Fishery managers are moving forward with proposals to slash harvest pressure on female blue crabs by 10 percent after an annual survey found their numbers had dipped slightly below the level deemed “safe” by scientists.

But managers and watermen alike are frustrated that the iconic blue crab population — after showing signs of recovery — has slipped back to low levels seen prior to ramped-up management efforts imposed in 2008 that included an initial 33 percent reduction in fishing pressure on adult females. There is no single, clear explanation for the decline.

“I’m still somewhat at a loss for an explanation,” said Tom Miller, director of the University of Maryland Center for Environmental Science Chesapeake Biological Laboratory, and a scientist who has overseen the last three blue crab stock assessments.

“Single explanations are unlikely,” Miller added. “It is much more likely to be a multifaceted answer that involves several different mechanisms that are going on there.”

This annual blue crab winter dredge survey, in which scientists sample crabs buried in winter mud at 1,500 locations, showed that the number of adult female blue crabs fell to 69 million this winter, the lowest since 2002 and slightly below the 70 million threshold that scientists consider “safe.” Historically, the survey has provided an accurate snapshot of crab abundance and is the primary tool for assessing the health of the crab stock.

This year’s result would have once been defined as “overfishing” but scientists now describe the fishery as “depleted.” The reason is that in recent years fishing pressure on blue crabs has been in the range that should have protected the female population.

“The fishery now is managed in a way that we think should be sustainable,” Miller said. “So I don’t think the cause is fishery-related unless there is just massive unreported harvesting, and I don’t honestly think that is the case.”

The preliminary Baywide blue crab commercial harvest for last year was 37 million pounds, the lowest in two-and-a-half decades.

Nonetheless, managers say fishing is the one thing they can most immediately manage to reverse the blue crab population slide. So their immediate focus is for the three management agencies — the Virginia Marine Resources Commission, the Potomac River Fisheries Commission and the Maryland Department of Natural Resources — to enact regulations that would reduce harvest effort on females by an additional 10 percent. Managing harvest effort aims to maintain harvests at a certain level relative to the overall population, usually by regulating the numbers of days open to fishing and the amount and kind of gear used to fish.

Those changes will have two objectives: To provide more protection for adult females so they get a chance to spawn, and to protect juvenile females so they can join the spawning population later this year.

Blue crabs spawn from late spring through fall, and females that will spawn this summer are already in Virginia waters. The VMRC was expected in June to consider new rules to protect those females so they can spawn, as well as additional measures to protect females that mature later this year but won’t spawn until early 2015.

VMRC officials also said the low number of crabs would likely doom any reopening of Virginia’s winter dredge fishery, which mainly catches females. It has been closed since 2008 although a number of watermen have sought to reopen it.

Pre-spawn adult females don’t arrive in Maryland waters until closer to summer, and will migrate back to Virginia to spawn after they mature later in the year, and the DNR is considering actions to protect those females.

The intent, said Lynn Fegley, deputy director of the DNR Fisheries Services, is to help give more females “safe passage” to Virginia this fall. Not all females complete the migration before burying themselves in the mud for the winter, so the agency is also seeking to boost protection in spring so females “can wake up and skitter down the Bay to join the spawning pool.”

Protecting female crabs to boost spawning has been the focus of blue crab management since 2008, after population levels hovered near record lows for a decade. Those initial efforts seemed to pay off quickly, as the number of females jumped from 91 million in 2008 to 246 million in 2010 — the highest counted since the winter dredge survey began in 1990. In 2012, the total blue crab population hit 765 million, the highest since 1993.

Yet those numbers did not translate into a sustained increased abundance in the Bay. Instead, crab numbers have fallen sharply. This winter’s dredge survey estimated that just 297 million crabs were in the Bay, the lowest since 2008 when the new crab protection measures took effect.

The bulk of the crabs in this year’s survey were juveniles. They numbered 199 million, an improvement from last year’s near-record low of 111 million, but still considered low.

Scientists and managers are at a loss to explain the population fall off. At least part of the this winter’s decline may have been caused by harsh winter conditions. In Maryland, scientists conducting the survey said that the cold may have killed 28 percent of adult crabs.

Yet that doesn’t explain why overall crab abundance has fallen so much the last two years. Increased predation by red drum, striped bass and even other crabs has been blamed, as has loss of habitat such as submerged aquatic vegetation, as well as generally poor environmental conditions that affect the success of spawning. Females release eggs near the mouth of the Bay and larvae float in coastal waters for several weeks before returning to the Chesapeake as small crabs. Offshore conditions heavily influence how many crabs return to the Bay.

This is why fishery managers and scientists describe the population as being depleted, rather than overharvested. “There are a lot of factors that are involved in what is going on here,” said Joe Grist, deputy chief of fisheries with the Virginia Marine Resources Commission and chair of the Chesapeake Bay Stock Assessment Committee, a panel of scientists and fishery managers that makes Baywide recommendations for crab management. “It is not just fishing. This is an environmental issue.”

The CBSAC, in an upcoming report, is expected to emphasize not only the need to protect adult and juvenile females, but also work to better understand other factors that may be affecting the population.

To help answer those questions, the CBSAC this summer is planning to begin examining spring and summer fish surveys, mostly collected in the Bay for other purposes, to see whether they can glean more information about what is happening to crabs between the annual winter dredge surveys.

Those surveys could offer new insights, such as whether crab numbers drop sharply as predators become abundant during the summer, or whether other factors might be contributing to the loss of crabs.

Although crab populations are not where managers and watermen want them to be, the blue crab management structure lets managers adapt their actions based on new, and better information, and managers believe the process will lead to a recovery. (See “Framework sets up rationale for blue crab management decisions” .)

“I really do think we need a chance to try this management approach.” said Rob O’Reilly, VMRC chief of fisheries management.

Unlike many other fisheries — and even many other Bay Program goals — the overall crab population is well-monitored through the winter dredge survey. Based on the results each year, managers across state lines can identify whether past actions were effective and coordinate on new regulations, as is the case this year — a level of coordination that would have been unimaginable little more than a decade ago.

The CBSAC provides a further review of the data, and makes independent recommendations to managers, as well as highlighting research needs to help fill information gaps — such as identifying new surveys that can better explain what is happening to crabs during the summer.

If the blue crabs don’t start showing signs of recovery soon, it could be a signal that other factors are influencing the population more than previously thought, or environmental conditions are changing. In that case, new management approaches may be needed.

“I don’t think the sky is falling,” Miller said. “I don’t think that there is panic, or that there should be panic, but I think there should be caution and increased attention to ensure that we do protect those females that are going to spawn and that we do watch very carefully what comes out the next time.

“The central issue is, will they recover next year, not have they just gone up a little bit. If they start showing indications that they are going to stay low again, then I think we have some serious concerns.”