Wildlife officials in Florida have reined back their optimism for the survival of a pod of whales stranded for several days in the shallow waters of the Everglades National Park, after a coastguard search crew spotted about 20 of the animals heading back towards the shore.

On Friday, rescuers had expressed their hopes that up to 35 of the original group of 51 might have escaped to sea after an earlier helicopter pass found no trace of them. But the discovery by marine biologists aboard a later flight of the smaller group one mile closer to the Florida coast than the larger pod's position on Thursday indicated that they had reversed course away from deeper water. A further search was scheduled for Saturday morning.

"We're less optimistic that they're going to continue to move offshore at this point," said Blair Mase, the Miami-based marine mammal stranding co-ordinator for the National Oceanic and Atmospheric Administration (NOAA).

She added that the whales' behaviour suggested they were suffering from dehydration and malnourishment after spending so much time in the shallow waters off Florida's south-west coast, and that a three-day rescue effort by federal and state agencies, including volunteers from wildlife groups, was drawing to a close. "Anything can happen. [But] at some point we have to stand back and let nature take its course, " she said.

An earlier search in a remote area of the Everglades turned up only seven whales not at first thought to be part of the larger breakaway group. They were found swimming in deeper waters six miles from shore and nine miles north of Highland Beach, where 11 of the original number died earlier in the week.

The rest of the group remained unaccounted for, Mase said, and had either escaped to deep water or died. What were thought to be two more whales struggling in shallow water much closer to shore turned out to be dolphins, she added.

Another helicopter flight was planned for Saturday morning to try to get a more accurate picture after poor weather forced a premature halt to Friday's operations. A coastguard cutter remained in the area ready to manoeuvre between whales and the shoreline if any continued back towards the beach and if further rescue efforts were thought beneficial. About 35 volunteers have also spent two days in boats trying to herd the survivors towards the ocean.

The survival of such a large number of the whales would have been an unusual outcome. According to NOAA, mass whale strandings rarely end well. The agency says success rates for survival are low and euthanasia is often the only option for beached whales, to spare the malnourished and dehydrated animals the pain of a slow death.

Mase said that necropsies had been performed on all 11 dead whales, four of which were euthanised. They included juveniles and adults of both sexes and all had empty stomachs, indicating that they had been unable to feed for several days and that their health had been compromised.

The full results of the examinations and a formal cause of death, however, might not be known for several weeks. Marine mammal experts have suggested a number of possible reasons for the beaching, including the US Navy's use of underwater sonar equipment, which can affect the animals' navigation, and an increasing number of cases of morbillivirus, which until recently was more common in dolphins.

"We didn't have anything jumping out at us," Mase said. "Particularly with mass beachings you don't often see any obvious signs of cause of death. Morbillivirus is definitely something we're looking into, particularly

because we've seen outbreaks along the Atlantic coast and pilot whales are known to be carriers."