© Alyssa A. Botelho/The Washington PostA bushel of "Number 1" male blue crabs, the largest crabs that the watermen sell.
For the second straight year, a harsh winter killed more than a quarter of adult blue crabs in the Chesapeake Bay.

But a baywide survey of the crab population released Monday said there was encouraging news in spite of the blow. The overall population of the beleaguered crustacean climbed modestly from a catastrophic low last year.

The yearly winter dredge survey conducted by Virginia and Maryland marine scientists estimates that 411 million crabs are in the main stem of the bay and its tributaries, a 38 percent increase from last year's critically low population.

Officials at Virginia's Marine Resources Commission greeted the news as a positive sign but said it's probably not enough to lift strict limits on the numbers of blue crabs that can be commercially fished.

"This is a step in the right direction, but we are not out of the woods," said Virginia Marine Resources Commissioner John M.R. Bull. Calling the gains modest, Bull said the state will continue to cautiously manage its crab stock. "More work needs to be done to boost us above modest abundance levels."

To recover, bay blue crabs need more breeding-age females, and by the dredge survey's count there are a few more. Their numbers increased from a number that meant the stock was depleted, 68.5 million in 2014, to 101 million this year.

The Chesapeake Bay Stock Assessment Committee ruled that a female population below 70 million is depleted and in danger of collapse, and any number below 215 million is unhealthy for the overall population.

The bay's spawning-age female stock has surpassed the healthy threshold only once in the past decade, and only twice in the past 15 years.

Each year females embark on an epic downstream trek baywide after mating in spring and fall to release sacks of eggs where the bay's fresh water meets the salty Atlantic Ocean in Virginia. They dodge commercial crabbers and predators through nearly the entire route.

Like crabs throughout the bay, they bury themselves in mud and dirt in an attempt to survive winter. Blue crabs are a tropical species that originated in the Caribbean Sea and migrated south as far as Argentina and north to the Chesapeake Bay and beyond. They are established as far north as Massachusetts but are sensitive to cold.

Last year, 28 percent died over the winter. This year's winter was worse, killing 29 percent. Maryland and Virginia conduct the dredge survey at 1,500 sites in the bay watershed during the overwintering period, December through March.

Females that manage to survive release their eggs in spring and return to various rivers and creeks upstream. Microscopic hatchlings spend their early lives at the mouth of the Atlantic before starting a deadly trek upstream in fall, whenmost are eaten by predators, including adult crabs.

Maryland and Virginia have two options that will likely hurt watermen who depend on the commercial crab fishery: continue to conserve females by reducing the number that can be taken in the March-to-November open crab fishery, and continue to conserve juveniles in the hopes that they reproduce in higher numbers in 2016.

This year's spawning female population is still well below the level of 160 million to 215 million that the National Oceanic and Atmospheric Administration says is needed to lift the overall population to its estimated strength of 828 million in 1991.

Juvenile crabs jumped from 199 million to 269 million, a 35 percent increase, and now comprise the bulk of the crab stock.

Crab populations naturally go up and down, but the number taken at harvest has an effect. The overall population reached 828 million in 1991, but dove to 367 million the next year after watermen harvested 90 million pounds.

History repeated itself a few years later in 1997, when watermen caught 77 million pounds after the stock rebounded to 680 million. The next year, the population dropped by 327 million.

A 21-year high of 765 million was reached in two years ago, then plopped to 300 million after watermen removed 56 million pounds.

Management is key, said Rom Lipcius, a Virginia Institute of Marine Science researcher who participates in the annual survey. "It's likely the collective management actions since 2008 enhanced the population's resilience."

Without it, Lipcius said, "The winter's impact on the crab stock could have been much worse."