The Chesapeake Bay’s blue crab population has declined by about 50% since 2010, a new study estimates, and while invasive blue catfish are responsible for some of that worrisome decline, the main cause or causes remain a mystery.
That’s the bottom line of a comprehensive stock assessment of the Bay’s crab population, which was launched in 2023 to figure out why numbers of the crustaceans have swooned in recent years, threatening the Chesapeake’s most valuable fishery.
Scientists from five different research institutions and fishery managers for Maryland, Virginia and the Potomac River pored over decades of harvest figures, survey data and dozens of reports and studies. They looked for changes in water quality or climate, among other things, and examined whether an influx of crab-eating fish could be responsible for depleting the population.
The basic problem, they agreed, is poor “recruitment,” meaning there aren’t enough young crabs surviving to produce the next generation. Their 281-page draft report concludes, though, that despite identifying some potential causes, they lacked the data to fully explain it.
“We suspect multiple causes, because the Chesapeake Bay is complex and the blue crab life cycle is complicated,” said Mike Wilberg, a fisheries scientist with the Chesapeake Biological Laboratory of the University of Maryland Center for Environmental Science. He led the computer-driven assessment.
“There’s no smoking gun,” agreed Rom Lipcius, a fisheries scientist with the Virginia Institute of Marine Science who was part of the assessment team.
The draft report is still being finalized, Wilberg said, but the key findings are unlikely to change. He is scheduled to present those Friday, May 8, to the Chesapeake Bay Commission, a tri-state legislative advisory body.
From rebound to decline
Stakes are high for figuring out what’s going on. For decades, the crustaceans have been the Bay’s most important commercial fishery, supplying about one-third of all the crabs harvested nationally and worth $74 million dockside in 2024. They’re also one of the region’s most popular recreational catches.
This is not the first time the Bay’s crabs have been in trouble. Watermen struggled through subpar harvests for about a decade beginning in the late 1990s before scientists convinced managers to clamp down on harvests of female crabs so more of them could survive to spawn. The population rebounded strongly over the next few years.
But warning signs began to appear again about a decade ago, as the survey that Maryland and Virginia scientists perform every winter reported below-average numbers of crabs more often than not. Those dips provoked little alarm at first, because in the past there had been wide swings in abundance every few years and the female population remained healthy.
But by 2022, after three consecutive years of below-average survey results, concern grew. Especially troubling was a deep dive in the number of juvenile crabs. Scientists and managers agreed it was time for a new assessment. With last year’s survey, the estimated crab population was below average for six years running.
No clear culprit
The assessment team created a new computer model that can help fishery managers in the years ahead, said Matt Ogburn, an ecologist with the Smithsonian Environmental Research Center in Edgewater, MD, who also participated in the study. The previous model had focused almost exclusively on female crabs because of their role in reproduction. The new model incorporates a broad set of population influences, including juvenile survival, growth, fishing and natural mortality.
This assessment finds little evidence that crabs are declining because of poor water quality, specifically the warm-weather condition known as hypoxia. Hypoxia occurs when nutrient pollution leads to oxygen-starved “dead zones” where fish struggle to survive. The scientists said there were no reports of mass crab die-offs in such conditions, and their analysis of water-quality data did not suggest it was a major factor.
They did find that blue catfish are contributing to the decline, as watermen have long contended — but the assessment ruled out the invasive fish as the leading culprit.
Introduced in a couple of Virginia rivers in the 1970s, blue catfish have spread to rivers all around the Bay region. As their population increased, the voracious finfish have consumed millions of juvenile crabs, according to the assessment. In 2023, the study estimated, blue catfish consumed roughly 8% of the little crabs that year.
“They’re not the primary cause,” Ogburn said. “There’s something five times bigger than that, and we’re not sure what it is.”
Even so, the assessment acknowledges that as blue catfish continue to spread throughout the Bay and increase in numbers, their impact is likely to increase.
Knowledge gaps
The intensive analysis has given scientists a better understanding of crabs, Lipcius said. But it has also highlighted some big gaps in knowledge that might help explain the current slump.
For instance, the assessment skipped the earliest, most vulnerable portion of a young crab’s life. Females release their eggs in salty water near the mouth of the Bay, and the millions of hatched larvae drift out into the Atlantic Ocean. After several weeks, they make it back to the Bay with the help of currents and tides. They spend their first year sheltered in shallow waters, growing to maturity among underwater grasses.
There was an episode several decades ago in which surveys detected a big drop in the number of crab larvae returning to the Bay, Lipcius said. The National Oceanic and Atmospheric Administration is now leading a study to see if there have been any changes in currents, climate or other conditions that might help explain the decline in young crabs. The results won’t be available for a couple more years, he said.
And though blue catfish have been cleared as prime culprits, predation might still be a significant factor, Lipcius suggested. Striped bass and red drum also feed on the young crustaceans, as do their own species.
Indeed, a March study by Anson “Tuck” Hines, a veteran crab researcher at the Smithsonian research center, concluded that adult crabs are the leading killers of juvenile crabs in mid-salinity waters. There’s no evidence that such cannibalism has increased over time, but Lipcius said that the topic deserves more study.
What’s happening Baywide with young crabs in the shallows isn’t clear, either. The vessels Maryland and Virginia use to survey their population in the winter are too large to get close to shore.
Next steps unclear
Troubling as the crab slump has been for several years now, Wilberg said the situation does not appear to be dire — yet. Though the number of spawning age female crabs is also down, there are still enough to produce a healthy crop of young under the right conditions.
So, he concluded, “it’s not time to panic, but the trend has been downward, which is worrying.”
Without a clearer picture of what’s happening, Wilberg said managers need to maintain a lid on crab harvests.
“My main takeaway from this assessment is that now is not the time to liberalize regulations,” he said. “Blue crabs are declining for largely unknown reasons, and adding more mortality through harvest would probably just lead to a faster decline.”
It’s too early to say how fishery managers will respond. The dredge survey results from last winter are expected to be released soon, providing the latest population estimates.
Some crabbers are already worried that the new assessment will be used to further limit their catches. Daniel Knott, a waterman based in Gloucester, VA, disputes that commercial harvests have thinned in recent years, suggesting that other factors are to blame, such as a drop in the number of crabbers doing the work.
“If they’re telling me the abundance has been declining since 2010, I’m not going to say that’s false, but I’m just not seeing the consequences of that,” Knott said.
The Chesapeake Bay Foundation, the largest Bay-specific environmental organization, called on fishery managers to integrate the report’s insights into their harvest decisions.
“It’s vital that this information drives future blue crab management to protect the species for decades to come,” said Allison Colden, the foundation’s Maryland executive director.
Mandy Bromilow, crab program manager for the Maryland Department of Natural Resources, said the draft assessment “provides significant new information about the blue crab population.” She said her agency will wait to see the final version and then discuss it with Virginia, scientists and others before deciding what changes in harvest rules might be warranted.