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, both sniffers and others, 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 last week. "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 both in closed and open waters, and sending it off for chemical testing, with more than 600 fish and shrimp processed to date.

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 only has "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.

She said the agency has deployed a mobile lab to Florida that is testing samples of fish caught in waters not yet believed to be impacted by oil, because fish don't stay in one place.

Gulf fishermen are already hurting from the perception that their product is tainted, said Ewell Smith, executive director of the Louisiana Seafood Promotion and Marketing Board.

"Some people also just think we're shut down altogether," Smith said, adding that higher prices for shrimp are causing smaller businesses to cancel orders simply because they can't afford it.

Smith said no oily seafood will ever make it to market.

"You're going to smell it, you're going to see it. It would be almost impossible for it to make it to market," he said.

Fishermen say they can't sell a tainted product anyway, whether it is inspected or not. Earlier in the week, fishermen brought in thousands of pounds of shrimp caught off Louisiana to the docks at Port 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 fishermen Mike Nguyen, who brought in 3,000 pounds of shrimp on Wednesday.

"When the shrimp get oily, they die and they stink," he said. "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."

Mississippi shrimper Richard Bosarge agreed, and said no one wants to sell oily shrimp.

"If we catch oily shrimp, the nets are coming up," Bosarge said shortly before heading out to sea.

He called the sniffers "ridiculous."

"They're going to smell it? No way," added Mike Triana, who works for a Mississippi gas company along the coast. "How they gonna know? I ain't eating any of it. I don't trust the nose."

Gerald Wojtala, director of the International Food Protection Training Institute, acknowledged that nosing around seafood may sound silly, but said it's a time-proven technique.

"The human nose has been used on a lot of (oil) spill response," Wojtala said. "There are a lot of sophisticated tests, but when you think about it, do you want to run a test that takes seven days and costs thousands of dollars?

"This saves a lot of time and money," he added, "and it puts more eyes and noses at different points in the system."

Still, Wojtala said, nothing is fail-safe. Even without an oil spill, people sometimes get sick from tainted seafood, or suffer illnesses from contamination in red meat such as E. coli.

"It's safe to say there is no 100-percent guarantee," he said. "There's never a 100-percent guarantee. We can only be as safe as we can be."

(Associated Press Writer Brian Skoloff authored this report.)