(USA TODAY) -- Wildlife officials say 20 whales believed to be part of the 51 pilot whales stranded off Everglades National Park were spotted moving closer toward shore Friday afternoon.
The movement toward the shore would be a reversal of the whales' movement yesterday toward deeper waters that had given scientists new hope for their survival.
The whales were about 5 nautical miles from the coast, 1 mile closer than Thursday. The development raises concerns the whales could be sick or becoming exhausted from the stranding.
Blair Mase, coordinator of the National Oceanic and Atmospheric Administration's marine mammal stranding network, said the animals appeared to be less organized and swimming slowly.
Officials planned to call off the rescue mission if the whales stayed on their path Friday, she said.
Rescuers from agencies including the National Park Service and the Coast Guard positioned their boats to corral the whales and used sound vibrations from aluminum pipes to drive them away from the shore on Thursday, Mase said.
Using these "gentle herding" techniques, scientists were able to get the 35 whales into 18-foot-deep water about 6 miles offshore. Happy with the progress, most of the rescuers returned to shore midafternoon Thursday.
"We are cautiously optimistic," Mase said. "Even though we are hopeful, this situation can go either way."
Of the 51 whales initially discovered stranded at Highland Beach on Tuesday morning, 11 are dead. Five that have not been located may have died and sunk to the ocean floor, she said.
Pilot whales usually swim in waters 1,000 feet deep about 15 miles offshore, but Mase said she would be satisfied once the whales moved into water 100 feet deep.
The scientists hope examinations of the dead whales will yield clues to why they beached themselves. Mase said likely causes could be changing currents, changing water temperature or the Morbillivirus, which has infected more than 800 animals along the Mid-Atlantic and Southeastern coast.
Animals' behaviors in mass stranding is unpredictable. Mase noted that a similar stranding occurred in the mid-1990s, when a group of beached whales re-stranded themselves after tentatively swimming back out to sea.
"We're doing our best to do what we can," Mase said. "Knowing what usually happens with these whales ... the outcome might not be good."
Mase said the rescue mission has been difficult: At one point, the whales scattered across 15 miles of remote beach an hour away from the nearest boat dock. There is no cellphone service, so rescuers could not communicate, and the warm waters are teeming with hungry sharks.
Four dying whales were euthanized Wednesday when scientists determined they could not be saved. The carcasses were left in the water to decay naturally, Mase said.
Short-fin pilot whales measure up to 20 feet. A tight-knit species, they often stay together as a group, and if one can't keep moving, they all stop.
"This is a very fluid situation," Mase said. "Things could change minute by minute or overnight."
Contributing: Associated Press