The mute swan, that graceful white bird much loved by Peter Ilich Tchaikovsky, Hans Christian Andersen and Marcel Proust (among others), is fast becoming an ecological ugly duckling on the Chesapeake Bay.

One day soon we may have to choose between mute swans and soft shelled crabs: It's that bad.

Once a docile, decorative species confined principally to zoos, park ponds and poetic images, the mure swan has literally gone wild in the bay in the past 15 years, bullying the local waterfowl, breeding profusely and gobbling up the eelgrass.

Its appetite for eelgrass is the big problem, William C. Baker, assistant to the director of the Chesapeake Bay Foundation.

"Few people realize the full importance of the submerged aquatic vegetation in the bay," Baker said. "It's a nursery for small fish. It's where crabs hide when they molt. It filters sediment form the water, puts oxygen in the water and against the shoreline. Almost everything in the marine environment depends on it in one way or another."

A full grown mute swan, however, can eat as much as 10 pounds of underwater vegetation per day, and this, biologists say, could endanger the bay itself if nothing is done.%T"Aquatic vegetation has been having a hard time in the bay recently anyway," said Richard Maldeis,a Bay Foundation biologist who has two years along 60 miles of shoreline in theeastern bay area below Kent Island.

"Herbicide runoff has killed a lot of it. Siltation and turbidity has prevented a lot more from even growing. The mute swan factor ia just the latest threat."

Unlike other grass-eating waterfowl, however, the mute swans do not migrate, but remains in the bay munching eelgrass all year. They are particularly destructive in the early spring, Maldeis said, when the underwater grass shoots are just getting started.

In the areas where the swans live, he said, submerged aquatic vegetation is virtually nonexistent.

Mute swans are native to the bay but have prospered there since accidental introduction in 1962.

Jan Reese, a St. Michaels, Md. biologist studying the birds for the past 10 years on a grant from the National Geographic Society, said a captive pair of swans escpaed from a penned ara on the Miles River during a March storm that year and began nesting nearby.

From those two he said, the population has grown "almost geometrically to the present population of 300 in just 15 years.

"Right now the swans are mostly a localized problems," he said. "The real problem is down the road a couple of years. When we get 2,000 or 3,000 of these birds in one area that ould be a real concern."

Despite the birds somewhat ethereal image among humans, Reese and Baker say swans are very aggressive, territorial birds tht not only prempt the food supply from the native waterfowl such as ducks and geese but attack them and drive them away.

"These are big birds - 32 to 35 pounds," said Baker. "They fly right at fishermen and knock them down. The researchers who try to band the cygnets have a terrible time."

"They seem to nhave settled into an ecological niche that was previously uninhabited," Reese said. "They have few natural predators,although turtles will often capture the young" during the March to October nesting season.

Reese said swans commonly lay from six to 10 eggs a yar and have been hatching and raising young at a rate of 3.5 per year per nest."Not all of those survive of course, but tht's a rather astonishing rate," Maldeis said. "They seem to be surviving here at a rate 50 to 60 per cent better than in Europe where they're native species."

The Chesapeake Bay Foundation is not yet ready to suggest shooting the swans, a solution from which they shrink for numerous reason.

"You can just imagine the public outcry if they did," said Reese. People form a very strong psychic bond with swans. I've been cursed and shot at the everything just trying to band the birds. If people think you're messing with swans they get very excited."

But some means of population control - probably egg destruction or some other means - will probably have to be tried within the next year or so, Reese said. Similar measures have been employed in hte past to curv similar swan population explosions in Rhode Island and Michigan.

But even then he and Baker said, the public - mindful of images from "The Ugly Duckling" to "Swan Lake," - won't be happy.

"These are extraordinarily beautiful birds, a real addition tothe environment," Reese said. "They just happen also to be doing extraordinary damage."