RIO VISTA, Calif. - Hope dimmed Wednesday for two lost, wounded whales as scientists spotted the humpbacks wildly slapping their tails on the water in possible distress as they lingered far from their ocean home.

Deep cuts on the mother whale and calf, likely caused by a run-in with a boat, were worsening after more than a week in fresh water that the pair are not physically well-equipped to inhabit, biologists said.

“I wouldn't say there's a lot of optimism right now,” said Brian Gorman, a spokesman for the National Oceanic and Atmospheric Administration.

“They may surprise us again. They may just take off and head down river. But as long as they continue doing what they're doing, we're very worried about them.”

For a third day, the whales frustrated efforts to herd them past a Sacramento River bridge about 70 miles from the Pacific, where they have lingered since Monday. They first were spotted in the Sacramento-San Joaquin Delta on May 13.

Biologists would not estimate how long the whales could survive in the delta but said the tail-slapping behavior, known as “lobbing,” was cause for concern.

“The whales were fairly quiet for a period of days. Then they weren't so quiet. So the question is, why have they changed?” said Trevor Spradlin, a NOAA whale biologist.

Boat crews resumed playing underwater recordings of humpbacks feeding to try to coax the whales downriver when startling them by banging on metal pipes failed to work Wednesday morning.

If the feeding sounds are not effective, scientists planned to try a new scare tactic - playing recordings of orcas, also known as killer whales, attacking a mother whale and her calf.

Rescuers planned to back off over the Memorial Day weekend if the twosome remained stranded.

The biggest concern remained the wounds on both whales, especially a 3- to 4-foot cut on the calf's side that appeared to pierce the blubber layer down to the muscle.

The freshwater environment was taxing the whales physically, turning their skin from its normally smooth, shiny texture to rough and pitted, “like when you sit in a bathtub for too long,” Spradlin said. The stress of that continued exposure “may be impeding their natural healing abilities,” he said.

The challenge facing the scientists trying to push the pair back to salt water was to encourage them to move quickly without causing them anxiety that could create more physical stress.

“Stressing even a healthy whale is not good. Stressing an injured whale is worse,” Gorman said.

More forceful techniques, such as using nets to drag the whales downstream or create a barrier across the river, could cause undue stress and threatened to harm the whales by entangling them, scientists said.

The humpbacks, which apparently took a wrong turn during their annual migration to feeding grounds in the northern Pacific, 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.

Scientists theorized the whales started circling because vibrations from traffic on the bridge upset them, though the pair continued to circle even when traffic was halted.