Scientists have only begun to understand the impact of the BP Plc oil spill disaster in the Gulf of Mexico, a top U.S. fisheries official said on Thursday.

While 37% of the Gulf's federal waters were closed to fishing in the wake of the Deepwater Horizon oil spill, only a fraction of 1% remains closed, a small block directly over the BP oil rig's wellhead, said Eric Schwaab, head of the U.S. National Oceanic and Atmospheric Administration's National Marine Fisheries Service.

But recent NOAA findings, such as dead and dying deepwater coral in the Gulf, make it clear scientists do not know what effect the spill, potentially the worse marine ecology disaster in history, will ultimately have.

"The long-term impacts to sensitive habitats and longer- term assessment of impacts on populations of fish, shrimp, crabs, oysters, sea turtles, marine mammals, we're still in the very early stages of understanding," Schwaab said in an interview after a keynote address to the 13th International Conference on Shellfish Restoration this week.

NOAA announced earlier this month that one of its research vessels had found dead and dying coral several miles (km) from the well that spilled more than 4 million barrels of oil into the Gulf from late April until mid-July.

The coral, at a depth of about 4,500 feet (1,370 meters), was covered by "what appeared to be a brown substance," NOAA said in a statement. Pennsylvania State University marine biologist Charles Fisher, lead scientist on the expedition, called the finding "the smoking gun" of the BP oil spill.

"The jury will be out for some time on the long-term ramifications," Schwaab said.

"We certainly know that the amount of oil in the marshes, while significant, could have been worse. We know that the movement of oil outside the northern Gulf, which was possible, did not materialize. ... The fate of oil that wasn't collected or hasn't weathered in the system remains to be determined."

But he added, "I think it's fair to say that the seafood coming out of the Gulf right now is very safe. It's well tested, probably amongst the best-tested seafood in the world right now."