Sea otters are swimming - and dying - in oily water, and goo-covered birds that cannot fly are dying a slow death on oil-blackened islands in Prince William Sound in America's biggest oil spill, biologists said Friday.

But the rescue effort was going as badly as the efforts to clean up 11 million gallons of oil that spewed from the tanker Exxon Valdez when it steered off course onto a reef March 24."We're never going to have a body count," said Everett Robinson-Wilson, a U.S. Fish and Wildlife biologist. "We're never going to be able to to tell how many animals die out there. Some sink. Some float."

But he also said that on Green Island alone, up to 1,000 birds had died.

One week after the spill, many hundreds of oiled animals have been found on beaches or seen during spill overflights. But only four had been taken to a makeshift rescue center by Friday noon - three birds and a sea otter, and one of the birds died.

Oil covered 862 square miles of Prince William Sound. The National Oceanic and Atmospheric Admiinistration reported "many" oiled birds near the southern reaches of Prince William Sound, but low clouds grounded most aircraft and made it impossible to map the spill and find wildlife.

Three fishing vessels with veterinarians aboard were converted to floating wildlife crisis centers in key places to retrieve oiled animals and have them flown to Valdez by float plane or helicopter.

Dozens of sea otters have been seen swimming in oil and biologists say that when their fur becomes matted with oil they will lose their insulation and die.

Other marine mammals with fur survive because they also have a thick insulating layer of blubber, biologists said.

Steve Zimmerman, of the National Marine Fisheries Service, said he saw seal coming up in area covered with tar balls and has seen whales and sea lions swimming in oily waters, but there have been no confirmed reports of dead seals, sea lions or whales.

There have been reports of dozens of sea otters getting bogged down in the oily mess.