The humpbacks, nicknamed Delta and Dawn, were stalled briefly overnight when they began heading upstream in the Sacramento River on their 90-mile homeward journey, forcing scientists and the Coast Guard to position boats in front of them to turn them around.
But early Monday, they were back on track, spotted just north of Rio Vista about 45 miles southwest of Sacramento.
"The plan is to keep boats behind them, trying to coax them in the direction we want them to go," Carrie Wilson, a marine biologist with the California Department of Fish and Game, said Monday.
Scientists continued to watch the whales closely because their route includes sloughs leading to muddy deltas that could trap them. The two, who appear to have been wounded by a boat's propeller but are in otherwise good health, also will have to get through the pylons of four bridges to reach the San Francisco Bay and will have to swim under the Golden Gate Bridge to return to the ocean, Wilson said.
"We were thrilled to get them down the channel yesterday, but I'm afraid that was the easy part," she said.
No one is certain why the whales decided to go back downstream, but Jim Oswald of the Marine Mammal Center said the change may have been spurred by tug boats. The tugs' engines fired up about 100 yards away from the pair, and the sound may have had an influence.
"The tugs were out in the basin and the whales decided to follow them. They've been on the move (ever since)," Oswald said.
During their journey, boats will be positioned at the mouths of tributaries where the whales could possibly go off course.
"We've got a bunch of metal pipes and hammers, and if we need to, ... we can give the other boats pipes to bang on to persuade the animals not to turn in the wrong direction," said Wilson.
The whales' plight has been followed closely, and hundreds of people have gathered along the banks of the Port of Sacramento to see the pair. People were also appearing on the banks of the river trying to get a glimpse of the retreating whales, Wilson said.
Wildlife experts decided on Friday to leave the whales alone for the weekend, since heavy recreational boat traffic in the delta could have complicated any efforts to move them.
If the whales don't continue on their current course toward the ocean, marine mammal rescue crews will resume trying to lure the pair in the right direction by playing recorded sounds of other humpbacks feeding. That strategy worked in the case of a humpback named Humphrey, who in 1985 swam in the delta for nearly a month before returning to the Pacific.
