By discovering the bay crabs' winter digs, scientists can now predict summer's bounty

A DEEP BLUE MYSTERY SOLVED

May 24, 1992|By Tom Horton

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.

Baltimore Sun Articles
|
|
|
Please note the green-lined linked article text has been applied commercially without any involvement from our newsroom editors, reporters or any other editorial staff.