River-Running Whale Disappears : Sea life: Scientists hope the 25-foot animal has headed back out to San Francisco Bay. They will search Petaluma waterway today before calling off rescue effort.

Phyllis the wandering whale abruptly vanished Thursday, raising hopes that the celebrity mammal has wearied of sightseeing and glided out of the Petaluma River toward the open sea.

Although the wily whale may have merely slipped undetected into a side slough for some rest, experts were cautiously optimistic that Phyllis--who may, in fact, be a Pete--had rejoined his or her migrating colleagues and plotted a proper course for Alaska.

“There is no sign of the whale,” said Peigin Barrett of the Marine Mammal Center, who organized efforts to coax the stubborn cetacean from the river. “We’re not sure where it is, but we would all be delighted if it’s heading home.”

The whale, a baby gray 25 feet long, was last spotted Wednesday evening near a marina about seven miles north of the river mouth. It came to a halt there after a flotilla of boats armed with noisemakers spent the day in a mostly futile effort to herd it downstream.

On Thursday, a team of scientists, veterinarians and acoustics experts had planned to observe the whale’s behavior and devise a new plan to escort the six-ton animal from the river, a murky waterway that averages just 15 feet deep.

Instead, the squad spent the day searching for their missing patient, assisted by Coast Guard boats and a television news helicopter that hunted for telltale spouts on San Francisco Bay.

By nightfall, the whale was still missing in action: “We don’t know how it got in and we don’t know how it got out,” said Joe Cordaro, a wildlife biologist with the National Marine Fisheries Service.

Cordaro said the team of scientists would make one more sweep of the river today before closing down the rescue operation.

The whale, believed to be 1 or 2 years old, first surfaced in the Petaluma River on Saturday, puzzling experts, who say such side trips into inland waterways are extremely rare.

Since then, scientists aboard a flotilla of vessels have tried a carrot and stick approach--attempting to lure the mammal seaward by broadcasting gray whale sounds underwater, and driving it downriver by clanging on steel pipes. Unperturbed, the whale mostly ignored the commotion.

Although somewhat frustrating for scientists, Phyllis’ odyssey thrilled hundreds of spectators, who trekked to Petaluma from all over Northern California for a glimpse.

“Whales are magnificent creatures,” Jacqueline Birch, a tourist from Schenectady, N.Y., said after viewing the mammal from the Petaluma breakwater Wednesday. “This one stopped to say hello, so the least we can do is welcome it.”

The saga also sparked an outpouring of volunteerism from whale-lovers concerned about the baby cetacean’s plight. Case Blazyk, for instance, donated his time and his 22-foot Boston Whaler boat to help get Phyllis back on track.

“This little whale doesn’t deserve to die at the end of a muddy brown river,” Blazyk said. “Humans have done so many awful things to whales over the years. Maybe this is one small way of paying them back.”