Gird yourselves, crab lovers. The Virginia Institute of Marine Science is predicting the smallest catch of blue crabs in the Chesapeake Bay in more than two decades.

This is a bitter pill for fans of the sweetest seafood there is. Blue crabs were thought to be riding a comeback after several troubled years.

In 1979 commerical crabbers in Maryland and Virginia landed more than 62 million pounds, the largest catch since 1974.

But W. A. Van Engel, head of crustaceology for VIMS, says hard crab times are coming. He expects a catch of only 25 million from January to mid-August this year, only two-thirds the normal amount.

Van Engel's agency runs trawl samples on lower stretches of the York, Rappahannock and James rivers, where little bitty crabs live until they grow up to be big ones.

Last summer very few little crabs showed in the trawl nets.

To understand why crabbing is expected to drop off dramatically this year it helps to know a little about the life cycle of the blue crab.

Female crabs spawn in the summer in the lower, salty stretches of the Bay. Toward the end of summer their larvae evolve into tiny crabs, which start swimming up the rivers, to their nursery grounds in brackish water.

It takes another year of growth before they reach catching size.

This year's keeper-sized crabs are ones that were spawned in the summer of 1978. There was a decent spawn that year and plenty of little crabs swam up the rivers.

The trouble came from January to May a year ago, when extraordinarily heavy rains turned all the rivers that feed the bay into surging, muddy drainage chutes.

If you remember last spring you'll recall, for example, that the Potomac was too high and muddy to fish almost until summertime.

No one knows exactly what the long stretch of high water did to the little crabs, but when it was over they were few and far between, marking the end of a stretch of steadily improving crab years.

"We had our weakest year in 1975. It improved in '76, and still better in '77, and even better in '78," said Van Engel.

"When '79 came along we thought we were still on this trend of improving environmental conditions. But the rains reduced salinity in the nursery areas and brought down herbicides, pesticides, organic matter, fertiilizer and sediment into the rivers and the Bay.

"All these things directly affect the juvenile crabs."

Worse still, Van Engel believes the floods of last spring and more floods last September could stretch the crab decline into a two-year phenomenon.

Though the spring floods had subsided some by June, when mature females began spawning again, conditions were still less than optimum for a good spawn. Then last September, just when the young of the year were evolving from larvae to the tiny crab stage, came more rains.

Those rains came when the tiny crabs were still in the lower bay and Van Engel is afraid they flushed the youngesters out of the bay into the ocean.

"We are unaware of mechanisms by which these crabs will get back into the bay if they're out on the continental shelf," the scientist said. "We'll get a second look at this year's class when we start our trawl samples up again in May and June. But our expectation is for another low population level."

Van Engel works for Virginia, but his predictions are for the entire bay, including Maryland waters, since almost all Chesapeake crab reproduction takes place in Virginia waters or the lower bay.

If indeed the crab catches drop to levels of 40 million pounds or less this year and next, as Van Engel predicts, the consumers are the ones who will pay the price.

Catches in the winter fishery in Virginia are down right now. Backfin crabmeat is selling for $7.75 a pound and up.

Wayne Brady, who runs Coleman's Sea Food in Rock Hall, Md., says the price might never drop if the bad catch continues through the summer.

Traditionally summer crabs start at about $35 a bushel and the price drops as they become more abundant in July, August and September.

Last year they dropped to about $16 a bushel when they were most abundant. Don't wait around for any $16 bushels this year.