The 90-mile (145-kilometer) journey of two lost whales back to sea raised hopes that they were returning to the Pacific Ocean more than a week after taking a wrong turn and swimming up the Sacramento River.
The mother and calf humpbacks, nicknamed Delta and Dawn, had traveled more than 20 miles (32 kilometers) toward sea Sunday night. But by Monday afternoon, the two were again going upstream.
Scientists and the U.S. Coast Guard tried to use more than a dozen boats to turn them around, but the whales appeared to be navigating by their own compass.
"They're at this point lost. We don't think they have any clue," Rod McInnis of the National Marine Fisheries Service said Monday.
The whales, who appear to have been wounded by a boat's propeller, risk getting trapped in muddy side deltas. Federal officials have authorized researchers to fire darts carrying a satellite tracking device to ensure authorities can locate the whales if they wander off.
The whales on Sunday left the Port of Sacramento, where they had attracted a crowd of thousands.
If the two continue to head upstream, authorities could resort to banging on metal pipes dangling underwater with hammers, said Steve Edinger, assistant chief of the state Department of Fish and Game.
The goal is to "make as much noise to be as obnoxious to the whales as possible" to get them to move back toward the ocean, Ed Sweeney, a member of the U.S. Coast Guard Auxiliary, said Monday.
No one is certain why the whales decided to go back downstream in the first place.
It was not the first time a West Coast whale has veered so far off course during the annual spring migration northward. A humpback named Humphrey swam in the delta for nearly a month in 1985 before scientists used recordings of whale songs to lure him back to the Pacific.