RIO VISTA, Calif. (AP) _ A wayward humpback whale whose instincts went awry was ''out there doing what he wants to do,'' refusing to head back to sea from its sojourn in central California waterways, a Coast Guard official said.

Sightseers crowded river banks Wednesday for a glimpse of the bus-size whale that began swimming up the Sacramento-San Joaquin Delta seven days ago away from its normal salt water element.

The whale, estimated at 45 tons and up to 45 feet in length, has been dubbed ''E.T.'' by some scientists and spectators after a movie extraterrestial who had to return home from Earth or die. Others call it Humphrey.

It was last seen by the U.S. Coast Guard about 6:30 p.m. Wednesday in the vicinity of Rio Vista, on the fresh water Sacramento River about 55 miles by water from the Golden Gate.

A fisherman reported seeing the whale today, still in the river. The Coast Guard and marine biologists said they would probably not follow the whale today, unless it gets into trouble.

''We're not disturbing him,'' said Coast Guard Lt. Tom Rydell. ''He's out there doing what he wants to do.''

At one time swimming as far as 70 miles from the Pacific Ocean up the river, the leviathan got no closer to salt water Wednesday than the Rio Vista drawbridge, which was vibrating from heavy traffic and highway resurfacing machinery.

As it approached the bridge, followed downstream by boatloads of Coast Guard personnel, scientists and spectators, the whale balked, swam in circles, then headed back upstream beneath the boats.

Motorists, toting cameras, binoculars and children, clogged narrow roads atop levees. Each time the whale surfaced and spouted, onlookers shouted nearly in chorus, ''There he is 3/8''

One sightseer, Barbara McCune, said, ''He's given a lot of people a real treat. I hope he gets back OK. ... The more you watch him the more you grow to love him and feel sorry for him.''

Wednesday's sighting startled the passengers of a private boat on Cache Slough, a deep stretch of a few miles between the main channel of the Sacramento River and the artificial Deep Water Ship Channel that serves the Port of Sacramento about 25 miles upstream.

Kathleen Potter, a Coast Guard spokeswoman, said passengers on the ''Sucker Fish'' saw the whale surface, then reported it to authorities.

The whale spent much of Wednesday in the vicinity of Cache Slough.

Marine biologists tried to influence the whale with underwater broadcasts of humpback whale sounds and music.

The whale was first spotted Friday in San Francisco Bay, apparently having strayed from its normal Alaska-to-Mexico fall migration in a failure of its instincts.

Suggestions telephoned to authorities include dangling a huge artificial fish from a helicopter as a lure and playing recorded sounds of a female whale in heat.

One caller suggested depth charges.