PASCAGOULA, Miss. (AP) — 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.”