Fifty pilot whales trapped in shallow water headed out to sea Friday after 250 volunteers, including schoolchildren, struggled throughout the night to save them. But another 40 whales died of exhaustion on the shore at Mangawhai, 50 miles north of Auckland.

The stranding was the worst since 1978, when 170 whales died after swimming ashore in Auckland's Manukau Harbor.Conservation officer Paul Irving said local volunteers spent the night in waist-deep water supporting the mammals. "Each of the whales had a group of between eight and 10 people supporting it while it waited for high tide," Irving said.

"The rescuers had to stop the whales from rolling on their sides, which would have prevented them from breathing through their blowholes.

"As the tide came in the rescuers guided and urged the whales out to sea, but as they moved away they left a trail of dead lying on their sides," he said. Children from the local school helped in the rescue.

Irving said whale beaching is a regular occurrence on the New Zealand coast. "But why whales, particularly pilot whales, beach themselves on shore is unknown," he said.