It remains unknown how long the damage could last, but Steiner says, 21 years after the Exxon spill, two-thirds of the injured population of fish and birds in that area have yet to fully recover.

Some disagree with that assessment, however. Ben Lieberman, senior policy analyst at the Heritage Foundation think tank, said the area affected by the Valdez spill recovered more quickly than many had forecast and many gloom and doom predictions never came to pass.

Meanwhile, scientists said oil could spread with the arrival of the hurricane season due soon. The spilt crude could travel wherever a storm surge takes it -- even to New Orleans, for example.

The National Center for Atmospheric Research reported Thursday that its computer model indicated oil could drift upward along the Atlantic coast this summer.

The simulation suggests the oil could reach Florida's Atlantic coast within weeks if it gets caught in the loop current of the Gulf of Mexico. Then the Gulf Stream could pull it as far as North Carolina.

Dolphins and sea turtles may also be in danger. The U.S. Fish and Wildlife Service and National Oceanic and Atmospheric Administration have confirmed at least one dead dolphin and one dead sea turtle found in the area showed signs of oil damage, although the majority have not been tested for oil contamination, according to the organization's website. Nearly 30 dead dolphins and 227 dead sea turtles have been collected in the area.

"The deaths of dolphins and sea turtles are particularly tragic, because we know we are only seeing the tip of the iceberg with those that have been washed up onshore and tested," said National Wildlife Foundation scientist Doug Inkley in a statement.

Researchers have reported at least two massive underwater plumes of oil, each hundreds of feet deep and stretching for miles. "The effect of these plumes on marine life remains hidden beneath the ocean's surface, making it impossible to capture the full scope of the oil spill's impact," the statement said.

Lieberman said it was too early to assess the full extent of the damage. With the last two major oil spills -- in Santa Barbara in 1969 and the Exxon Valdez in 1989 -- the damage was not as severe as some had predicted, he said.