RIO VISTA, Calif. (AP) _ Fearing their efforts may cause fatal stress and having exhausted rescue funds, researchers say they are giving up their efforts to entice Humphrey the wayward humpback whale back to the ocean.

Instead, scientists who have been working to lure the 45-ton mammal out of the Sacramento River say they will let Humphrey try to find his own way to freedom now that he has passed the Rio Vista Bridge, a major obstacle.

Humphrey, who remains more than 50 miles northeast of the Golden Gate Bridge and the Pacific Ocean, could survive indefinitely in the freshwater river, one expert said Tuesday.

''There is no information on humpbacked whales in fresh water, so we don't know for sure'' how long he can live, said Sheridan Stone, wildlife biologist for the National Marine Fisheries Service. ''He should cause no problem to traffic or to people who want to fish.''

The whale was three miles south of the Rio Vista Bridge late Tuesday, its 16th day out of salt water. Earlier in the day, gusty winds foiled efforts to attach a transmitter to Humphrey's back to track him.

''We tried our best to tag him,'' said Jim Slawson of the National Marine Fisheries Service. ''But we need calm conditions and everything in our favor. ... He's just too big and dangerous.''

Asked about earlier reports that the whale has been plagued by skin blisters and eye infections due to the fresh water, Slawson said no one really knows how the whale is holding up.

''We've had experts give the opinion that it would be six days before it develops problems, and others say it could be six months,'' he said.

Marine scientists gave up the pipe-banging and whale-chasing boats to try to coax the 40-foot Humphrey back to the Pacific because of possible stress to the whale and dwindling funds, Slawson said.

Whales ''as a rule do not handle stress well,'' Stone said. ''It could cause internal problems that could definitely be fatal to the animal.''

Stone said stress could cause irregularities in the circulatory, digestive, mental and adrenal processes of the whale. He said the fresh water may make the whale's skin swell and that ingesting fresh water could make more demands on its kidneys.

He said $50,000 in state and federal funds have already been spent and that no other efforts to herd the animal to the ocean will be undertaken until more money is raised.

''To push him out into a marine environment where he'll stay, we'll need a lot more resources and a lot more planning,'' Stone said.

Exasperated experts have seen Humphrey heading toward the open sea several times, only to watch him do an about-face and spout his way back upriver.

Humpback whales are an endangered species.

Some scientists speculate Humphrey is defining his territory with his back- and-forth cruises.

''We're hopeful we've taught the whale he can swim south,'' said James Lecky, a Marine Fisheries Service biologist.

State Sen. John Garamendi has called experts to convene in Sacramento Thursday to review the situation.