Chesapeake blue crabs are disappearing. The reason remains a mystery.

After hatching, minuscule blue crabs swim toward the light at the surface of the Chesapeake Bay, where outgoing currents sweep them into the ocean. They spend weeks developing in the Atlantic’s salty waters before returning to the bay, where the lucky ones grow big.

In recent years, fewer young have survived those crucial early months, depleting an iconic bay species and hampering one of Maryland’s signature fisheries, which produces tens of millions of dollars dockside each year.

But despite a sweeping, two-year investigation, the reason for this decline remains largely mysterious.

From a peak in 2010, juvenile blue crabs in the Chesapeake Bay dropped about 50% through 2023, said Michael Wilberg, a University of Maryland Center for Environmental Science fisheries scientist who led the new stock assessment alongside state regulators and scientists from five different institutions.

Wilberg presented draft findings of the 282-page report at a meeting of the Chesapeake Bay Commission last week.

The report, the first assessment of its kind since 2011, underscores how little experts still understand about one of the bay’s biggest challenges. The decline of blue crabs has signaled broader changes in the bay ecosystem and hampered the Maryland seafood industry, whose restaurants have increasingly turned to fisheries in Louisiana and Venezuela.

Some fear that the new findings could lead to tighter restrictions on the bay’s struggling fisheries.

Robert T. Brown, president of the Maryland Watermen’s Association, pointed to year-to-year swings in the crab population that he said shouldn’t sway regulation. He recalled one recent season when experts predicted a poor catch, and by August, “all of a sudden it was just like it rained ‘em,” with crabs showing up from one end of the bay to the other.

Early indications suggest this should be an average or above-average year, said Brown, who lives along the Potomac River in St. Mary’s County.

Still, the longtime waterman worries about the impact that blue catfish have had on crabs. Native to the Mississippi River watershed, blue catfish have taken over many Chesapeake Bay tributaries since their introduction to Virginia waters in the 1970s. They can grow to over 80 pounds and eat just about anything in their path.

Wilberg’s research team ruled out blue catfish as the leading driver of the crab decline, but he said they’ve contributed to it. Researchers estimate that in 2023, blue catfish consumed roughly 8% of the bay’s young crabs — accounting for nearly a fifth of the decline — and Wilberg expects their impact could only grow as the invasive fish further infiltrates the bay.

That still leaves a big share of the decline unexplained.

A study released earlier this year found that in the bay’s extensive mid-salinity waters, many young crabs are killed and eaten by bigger ones. Wilberg, however, doesn’t think this phenomenon has increased with time, in part because the bay’s population of male crabs is in decline.

The bay’s stubborn water-quality problems — the focus of a more than four-decade-long, watershed-wide restoration effort — also don’t seem to be the prime culprit. Wilberg’s team found that crabs seem to have largely figured out how to live around oxygen-dead zones that stifle underwater life.

State surveys have shown a crab population struggling for years. Last year’s report estimated that there were about 238 million blue crabs in the bay, the second-lowest count since annual assessments began in 1990.

Still, the crab population is in better shape today than it was in the 2000s, when a decade of record-low harvests prompted Maryland and Virginia to impose stringent new limits on harvesting female crabs.

Scientists credit that step with helping to rebuild populations in the early 2010s, when the blue crab population reached nearly 800 million.

Whether the latest findings could lead to new harvest restrictions isn’t clear. Crabbing season officially opened on April 1, and the latest population count is expected later this spring.

Mandy Bromilow, blue crab program manager for the Maryland Department of Natural Resources, said in a statement that the new assessment provides “significant new information” about the health of crabs in the bay. Officials will review the final version and coordinate with other jurisdictions, scientists and watermen over the next year, Bromilow said.

Blue crabs tend to live short lives — two to three years in the Chesapeake Bay — and their populations can boom and bust.

The odds of revival hinge on the bay’s spawning female crabs, said Matthew Ogburn, an ecologist with the Smithsonian Environmental Research Center in Edgewater. More females increases the chances of plentiful young. This spawning population has also seen a concerning slip, said Ogburn, but there remain enough adult females in the bay to produce a healthy population of young crabs.