Fishing for data on underwater life


October 15, 1994|By TOM HORTON

What would it be like, the ultimate machine for finding fish in Chesapeake Bay?

For a few thousand bucks, you can install electronics that show what's swimming below your pleasure boat with a precision that once couldn't have been had this side of a nuclear sub.

Maybe 10 times that will get you equipment worthy of big commercial trawlers, with systems so good they can watch their nets encircling fish and reconfigure them to catch more.

So what could you do if you had a few million to spend?

Walter Boynton has the money -- a recent National Science Foundation grant for him and a few other University of Maryland researchers to mount the neatest fishing trips the bay's ever seen.

From their proposal's title, you will surmise this is about more than filling the cooler with rockfish: Trophic Interactions in Estuarine Systems -- Pulsed Inputs, Physical Structures and Biological Patchiness. Translation: The scientists are intrigued by why there are so many fish in estuaries such as the Chesapeake Bay.

It is more than just abundance. It is an abundance so high as to be improbable. Consider:

Estuaries (essentially coastal embayments where rivers meet the sea) are a minuscule half of 1 percent of the world's marine areas.

You would expect them to support more than half a percent of global fish production, because they are relatively rich in nutrients such as nitrogen and phosphorus, washed from coastal uplands and wetlands.

These nutrients do translate into a disproportionately large production -- about five times average -- of plankton, the tiny plants and animals that support fish.

But actual estuarine production of fish is higher still -- about 10 times average -- and among estuaries, Chesapeake Bay is a star, BTC with a fish production (per unit of plankton) a hundred times or more that of the oceans as a whole.

How come? Perhaps the answer begins with something as basic as the Latin root of the word "estuary" -- meaning a surging, heaving, boiling and seething where sweet water and salt collide.

Dr. Boynton and his colleagues characterize the bay as a "pulsing" ecosystem where the main constant is constant flux.

From the ebb and flood of ocean tides, to spring freshets and summer dribbles of its rivers, to the quixotic impacts of wind rioting across its shallow waters, to the exodus and homecoming of its migratory fish, the life of the estuary proceeds in a jerky, twitchy, jumpy manner.

At the other end of the spectrum are "steady state" ecosystems such as a climax, old growth forest and a coral reef.

These are marvelous places too, but characterized by stability, longevity and efficient recycling, producing little above what is needed to maintain themselves.

Pulsing systems, on the other hand, are known to produce more excess energy.

A way it might work in the bay, Boynton says, is that all the pulsing keeps the system out of sync. So, when conditions are ripe to produce, say, a large bloom of nutritious algae, bacteria that might naturally consume it are off balance and lag in doing their job, leaving a rich food source available to fuel fisheries production.

Exploring 'structure'

Another key component of the bay that the scientists especially want to explore is "structure."

Fishermen have known for a long time the benefits of locating structure -- from submerged logs to reefs and offshore oil rigs, and even changes in the bottom contour. Fish congregate there.

The structure the scientists will examine in the bay is a little different from popular concepts but may be just as effective for fish production.

The structure they are interested in is known mostly to oceanographers and experts in fluid dynamics. It includes something called the pycnocline, the depth at which the bay's upper, seaward-flowing river water forms a layer over the heavier, denser saltwater always pushing up-bay from the oceans.

Also, there is the hydraulic control point, an area off the mouth of the Potomac where complex currents develop and the bay channel abruptly deepens.

Further north is a large zone known as the bay's turbidity maximum, where the interface of salt and fresh water forms a physical and chemical trap, settling out sediments and minerals.

Mix in with all these any number of so-called river fronts, where tributaries collide with the main bay; and a larger "lateral front," caused by the earth's rotation slinging saltier water toward the Eastern Shore side of the estuary.

Thus the theory that underlies the $3 million fishing trip:

Estuaries, and especially the Chesapeake, with their complex bathymetries, and constant collisions among wind and water, ocean and river, contain a rich mosaic of structure and pulsing that translates into an astounding ability to grow fish.

To test that, the scientists, for the next six years, will "troll" up and down the bay, especially around the estuary's fluid "structure," trailing from university research vessels an array of instruments.

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