BENICIA, Calif. – Two whales lost in the Sacramento River encouraged biologists Monday by making progress toward their ocean home, but then abruptly halted their journey at a busy bridge.

Unlike the relatively narrow section of the river where the two had lingered for the past week, the whales were in a holding pattern in a much wider waterway teeming with traffic.

“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 shift came after an encouraging spurt in which the pair traveled about 24 miles in 24 hours. They were first spotted May 13 and got as far as 90 miles inland to the Port of Sacramento before turning around.

The mother humpback whale and her calf were seen Monday morning near the Benicia-Martinez Bridge, about 45 miles from the Pacific, said Carol Singleton of the Governor’s Office of Emergency Services.

Sightseers swarmed the waterfront near the bridge to catch a glimpse as the U.S. Coast Guard tried to maintain a 500-yard safety zone around the whales. About 100 boats surrounded the pair, 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, and 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 whether the whales would 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 around 2 p.m. Sunday, which was around the same time of their equally mysterious departure from the port exactly a week earlier.

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

The calf also appears to be more active, which is an encouraging sign, they said.