Fishing authorities for the Potomac River yesterday imposed emergency regulations to limit this autumn's harvest of the blue crab, which has been in such short supply that a prominent crab festival in Annapolis recently had to buy bushels from North Carolina.

Top natural resources officials from Maryland and Virginia joined in yesterday's 7 to 0 vote by the Potomac River Fisheries Commission to establish a temporary daily harvest limit of 30 bushels. The Potomac supplies one-tenth of the Chesapeake Bay's crab harvest.

The vote is among the first official reactions to two straight years of erratic harvests. Scientists increasingly believe the crustacean is under dangerous pressure from fishing.

After a good spring, harvests dropped sharply in mid-summer of 1998, and scientific surveys last winter revealed relatively few crabs nestled in the muddy bottom of the Chesapeake Bay. By spring, Maryland and Virginia watermen were reporting harvests that were one-third to two-thirds lower than the corresponding May-through-June totals from the year before, according to statistics from both states.

The poor harvest is felt in watermen's wallets, and at marketplaces and even festivals. Last week, the 54th annual Annapolis Rotary Club Crab Feast, a celebration of Maryland's official state crustacean, had to settle for serving Carolina crabs to its 2,600 patrons.

"We couldn't get enough Maryland crabs," said ticket chairman Jim Powers, an Annapolis businessman. He said the festival's supplier could fill only half the bill with bona fide Maryland crabs.

On the Potomac River, watermen this year took roughly 26,000 bushels of hard crabs from May through July--less than half the long-term average of 56,000 bushels for that period, according to the fisheries commission.

But watermen, many of whom oppose restrictions on fishing, said that situation promises to improve. Several told the commission they see many crabs that are one molt away from growing to the legal harvesting size of five inches from tip to tip.

Some of the 70 people who packed the commission's meeting in the Charles County seat of La Plata pleaded against any change in regulations.

"Now is not the time to do this," said St. Mary's County waterman Robert T. Brown. "We went through the hardest time. Now there's a chance to make a few dollars."

Those favoring the new limits also talked of economics. They said crabs spared by the temporary regulations can be captured next spring, when market prices typically are higher than in the fall.

"We like to have a little something to sell in the spring," said William L. Rice Sr., a Charles County waterman and commission member. "If everybody has to conserve a little bit, hopefully we'll have something for the future."

The effects of the restriction are likely to be modest. The commission estimates a 30-bushel limit will result in an autumn harvest reduction of 8 percent.

Maryland Natural Resources Secretary Sarah J. Taylor-Rogers, who as a commission member participated in yesterday's vote, called it a small but symbolically important step.

"The crab population is not going to be made or broken by what happens on the Potomac River," Taylor-Rogers said. She said the decision "sets the tone" for further decisions.

William A. Pruitt, chairman of the Virginia Marine Resources Commission, participated in the vote and called it "an economic decision." He said Virginia and Maryland are jointly studying what steps might be needed to protect the crab.

"I don't think we need to get confused with that long-term work today," Pruitt said.

Virginia officials in May banned the sale or transfer of crabbing licenses in a bid to keep more people from entering the fishery.

The Potomac restrictions, in effect for the 1999 season only, begin Sept. 15 and expire Nov. 30, when the crabbing season ends. Those fishing in the bay or in rivers that empty into the Potomac are bound by separate state requirements.