A huge swath of the Chesapeake Bay has been declared off-limits to crabbers by Virginia as part of an ongoing effort to stabilize the population of the Bay’s most valuable fishery.

The Virginia Marine Resources Commission in June created a 660-square-mile deep-water sanctuary which scientists believe will protect 40 percent of the Bay’s spawning females.

The sanctuary, generally located in water more than 35 feet deep, will be closed to crabbing from June 1 through Sept. 15 each year, a time frame that corresponds with the crab’s spawning season.

This year’s closure was effective July 1 — just four days after the VMRC approved it, on a 6-1 vote. Officials said they would enforce the closure with boat and plane patrols.

The sanctuary incorporates most of two previous spawning sanctuaries that totaled 195 square miles, and had been recommended by scientists from the Virginia Institute of Marine Science. VIMS research has indicated that the population of mature female crabs in Virginia’s portion of the Bay has plunged 70 percent in the last dozen years.

Concern has grown among scientists in recent years that crabs are on the verge of being overfished — something that could push the population into a long-term decline. By protecting spawning females, the VMRC hopes to bolster reproduction which, in turn, will help build future crab stocks.

“This is kind of a preemptive strike,” said Jack Travelstead, the VMRC’s fisheries manager. “It is action we felt we needed to take to keep things from getting any worse.”

Although Maryland and Virginia usually harvest similar numbers of crabs, females tend to stay in Virginia waters, while males prefer Maryland’s cooler and less salty portion of the Bay.

In the past, the deep area of Virginia’s part of the Bay was not heavily used by crabbers. But as crab populations have declined, more watermen began using what is now the sanctuary area.

Maryland Gov. Parris Glendening was pleased by the action, declaring at the Executive Council meeting, which took place the day after the VMRC’s action, “I praise Virginia for its actions yesterday.”

Both states have taken several actions in the past decade to stabilize the blue crab catch, which had been under growing pressure as the populations of other Bay fisheries, such as oysters, declined.

Virginia’s action had been endorsed by most, but not all of the watermen’s groups in the state. “That speaks well for the watermen’s realization that we need to be concerned and not let the fishery fall any further,” said Wilford Kale, a spokesman for the VMRC.

Bill Goldsborough, senior scientist with the Chesapeake Bay Foundation, said the sanctuary was an “important tool” in managing the blue crab population, but that it should be viewed only as part of an overall program to manage fishing pressure on the crab population. “We don’t think this is the be-all and end-all,” he said.

The commission’s action followed the latest annual report on the blue crab stock from the Chesapeake Bay Stock Assessment Committee. As it has in recent years, the report continued to say the Bay’s blue crab stock was “fully exploited” and continued to warn that current catch rates allowed little margin for safety.

“As pointed out in last year’s report, there is an urgent need to establish target fishing mortality rates, which are distinctly lower and more risk-averse than current threshold rates,” the report said.

Threshold mortality rates are considered the maximum the stock can sustain. The actual catch target should be lower than the threshold to allow for a margin of safety, in part because it is hard to precisely measure the blue crab population, said the CBSAC, which is coordinated by the National Oceanic and Atmospheric Administration’s Chesapeake Bay Office.

To help set new targets, a separate Bi-State Blue Crab Advisory Commission — made up of lawmakers, fishery managers, waterman and conservationists from the two states — is convening a group of top scientists for three days in July. “This really is designed to hunker down and get the work done,” said Ann Swanson, executive director of the Chesapeake Bay Commission, a panel that represents the legislatures of the Bay states and is coordinating the Bi-State crab panel.

Scientists have expressed concern not only about the total harvest in recent years, but also that too many crabs are caught when they are small. By managing fishing pressure in ways that allow crabs to grow larger, they would not only be more valuable, but would help maintain the population because large crabs produce more eggs than small ones.

The new Chesapeake 2000 Agreement directs the states to establish harvest targets and management plans for blue crabs by next year which would help “restore a healthy spawning biomass, size and age structure” within the blue crab population.