Deep under the cold, dark waters of the Chesapeake Bay, the answer to whether the decimated blue crab population can survive lies buried in mud.

Tens of millions of female crabs are scattered across the floor of the lower bay in southern Virginia, where the estuary pours into the Atlantic Ocean, waiting out winter for one of the most important events in their short lives. When spring comes, they will inch closer to the ocean with billions of eggs.

It’s a critical time because the blue crab population is reeling, facing some of the lowest numbers in history. Officials are desperately hoping that steps taken to protect females last year will allow the fishery to rebound from the edge of disaster. But those efforts are also mired in a debate over the best way to protect the crabs.

As females go, so go blue crabs. Last year, scientists estimated that there were only 68.5 million females old enough to spawn, far below the 215 million that officials say are needed to overcome natural threats such as predators and cold — and human threats such as commercial overfishing — without depleting the population. This year’s count is underway by Maryland and Virginia scientists at 1,500 locations.

The blue crab, Maryland’s state crustacean and a symbol of pride for the region as much as a resource, is more threatened now than at any time since biologists started to record numbers in the late 1940s. Both watermen and state officials are deeply worried about blue crabs’ future. Sixty years ago, the Chesapeake Bay yielded 75 percent of the crabs harvested in the United States; now the withered stock yields about 35 percent, according to a report by the Maryland’s natural resources department.

The blue crab life cycle takes about two years to complete.

Some watermen say the steps taken by Virginia, Maryland and the Potomac River Fisheries Commission to rescue crabs after the population’s free fall in 2008 haven’t worked. The states cut the number of females that can be fished by about 30 percent. When the stock rose only to fall hard again last year, Virginia cut the number of females that could be fished by an additional 10 percent during the spawning run.

Smithsonian scientists worry that efforts to save females might produce a serious side effect: overfishing males. When male crabs decline, those remaining mate more often, and sometimes can’t regenerate their sperm supply quickly enough, according to a study by the Smithsonian Environmental Research Center.

“I don’t believe it’s going to get better, doing what we’re doing,” said Bill Mullis, chair of the Virginia Blue Crab Industry Panel of commercial fishermen and owner of B&C Seafood, based in Newport News, Va. “We’ve been doing this eight years and the stock is the same as it was when it was declared a disaster. It’s like disaster has become the status quo.”

Virginia Marine Resources Commissioner John Bull disagreed, saying regulations worked. The population rose over four years to a 21-year high of 765 million in 2012, then dropped like a stone the next two years.

“We’re not happy with the status of the crab stock,” Bull said. “But fishery managers are stunned by the curveball Mother Nature has thrown at us.”

Bull said female crabs and their offspring were hit by forces managers couldn’t control in the past two years. A plague of red drum fish that prey heavily on juvenile crabs decimated them in 2013. At the same time, grasses that conceal juveniles diminished.

“Baby crabs didn’t have a place to hide, and they were dinner,” Bull said. The next year, things got worse. Managers worked successfully to reduce the red drum by fishing them out, but another problem came out of nowhere.

A harsh and sudden winter wiped out nearly 30 percent of adult crabs, marine scientists in Maryland and Virginia said, ending any hope for a fast rebound. The total weight of crabs watermen harvested in 2014 was 37 million pounds, the lowest ever recorded.

The low supply and high demand for crabs caused prices to spike, said John Rorapaugh, the sustainability director for ProFish, a wholesale seafood supplier in the District.

A pound of lump crab meat ranges between $19 and $26 in the April-to-December season. But last year, it started at the high mark and stayed there, Rorapaugh said. For one restaurant, Canton Dockside in Baltimore, the price of crab meat jumped 40 percent, eatery managers said.

The total number of males, females and juveniles was a mere 297 million last year. The last time it was that low — at 293 million in 2008 — the U.S. Chamber of Commerce declared the Chesapeake’s commercial crab fishery a failure. The estimates paled in comparison to the 852-million count in 1993.

Heavy lifting for females

Adult male crabs are brawny, but the weight of maintaining the habitat falls on females. They do the heaviest lifting, by far, in a yearly trek to replenish the species.

After they mate from May to early October, nature beckons them to the mouth of the bay. Depending on where they depart in the estuary, pregnant females known as sooks migrate as far as 150 miles to get to a spot in the lower bay near the salty Atlantic. Their offspring have a higher rate of survival in waters with elevated levels of salinity.

Walking on six skinny legs and swimming with two tiny flippers, the perilous trip takes the entire fall.

“They’re all moving down . . . waves of females” from as far north as Havre de Grace, Md., said Romuald Lipcius, a professor of marine science at the Virginia Institute of Marine Science, who studies crabs. The epic migration puts the more storied march of emperor penguins in Antarctica to shame.

In a bay choked by pollution, killer invasive species and hundreds of fishermen, every step could be their last. They dodge crab pots, predators such as striped bass and red drum, and polluted dead zones where water lacks oxygen.

“It’s an amazing thing,” said Anson “Tuck” Hines, director of the Smithsonian Environmental Research Center. “You can think of it like a migration of birds, except they’re not moving in a flock.” The bay is huge, so “they’re not shoulder to shoulder,” but scattered out, “moving as individuals.”

Females burrow in mud and stay almost perfectly still in winter to conserve energy at a time when food — oysters, clams, mussels, snails, insects, worms and some fish — is scarce. After the thaw, they move closer to where the bay’s fresh water meets the salty Atlantic.

Each female lays between 750,000 and 3 million eggs starting in late April, but an average of only one in each brood is likely to live.

The hatchlings take the form of microscopic shrimp that fan out to the continental shelf in the Atlantic in the early stage of life. After a month, they grow to look like lobsters the size of pin heads and ride flood tides northward into the bay. At the end of summer, they become juveniles that finally take the typical shape of a blue crab as they walk up the bay in grasses on the floor.

They journey north on fall flood tides — and at every stage they are a tiny crab feast, disappearing into the mouths of virtually everything that swims, including bigger crabs.

Considering the importance of the event, Mullis said he doesn’t understand why officials allow females to be harvested shortly after the fishery opens in March.

“I feel there are too many sponge crabs going to market,” Mullis said. Females carry up to 2 million eggs each in a foamy sack called a sponge. There should be a moratorium on collecting them, he said: “Somebody needs to stop carrying the pregnant mothers to market. They are keeping millions of eggs from hatching.”

Bull said that in cutting the number that can be harvested by 10 percent, Virginia was trying “to not put a debilitating hardship on commercial crabbers” trying to make a living. “A rule of thumb,” he said, “has been that if you ask 10 watermen their opinion on one issue, you get 30 back.”

Beautiful, savory swimmer

Blue crabs are a tropical species called Callinectes sapidus, meaning beautiful and savory swimmer. Their origin dates back tens of thousands of years in the Caribbean, long before the Chesapeake Bay formed. The warmth-loving species stretches from Argentina and around Bermuda to Cape Cod.

Even for male and female blue crabs known to eat each other, there can be touching moments during the May-to-October mating season. Hines has watched males do a little wobbly underwater dance to lure females out of hiding.

During courtship, the female is molting, meaning her top shell softens as she grows a new and larger one, making her more vulnerable to predators.

Males climb atop females and latch on like a backpack to protect them. This happens only once in a female’s life. They store sperm and can use it several times to fertilize eggs.

Once mating is over, sometime before summer ends, the long southern trek of females in the Chesapeake Bay begins.