Hurricane spawns crab windfall Weather: The offshore winds of Hurricane Felix last year blew young crabs into the bay in unprecedented numbers, illustrating the unforeseen effects of natural phenomena.

On the Bay

June 14, 1996|By Tom Horton | Tom Horton,SUN STAFF

LIKE WEATHER forecasting, long-range, scientific prediction of blue crab availability in Chesapeake Bay is improving remarkably.

But anyone dreaming of a time when foretelling next winter's snowstorms -- or next summer's crab cake prices -- will be a lock might consider two not-unrelated texts:

"Does the Flap of a Butterfly's Wings in Brazil Set Off a Tornado in Texas?" -- a 1979 lecture by MIT meteorologist Edward Lorenz.

Also, the less scholarly dissertation by Professor Robert Dylan -- "The answer my friend, is blowin' in the wind."

First, Lorenz: His whimsical title cloaked a hard truth he had discovered doing computer simulations that let him set crude weather systems in motion and watch them develop over weeks.

Weather behaved contrary to a core assumption of conventional physics, one that said tiny errors in calculating anything from the structure of a bridge to the trajectory of a moon rocket stayed tiny and were no big deal.

But with weather, any small initial variation from one computer run to the next turned out to be a very big deal, quickly and chaotically magnifying into vastly different outcomes.

The 'butterly effect'

The bottom line of this "butterfly effect" (initially Lorenz used the analogy of a seagull's wings flapping) meant daily weather would always be unpredictable beyond a couple of weeks.

And even with weather computers today, thousands of times more powerful than anything Lorenz had, the accuracy of daily forecasts declines sharply beyond five days.

Which brings us to how the winds blow, which is more important to the bay's crabs than anyone used to dream.

Although prediction of daily weather may forever have severe limits, real strides are being made in predicting, a year and more in advance, the general framework for weather, which we commonly call "climate."

Ominous signs

Thus it was that hurricane forecasters at Colorado State University as early as spring 1994 began to think the summer and fall of 1995 might be perilous for the U.S. coastline.

They saw an ominous convergence building among far-flung events known to promote hurricane activity: equatorial winds reversing direction; the periodic El Nino warming of the tropical Pacific ending; rainfall and air pressure shifting across the West African Sahel; temperature oddities in Singapore and salinity increasing in the North Atlantic off Greenland.

As predicted, 1995 saw the most hurricane activity in nearly half a century.

However, because of the butterfly effect, no one could have predicted individual storms or how they might behave -- least of all the oddest and most threatening of all, a Category 4 monster named Felix.

With winds to 120 knots, Felix terrorized the Chesapeake during its Aug. 8-25 life span. It seemed set to roar straight up the bay, and evacuations began throughout Virginia and into Maryland.

But it's an ill wind that blows no good, and Felix, unpredictably, sat at sea and churned and spun and finally sputtered out, causing scarcely a puff or a ripple in the bay.

But something else quite amazing was happening as a result of Felix's winds (augmented by winds of a northeaster that preceded the hurricane).

Trillions of larvae

Both blows overlapped the summertime blue crab spawning that occurs where the Chesapeake meets the ocean. There, by the trillions, crab larvae (zoeae) are hatched on the ebbing tide, ensuring that they are washed out to sea.

Seaward would seem the wrong direction for an animal that flourishes the whole length of the bay as in few other places in the world, but in their first few weeks of life, larval crabs need ocean-strength salinity to develop properly.

So they float out to sea, as much as a hundred miles, and depend upon the caprice of winds and currents to eventually get them back into the estuary. By that time, they have changed to the next stage in their growth, called megalops.

Somehow, the tiny crabs "smell" when they are once again close to shore, and only then do they complete their transformation to true crab form. Scientists think this "land-ho" cue is the odor of humic acid from decaying vegetation.

Such dependence on events as quixotic as wind for delivery back into the bay's embrace may explain some of the astounding variability of crabs from year to year.

In fact, 1995 was not shaping up as a banner year. Scientists monitoring near-shore waters for incoming crab larvae were picking up almost nothing through July.

But in August, their collecting filters suddenly clotted with larvae. You could dip them by the hundreds with a bucket in bay-side harbors. At some locations, the little crabs clogged water intakes.

Apparently, the hurricane's improbable offshore spinning was setting up currents that moved little crabs back into the bay in unprecedented numbers.

It was a good thing, given that the severe winter of 1995-1996 would turn out to be a huge killer of larger crabs that were overwintering in the bay muds.

Only the masses of windblown larval crabs from 1995, just now reaching an inch or so in size, promise to bring the current sparse harvests back to healthy levels.

The Felix crabs will reach legal size between late summer and next spring.

What would have happened had hurricane conditions not converged worldwide -- or if Felix had taken another course?

Such imponderables don't mean we shouldn't manage crabs for long-run conservation, but in any given year, we can almost count on being surprised.

Pub Date: 6/14/96

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