RIO VISTA, Calif. (AP) _ Scientists hope recordings of orcas attacking a mother whale and her calf will persuade a pair of ailing humpbacks meandering in a freshwater river to head back toward the ocean.

The tactic is just the latest that increasingly concerned marine biologists have come up with to coax the two lost, injured whales back to the ocean. They have spent more than a week in freshwater, which they are not physically equipped to inhabit.

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

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 a Sacramento River bridge about 70 miles from the Pacific and began swimming in circles.

For a third day Wednesday, the whales _ a mother and calf _ did not respond to a gauntlet of boats that tried to herd them past the Rio Vista Bridge.

Boat crews resumed playing underwater recordings of humpbacks feeding _ a method tried last week _ after attempting to startle them by banging on metal pipes.

Scientists were hoping the sound of blood-thirsty orcas might frighten the whales back into the ocean.

``We've done the best we can with the piping, so now we have to look at something else,'' said Carrie Wilson, a marine biologist with the California Department of Fish and Game.

Scientists acknowledged that hope was dimming for the pair, who were spotted slapping their tails on the water, a possible sign of distress. Their deep cuts, likely caused by a propeller, were visibly worsening. A 3- to 4-foot cut on the calf's side appeared to pierce the blubber layer down to the muscle.

``They may surprise us again,'' Gorman said. ``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.''

The two whales first were spotted in the Sacramento-San Joaquin Delta on May 13.

Although the freshwater delta is devoid of the whales' saltwater food sources like krill and plankton, starvation was not an immediate concern, biologists said, because humpbacks usually fast during the winter months.

Rescuers planned to back off over the Memorial Day weekend if the twosome remain stranded by then. U.S. Coast Guard crews would keep a 500-yard buffer zone around the whales during the popular boating holiday.

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 freshwater environment was taxing the whales physically, making their normally smooth and shiny skin become rough and pitted, ``like when you sit in a bathtub for too long,'' said Trevor Spradlin, a NOAA whale biologist.

The stress of that continued exposure ``may be impeding their natural healing abilities,'' he said.

The challenge facing the scientists was to encourage the whales 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.