Marine biologists temporarily halted efforts to coax two lost, injured humpback whales back to the Pacific on Thursday morning, after recordings of clanging pipes, feeding humpbacks and killer orcas failed to induce the pair to head back to saltwater.

The mother whale and her calf were last spotted about 70 miles from the ocean, in an area of the Sacramento River where they have been circling for several days, said Carrie Wilson, a biologist with the state Department of Fish and Game.

Marine biologists had planned to broadcast recordings of attacking orcas to scare the wayward whales back to saltwater Thursday morning, but decided instead to give the pair a break.

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"We don't want to do anything that will impose stress on them today," Wilson said. "We do not want them to become habituated or desensitized to the different types of things we're doing to coax them down the river."

Scientists planned to observe the whales and inspect their wounds Thursday before deciding what, if anything, they should do on Friday if the pair still hasn't moved.

Scientists tried playing different underwater sounds on Wednesday evening.

The lost whales did not respond to recordings of fellow humpback whales or blood-thirsty orcas, but a noisy mix of unnatural sounds caused the pair to back away, Wilson said.

The stranded whales also didn't respond to recordings of banging metal pipes or a small fleet of boats trying to herd them downstream earlier Wednesday.

On Thursday, biologists planned to take boats out to the whales to better assess their health. Both animals are injured, apparently from a run-in with a boat, and their wounds do not appear to be healing.

"It's important that we get those animals back into salt water in order for them to heal properly," Wilson said.

Rescuers had already planned to back off over the Memorial Day weekend as well if the stranded whales have not moved downriver.

The U.S. Coast Guard crews would keep a 500-yard buffer zone around the whales to keep boats away. They expected crowds to gather along the riverbanks to catch a glimpse of the humpbacks.

The humpbacks apparently took a wrong turn during their annual migration to feeding grounds in the northern Pacific. They traveled 90 miles inland to the Port of Sacramento before turning around.

They were making progress Monday until they reached the Rio Vista Bridge and began swimming in circles.