U.S. scientists have found damage to deep sea coral and other marine life on the ocean floor several kilometres from the blown-out BP well in the Gulf of Mexico.

The discovery made by a government-funded expedition is a strong indication that damage from the spill could be significantly greater than officials had previously acknowledged.

Tests are needed to verify that the coral died from oil that spewed into the Gulf after BP's Deepwater Horizon rig exploded in April, but the chief scientist who led the expedition said Friday he was convinced it was related.

"What we have at this point is the smoking gun," said Charles Fisher, a biologist with Penn State University who led the expedition aboard the Ronald Brown, a National Oceanic and Atmospheric Administration research vessel.

"There is an abundance of circumstantial data that suggests that what happened is related to the recent oil spill."

Data contradicts officials' rosy picture

For the government, the findings were a departure from earlier statements. Until now, federal teams have painted relatively rosy pictures about the spill's effect on the sea and its ecosystem, saying they had not found any damage on the ocean floor.

In early August, a federal report said that nearly 70 per cent of the 643.5 million litres of oil that gushed from the well into the sea had dissolved naturally, or was burned, skimmed, dispersed or captured, with almost nothing left to see — at least on top of the water. The report was blasted by scientists.

Most of the Gulf's bottom is muddy, but coral colonies that pop up every once in a while are vital oases for marine life in the chilly ocean depths.

Coral is essential to the Gulf because it provides a habitat for fish and other organisms such as snails and crabs, making any large-scale death of coral a problem for many species. It might need years, or even decades, to grow back.

"It's cold on the bottom, and things don't grow as quickly," said Paul Montagna, a marine scientist at the Harte Research Institute for Gulf of Mexico Studies at Texas A&M University in Corpus Christi. He was not on the expedition.

Montagna said the affected area is so large, and scientists' ability to explore it with underwater robots so limited that "we'll never be able to see everything that happened down there."

Using a robot called Jason II, researchers found the dead coral in an area measuring up to 40 metres by 15 metres, about 1,402 metres under the surface.

"These kinds of coral are normally beautiful, brightly coloured," Fisher said. "What you saw was a field of brown corals with exposed skeleton — white, brittle stars tightly wound around the skeleton, not waving their arms like they usually do."

Damage could be at 25 other sites

Fisher described the soft and hard coral they found 11 kilometres southwest of the well as an underwater graveyard. He said oil probably passed over the coral and killed it.

The coral has "been dying for months," he said. "What we are looking at is a combination of dead gooey tissues and sediment. Gunk is a good word for what it is."

Eric Cordes, a Temple University marine scientist on the expedition, said his colleagues have identified about 25 other sites in the vicinity of the well where similar damage may have occurred.

An expedition is planned for next month to explore those sites.

When coral is threatened, its first reaction is to release large amounts of mucus, "and anything drifting by in the water column would get bound up in this mucus," Cordes said. "And that is what this [brown] substance would be: A variety of things bound up in the mucus."

About 90 per cent of the large coral was damaged, Fisher said.

The expedition was funded by the Bureau of Ocean Energy Management and the National Oceanic and Atmospheric Administration. The mission was part of a four-year study of the Gulf's depths, but it was expanded this year to look at oil spill damage.

In a statement released Thursday night, NOAA Administrator Jane Lubchenco said the expedition underscored that the damage to marine life from the oil spill is "not easily seen." She added that more research was needed to gain a "comprehensive understanding of impacts to the Gulf."

"Given the toxic nature of oil, and the unprecedented amount of oil spilled, it would be surprising if we did not find damage," she said.

NOAA did not provide any officials or scientists of its own who went on the expedition. The Bureau of Ocean Energy Management said its researcher on the expedition was unavailable.