Two wayward humpback whales made it 20 miles back toward the ocean Monday before balking at a Sacramento River bridge and swimming in circles, apparently upset by vibrations from the traffic.

"They're wild animals and they're going to do what they want," Coast Guard Petty Officer Brian Leshak said late Monday as officials decided what to do next.

"We're working on a plan to actually herd the whales," he said.

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The humpbacks, nicknamed Delta and Dawn, had traveled 90 miles inland before scientists got them turned around at the Port of Sacramento on Sunday. They were making progress Monday until they reached the Rio Vista Bridge and began swimming in circles.

Scientists theorized that the whales began circling because vibrations from traffic upset them. They pair could not be coaxed forward even when the drawbridge was raised to halt the flow of vehicles.

The U.S. Coast Guard tried positioning more than a dozen boats in front of them to turn them around, but the whales appeared unfazed. Banging metal pipes in the water to create vibrations didn't work, either.

Scientists have been watching the two closely because their route includes sloughs leading to muddy deltas that could trap the whales, both already apparently wounded by a boat's propeller. The pair also face a couple more highway bridges between Rio Vista and San Francisco Bay.

Federal officials have authorized researchers to fire darts carrying a satellite tracking device beneath the mother's fin to ensure authorities can still locate the whales if they wander from the river into the delta's maze of tributaries.

"They're at this point lost," Rod McInnis of the National Marine Fisheries Service. "We don't think they have any clue."