Image: Stranded whales and bottlenose dolphins
Tony Ashby  /  AFP - Getty Images
Rescuers prepare to herd a pod of long-finned pilot whales and bottlenose dolphins back out to sea from Hamelin Bay, south of the western city of Perth, Australia, on Tuesday.
updated 3/25/2009 6:36:38 PM ET 2009-03-25T22:36:38

Six whales believed to be part of a pod that was rescued from a mass stranding in southwestern Australia earlier this week died after they re-beached themselves, the government said Wednesday.

Two of the long-finned pilot whales were already dead when they were spotted by airplane on a beach about four miles away from where a pod of 10 had been released a day earlier. Veterinarians were sent to euthanize four others that were deteriorating rapidly, the Western Australia state conservation department said.

By the time officials arrived Wednesday, one of the four beached whales had died on its own. Veterinarians shot the remaining three ailing creatures, the department said.

"We believe they are part of the rescued group from yesterday, so it's very disappointing," said John Carter, operations officer with the department.

He said they thought the other four rescued whales were safe at sea.

"At this stage we're assuming they are OK but we'll be monitoring the coast and waters over the next few days," Carter told The Associated Press.

The whales were part of a group of about 90 whales and five bottlenose dolphins that became stranded on a beach in Western Australia state early Monday. Most of the animals died, but rescuers were able to push four dolphins and four whales out to sea at the stranding site and truck 10 surviving whales overland to deeper waters Tuesday.

Interactive: Swimming with whales The saved whales appeared disoriented at first, trying to swim back to shore, but rescuers guided them to deeper waters and the animals began swimming away. Officials had hoped they were swimming to safety.

Carter said three dead whales were reported Wednesday on the beach near the site of the original stranding. But they were part of the original group, not saved whales returning to shore, he said.

At least one carcass along the beach had been chewed by sharks, and the beach would remained closed to swimmers until the danger of attacks passed, the department said.

This week's mass beaching was the fifth in Australia in as many months; nearly 500 whales have died.

Scientists say the types of whales that beach themselves are extremely social groups that follow pod members into danger. But they cannot explain what draws the deep-sea animals so close to shore.

There are a number of possible theories. The whales may be chased by predators such as killer whales, or they could be following prey themselves. The sonar they use to navigate the dark seas could be hindered by natural geomagnetic factors such as iron ore deposits. They may swim into an area where sandbars or peninsulas block their exit. Or they may follow one ill or injured pod member.

Human activity such as undersea exploration for petroleum or the sonar of submarines can also interfere with whale and dolphin navigation.

Whatever the reason, once one animal heads for the dangerous shallows, the rest are likely to follow.

"Certain species of whales are more prone to mass strandings because the social bonds between them are incredibly strong," said Mike Bossley of the Whale and Dolphin Conservation Society. "If one animal is in trouble, the others won't leave him."

Once stranded, some are battered by rocks and surf, while others die of overheating or have their organs crushed by their own body weight after leaving the buoyancy of water.

The mass strandings occur most often in the island state of Tasmania, in Australia's southeast, and in Western Australia.

Copyright 2009 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

Video: Whales returned to sea

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