The federal government has closed 35 percent of the gulf waters under its jurisdiction to fishing. Officials in Louisiana, Alabama, Mississippi and Florida have also closed hundreds of miles of water along the coast. The idea is to make the margins around the spill so wide that even areas several miles from the site are off limits.

"The main tool is that closed area, that red line,” said John Stein, the head of the NOAA Deepwater Horizon oil spill seafood-safety program, who spoke with reporters in Pascagoula, Miss., last week. “No fishing inside that red line.”

Image Workers carry specimens of deep-water fish to the lab. Credit... Jeff Haller for The New York Times

Although the NOAA regularly keeps a handful of vessels in the gulf pulling samples to assess seafood stocks, in the months since the spill, the agency has provided 16 boats just to sample for evidence of oil. Its laboratories in Mississippi and Seattle have processed 1,576 samples to date. That includes oysters, which can’t move through oiled waters, and big fish like swordfish and tuna, which can.

Late last month, federal and state agencies responsible for keeping tainted seafood from the market announced new rules to help monitor gulf waters and determine when some areas can be reopened. Initial responsibility now falls to a panel of seven trained analysts who smell samples of seafood from the area. If three of the seven testers detect the smell of oil or other chemicals, the sample is deemed tainted and the area where it was found is closed to fishing or kept from reopening, said Christine Patrick, a NOAA spokeswoman. If there is no taint, the sample is sent to labs in Seattle for chemical analysis. Once the oil stops flowing and pressure mounts to reopen the closed areas, the panels could be called upon to test up to 100 samples a day.

It might seem a surprisingly unscientific method, but sensory testing is considered the gold standard. “The nose, believe it or not, is a sensitive organ and is capable of detecting low levels of hydrocarbons,” said Joan Bowman of the International Food Protection Training Institute, a nonprofit organization financed by the W. K. Kellogg Foundation. “Some people have the DNA to do it and some don’t.”

So far, the organization has paid for 56 special sniffers to travel to the gulf for NOAA training.

The chemicals in petroleum have been linked to cancer if eaten at high levels over time, according to the F.D.A. But since the chemicals are not believed to accumulate in the body, the F.D.A. has said that low levels are not necessarily harmful. “However, it should not be present at all,” the agency said in the statement.