so many factors contributing to the survival of blue crabs, scientists still don't know the exact causes behind increases and decreases in their population.

"Blue crabs are microscopic when they start life, and so they are hard to study. They're hard to find in the ocean," Wilberg said. "There's lots of challenges, but I'm convinced that we can make progress on this and gain a better understanding of what's happening."

What's at stake?

While there's no smoking gun that explains the swing in this year's numbers — scientists say it's likely a mix of environmental and human-caused stressors — these numbers can be used as an overall indicator of the ecosystem's health.

"If we have species that are really declining, that might be a symptom of an ecosystem that's under stress, and we should be trying to do something to make that environment better and healthier," Wilberg said.

If the blue crab population continues to see a decline, experts say it could affect the bay's ability to produce its own food, limiting a significant source of revenue for the Mid-Atlantic.

"Fish and shellfish are a really important part of our food supply. We want to maintain our ability to produce our own food," said Wilberg. "With each of them, we're trying to make sure how we're harvesting them is sustainable."

Ihde, Bromilow and Wilberg have spent their careers seeking solutions to protect the bay and its delicate species. Recreational and commercial fishing was once a major driver of mortality among blue crabs, until Maryland and Virginia passed catch limits.. The 2026 Chesapeake Bay Blue Crab Assessment and Advisory report, which Bromilow, Ihde, and Wilberg worked on together, is separate from the winter dredge survey and found no overfishing in the bay.

"If you leave some crabs this year, that's going to ensure that you will have a fishery that's sustainable for the future for yourself and others," Bromilow said.

Bromilow, Ihde and other scientists working on the assessment believe that, among other actions, expanding monitoring into new areas and shallower waters, surveying the crabs more often and closely monitoring the effects of extreme temperatures on growth and mortality will increase the chances of a full blue crab rebound.