PASCAGOULA, Miss. - William Mahan bends over a bowl of raw shrimp and inhales deeply, using his left hand to wave the scent up toward his nose. Deep breath. Exhale. Repeat. He clears his palate with a bowl of freshly cut watermelon before moving on to raw oysters. Deep breath.

Exhale. Repeat.

He's one of about 40 inspectors trained recently at a federal fisheries lab in Pascagoula, Miss., to sniff out seafood tainted by oil in the Gulf of Mexico and make sure the product reaching consumers is safe to eat.

But with thousands of fishermen bringing in catch at countless docks across the four-state region, the task of inspectors is daunting. It's certainly not fail-safe.

The first line of defense began with closing a third of federal waters to fishing and hundreds more square-miles of state waters. Now comes the nose.

Mahan is an agricultural extension director with the University of Florida based in Apalachicola, where some of the world's most famous oysters are culled.

"We're being trained to detect different levels of taint, which in this case is oil," Mahan said recently. "We started out sniffing different samples of oil to sort of train our noses and minds to recognize it."

So what does an oily fish smell like?

"Well, it has an oil odor to it," Mahan said. "Everyone has a nose they bring to it. . . . Everybody's nose works differently. For me, the oysters are a little more challenging."

The human nose has been used for centuries to aid in making wine, butter and cheese, and is a highly efficient and trustworthy tool, said Brian Gorman with the National Oceanic and Atmospheric Administration, which is hosting the courses along with the nonprofit Battle Creek, Mich.-based International Food Protection Training Institute.

"Properly trained noses are really remarkable organs," Gorman said.

Even so, inspectors can't be everywhere. The trained sniffers will be deployed where needed, when suspicions are raised about seafood being illegally culled from closed waters, or even to test fish from open waters. No agency has yet reported finding or stopping any tainted seafood from getting to market.

The National Oceanic and Atmospheric Administration has also been sampling seafood in closed and open waters, and sending it off for chemical testing.

State and local inspectors are fanning out across the region to docks, seafood processors and restaurants, some now armed with specially trained noses. NOAA currently has 55 inspectors at its Mississippi lab, with another 55 in training.

"The message we're delivering is simple: The seafood in your grocery store or local restaurant is safe to eat, and that goes for seafood harvested from the Gulf," said Kevin Griffis of the U.S. Department of Commerce.

The U.S. Food and Drug Administration also has a role with its own inspectors, though the agency said it has only "several seafood specialists" currently in the Gulf area.

"We are ramping up inspections at facilities in the region," said FDA spokeswoman Meghan Scott, adding that inspectors would be present at seafood processors throughout the Gulf states.

Fishermen say that they can't sell a tainted product anyway, whether it is inspected or not. One recent weekday, fishermen brought in thousands of pounds of shrimp caught off Louisiana to the docks at Pass Christian, Miss., where the catch was offloaded and sold to processors and customers on site.

No inspectors were present.

"No oil, not even a drop," said fisherman Mike Nguyen, who brought in 3,000 pounds of shrimp. "When the shrimp get oily, they die and they stink. See? They're alive."

Joe Jenkins owns Crystal Seas Seafood Company on the docks at Pass Christian. He'll be buying thousands of pounds of shrimp.

"Here, we don't have inspectors on any level, so we have to inspect our own seafood products to make sure they're safe and oil-free and good to eat," Jenkins said. "We're not going to have inspectors everywhere. Everybody's got to do their own job . . . to make sure they don't have a problem with oily shrimp whatsoever."