BP consultant Robert Nailon measures how high oil from the Gulf of Mexico spill has soaked marsh grasses in marshes around Bay Ronquille, La., in this July 15 photo. Nailon is part of a joint government-BP team trying to gauge the impact of the oil on the region’s fragile saltwater marshes.

Bay Ronquille, La. ? The marsh is soaked with oil and the grass is dying. It’s a common sight on the Gulf coast these days, and it’s nothing new for Robert Nailon.

The BP-hired environmental consultant kneels as he has done many times on the Louisiana coast, assessing the damage in a task now taking on new importance as the world’s attention turns from the ubiquitous images of gushing oil to the daunting task of restoration.

He dips his hand, covered in a blue rubber glove, into the muddy ground. It comes up streaked brown with crude. “You’ve got sheen throughout,” he says, and calls out his findings to a government scientist: Oil covers about 95 percent of the grass, reaching about 15 feet inland.

Both men nod, agreeing to add this stretch to the growing and painstaking census of the dead from the Gulf of Mexico oil spill. About 40 BP-government teams are cataloguing seemingly everything touched by the oil, from poisoned plankton and fish to lost marshes and stained beaches.

BP PLC will eventually be given two options: Restore everything itself, or pay the government to do it. Before a final bill is written, however, those tallying the damage must still account for things they can’t see — from contaminated fish eggs that never hatch to impacts that may take years to show.

Some experts worry BP could exploit the uncertainty to minimize its responsibility.

“If you end up with a bunch of dead fish five years from now, it becomes very hard to prove BP killed them,” said Mark Davis, director of Tulane University’s Institute on Water Resources Law and Policy.

BP spokesman John Curry declined to detail any potential challenges his company might make regarding wildlife and habitat claims.

“We’re not trying to run and hide from the situation,” he said. “Bottom line is we want to know exactly what the impact is, too.”

So far, about 4,000 birds, more than 700 sea turtles, dozens of dolphins and one whale have been found either dead, or alive but oiled. Oil has hit some 600 miles of shoreline and at least 44,000 square miles of the Gulf. The count doesn’t include the hundreds of oiled birds left in the wild to avoid disturbing their nesting grounds.

Pinpointing damage beneath the Gulf’s surface, however, is turning into an even bigger problem.

“It’s a 3-D challenge,” said Tom Brosnan, chief of the National Oceanic and Atmospheric Administration’s assessment and restoration division. “It’s not just on the shoreline, it’s at depth, down to 5,000 feet in the Gulf.”

The government is deploying remotely operated submarines to get snapshots of what is happening in the deep, as well as collecting water samples to assess the populations of plankton and other small organisms.

Computers will use the information gathered to produce estimates of how many plankton, fish or shrimp are killed based in part on how much habitat is ruined.

Gauging the consequences could take years and require some calculated guesswork to account for wildlife that dies or suffers unseen.

Federal officials haven’t said whether they’ve assigned a cost to everything.

In some cases, however, arriving at a cost can be as straightforward as similar efforts during the 11 million-gallon Exxon Valdez spill in 1989 in Alaska. The state priced each seagull at $167, eagles at $22,000, harbor seals at $700 and killer whales at $300,000.

The scope of the latest census is enormous — the Gulf spill has so far unleashed between 91 and 179 million gallons of oil — and the cost of that tally will likely prove expensive in itself.

In the case of the Valdez, $125 million has been spent on scientific research since the spill in Prince William Sound, said Stan Senner, Alaska’s restoration program manager following the spill and now director of science for the Ocean Conservancy.

Exxon settled with the government for its restoration costs in 1991, for $900 million. Another request 15 years later for $92 million more is pending.

In what could be a cautionary note for those working the BP spill, the settlement with Exxon never addressed a major impact tied to the Valdez by some scientists — the collapse of the Pacific herring population. That’s in large part because the collapse came two years after the settlement.