A DEEP BLUE MYSTERY SOLVED By discovering the bay crabs' winter digs, scientists can now predict summer's bounty

THE BALTIMORE SUN

It was around the second week of January, when the workboat Loni-Carol II, under contract to a scientific survey, dredged something from a deep trench in the Chesapeake that was as intriguing to bay fisheries researchers as the dark side of the moon is to astronomers.

The dredge, plowing through the soft, sandy muds of a 60-foot hole off the Manokin River near Deal Island, filled again and again with gobs of big blue crabs, nearly half of them pregnant $H females, dug in for the winter.

Now, finding crabs anywhere in the Chesapeake Bay might not seem exciting. After all, the bay annually yields about a quarter-billion of the creatures to fishermen. But strangely, after more than a century of commercial crabbing on the bay, and more than half a century of scientific study of the blue crab here, no one had ever systematically looked to see where crabs went in the winter.

Conventional wisdom had it that in the autumn the great bulk of the bay's "sooks," or mature female crabs, on whom the next summer's spawning success depended, swam to Virginia, to bury near the bay's mouth. That is the only place consistently salty enough to ensure survival of baby crabs, which must bathe in near-oceanic salinities in their early life stages (as adults, crabs are tougher customers, and probably could thrive in your bathtub).

The rest of the crabs, males and immature females, so far as anyone knew, dug in randomly across 4,000 square miles of bottom in the bay and its tidal tributaries. But the Deal Island discovery, buttressed by thousands of other survey samples taken by Maryland and Virginia scientists from the Susquehanna Flats to the Virginia Capes, has changed those views.

L In the process it also has cleared up a couple of mysteries:

During the hard winter that froze the bay nearly shut in 1978, there was near-panic when Maryland scientists announced the cold had killed as many as 80 percent of the bay's crabs. Only the crabs seemed not to be perturbed, as they yielded a bountiful harvest the following summer.

The dire conclusions in 1978 were based on samples of crabs wintering in waters of less than 40 feet in depth, which in the shallow Chesapeake account for a good three-quarters of the total bottom area. "But now we have finally found the mother lode, and it turns out to be in the deep channels . . . deep as 140 feet," says Brian Rothschild, one of the University of Maryland researchers heading the bottom survey in the northern half of the bay.

It makes sense from the crabs' point of view, he says. The bay's deeps are its most stable environment, buffered from climate changes, like winter freezes, that can have dramatic impacts nearer the surface.

A second mystery had centered on Virginia's winter commercial crab dredging near the bay's mouth. The dredging is conducted in the heart of the crab's spawning grounds, and almost every crab caught is a female that otherwise would have released up to 3 million eggs the following summer. The dredging fleet has grown in the last two decades from 70 vessels to more than 300, and dredges have evolved from 5 1/2 feet wide to 8 feet wide. Each boat drags two dredges. In a single day that is equivalent to dragging nearly a mile-wide of dredge for weeks on end, back and forth through the heart of the world's greatest crab nursery.

Maryland bans such fishing in its waters on conservation grounds. Yet, curiously, the Virginia dredging to date seems to have had little impact on crabbing the following year. Again, the bottom survey is turning up much larger than expected numbers of pregnant females all over the bay bottom -- not just concentrated, as many observers thought, in the spawning grounds. The females emerge in the spring and complete their journey to the bay's mouth to release their eggs after the dredging season. Perhaps as few as 10 percent to 15 percent of the bay's expectant crabs are even susceptible to the much-maligned Virginia dredging.

Indeed, the ability of the blue crab to endure and even thrive in the Chesapeake environment is almost eerie. It is now caught every month of the year, from Smith and Tangier islands to the inner harbors of Baltimore and Norfolk; taken by dredges, pots and scrapes; by dip net, trotline and chicken neck; taken hard and taken soft; torn from semi-hibernation in the mud, and ripped in the act of mating from the sheltering bay grass beds it seeks in summertime.

Yet, throughout the last decade, commercial harvests of crabs baywide have remained at some of the highest levels in history, nearing 100 million pounds annually some years. But harvest levels are only part of the story. How much effort goes into maintaining those levels is the other part, and every crabber on the bay will tell you that it gets harder every year. In just the last seven or eight years, the number of crab pots in use on the bay is estimated to have risen from about 650,000 to a million. Recreational catches of crabs are not reported, but Maryland and Virginia officials estimate that this harvest is from a quarter to a half of the total commercial take -- and probably growing faster, as population around the bay swells.

Meanwhile the oyster, the shad, the rockfish -- the species that once took some of the pressure off harvesting the crab -- all are in decline. Crabs surpassed oysters as Maryland's most valuable commercial fishery in 1983, a position not likely to be reversed as shellfish stocks have declined to about 1 percent of their historical abundance.

It is notable that at one time or another, the oyster, the shad and the rockfish all were described as virtually limitless resources, so successful and well adapted to the Chesapeake that it would be virtually impossible to fish them into oblivion. None of them now seems likely to support a major commercial fishery in the foreseeable future.

"The crab is the last great fishery of the Chesapeake Bay, and there is a feeling that we've got to learn enough to protect it while it is still healthy," says Rom Lipcius, a marine scientist who is Dr. Rothschild's counterpart at the Virginia Institute of Marine Science. The livelihoods of nearly 20,000 people now depend, full or part time, on catching crabs, and if crabbing collapses, there is nothing left to take up the slack, the scientists say.

There are preliminary indications, the researchers say, that crabs may be about to dip to numbers well below their recent levels of abundance. Such fluctuations are, in fact, more the norm than the exception. A look at half a century of crab harvest data produces a graph as jagged as the Himalayas.

"The problem could come when we hit a natural low point combined with very strong fishing pressure," says Dr. Lipcius. "Then we could drive the crab population to collapse. It happened to Dungeness crabs on the West Coast 10 years ago, and they have not recovered."

Thus the survey to find out where crabs spend the winter. A day spent aboard the Loni-Carol II, with a sopping, thick February fog closing in, shows that there is nothing exotic about the work. It is basic, grind-it-out science. Over and over again the dredge is pulled for precise intervals of a minute. The length and sex of each crab is recorded, as well as latitude, longitude and water depth, bottom type, salinity and a host of other environmental conditions. For the captain, Loni Moore, this is child's play, and a little frustrating. If he were not hired out to the survey, he would be dredging crabs for real near the bay's mouth, where for years he has been considered one of the top watermen in the game.

It has been, nonetheless, an interesting experience, Mr. Moore said. Crabs, the survey dredgers have found, are hardly the only creatures using the bay's bottom muds and deep-water trenches in the winter. Ocean-going species such as hake and Atlantic herring have turned up -- to the surprise of scientists -- and so have sturgeon, almost extinct from the Chesapeake in recent decades.

Eventually, the survey, which involves three boats, will yield a basis for comparing from year to year how many crabs are out there -- comparable to a store owner knowing what he has in inventory. Along with other recent research on crabs, the survey aims ultimately to give Maryland and Virginia the ability to do what up to now has been impossible -- predict how many crabs watermen can take each year without risking a collapse of the industry.

A great deal more than crab feasts or even the jobs of watermen is at stake. Virtually everything in the Chesapeake ecosystem is tied to the blue crab in some way. Just a partial list of which bay dwellers eat the blue crab includes eels, cobia, red drum, black ++ drum, sharks, rays, speckled trout, weakfish, catfish, garfish, largemouth bass, loggerhead turtles, rockfish and croakers. Other crab eaters include herons, egrets, gulls, diving ducks and raccoons.

Crabs in turn eat tremendous numbers of clams and small oysters. Large male crabs have been observed eating as many as 142 oysters (of less than half an inch in size) in a day. Many scientists are betting on them to help combat the small shellfish known as zebra mussels, exotic pests that have spread from the Great Lakes into the Chesapeake drainage basin, and are making their way down the Susquehanna River toward the bay. Any dramatic change in the population of crabs would have dramatic repercussions throughout the system.

Both Drs. Rothschild and Lipcius say they are confident of having at least a fair ability to predict crab populations within the next two years. That would represent major progress, but a much tougher step will remain -- mustering the political will in both states to use such data to manage crab harvesting.

Both states are officially committed in a 1987 Chesapeake Bay Agreement to develop modern management plans for a range of bay species. Among important bay fish, only the rockfish is the beneficiary of a fairly complete conservation plan, and that plan came about only after a dramatic decline in the species. There is virtually no history of politicians and natural resources officials doing this before a Chesapeake species gets in trouble. Now, the blue crab represents one of the last chances to get it right.

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