Two humpback whales lost in the Sacramento River once again halted near a busy bridge, a day after making considerable progress in their quest to return to the Pacific Ocean.

Though they had stopped in a relatively narrow section of the river for the past week, the whales stopped Monday in a much wider waterway teeming with ship traffic.

The shift came after an encouraging spurt in which the pair traveled about 24 miles in 24 hours.

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"Right now they are meandering, but we're hoping that they get it in their minds to keep going," said Rod McInnis, a spokesman for the National Oceanic and Atmospheric Administration.

The mother humpback whale and her calf spent Monday near the Benicia-Martinez Bridge, about 45 miles from the Pacific, rescuers said.

They were first spotted May 13 and got as far as 90 miles inland to the Port of Sacramento before turning around.

Memorial Day sightseers swarmed the waterfront near the bridge to catch a glimpse. The U.S. Coast Guard worked to maintain a 500-yard safety zone around the whales.

About 100 boats carrying would-be whale watchers surrounded the pair as news of their location traveled, and Coast Guard crews hauled several swimmers out of the water as they tried to approach the whales, Lt. Larry Curran said.

Biologists hope that getting the whales into more brackish water after more than two weeks in the river could help ease the physical strain they have suffered from long exposure to fresh water.

Scientists were particularly concerned about lesions that have appeared on the humpbacks' skin over the weekend. They were awaiting test results from skin samples to determine the cause.

The humpbacks' long exposure to fresh water has led to serious skin damage, making them vulnerable to germs they would not face in their native saltwater habitat.

Despite the pair's health problems, officials did not plan to take any action to prod them toward the Golden Gate Bridge. They also said they could not predict when the whales might move again.

"It's still anybody's guess. The whales are going to decide what they're going to do and how they're going to do it," said Bernadette Fees, deputy director of the California Department of Fish and Game.

Scientists acknowledged they had little idea why the whales started moving again Sunday around 2 p.m., about the same time they had mysteriously departed from the port exactly a week earlier.

Both whales have gashes — likely suffered from a run-in with a boat's keel — that have worsened during their river stay. But veterinarians believe antibiotics injected into the whales Saturday could slow the damage. Scientist were ready to administer a second dose.