COCODRIE, La.—Eating at North America's southern rim, where the land fades into the water, demands a stomach for seafood—particularly shrimp, crab and fish, such as sea bass. After all, Louisiana alone pulls in some six million metric tons of seafood per decade, and Terrebonne Parish, which encompasses Cocodrie, is responsible for 20 percent of the state's oyster, crab and shrimp harvest. But the dark line that runs along the underbelly of locally caught fresh shrimp—the aquatic creature's gut—is full these days of the residue from last year's BP oil spill, along with the microscopic plants and animals that make up the shrimp's diet.

The official line, however, is that the seafood is safe for human consumption.

"Our seafood has to be the most tested seafood anywhere," says biologist Martin Bourgeois of the Louisiana Department of Wildlife and Fisheries. Oil contaminants "have been detected but at levels well below any threat to human health and safety."

The U.S. Food and Drug Administration (FDA) agrees, due to a battery of sniffing, tasting (or sensory tests, in agency parlance) and chemical testing since April 30, 2010 to detect the presence of either oil or the dispersants used to break up the slick. The National Oceanic and Atmospheric Administration (NOAA) opened the last of the federal Gulf waters closed to fishing on April 19. Testing at the start of this month reconfirmed via the same tests that oil was no longer present in the fish caught in the area immediately around the Macondo well.

"The seafood coming out of the Gulf is perfectly safe to eat," says retired Coast Guard Adm. Thad Allen, who was the national incident commander for the worst oil spill in U.S. history. "The American public needs to understand that."

There is no question that most fish at least can process—transforming the contaminant from a compound that is fat-soluble into one that is water-soluble—and excrete the nasty stuff found in oil: polycyclic aromatic hydrocarbons (PAHs), such as benzene, toluene and more, although studies after previous oil spills suggest that other marine life can accumulate these toxicants and even concentrate them as they pass up the food chain. "Those are long-term persistent and some are considered carcinogenic," says toxicologist Scott Miles of Louisiana State University, "but you find that in your gasoline every day" as well as in the sticky black burned fat left over on barbecue grills.

Such compounds are not found in the fish tissue on which we dine. "You can expose fish [to PAHs in water] for two years and, at the end, the tissue burden is no higher," notes toxicologist Joe Griffitt of the University of Southern Mississippi.

When marine animals are exposed to the oil—whether through breathing oil-laced waters or eating oil-carrying plankton—it can cause serious side effects, however, such as dampening the immune system, which can allow infections or tumors to develop. That list is similar to the impacts on people. For example, the PAH known as benzo(a)pyrene is both highly toxic and breaks down into cancer-causing byproducts.

The guideline for FDA and NOAA testing is an exposure level to benzo(a)pyrene, for example, that will not result in more than one excess cancer per 100,000 people exposed over a lifetime. That is assessed based on an individual who weighs 80 kilograms and eats four jumbo shrimp four times a month. And to determine the contamination level of any seafood, at least 10 trained personnel sniff and taste samples for petroleum "taint" as well as apply a chemical solvent to dissolve and capture any contaminants.

But the FDA only tests the "edible portion" of the shrimp, which, for FDA purposes, is the muscle tissue; it does not test, for example, the gut contents of the shrimp. PAHs and oil are in the gut, says ecologist Paul Sammarco of the Louisiana Universities Marine Consortium, who is involved with such gut testing, and at very high levels, as well as in the fat, which clings to the interior of the exoskeleton near the head, among other places. And, at least in Louisiana, if not elsewhere, people most often eat the shrimp gut and all—and they eat a lot more than four shrimp in a serving. "That's what falls off my po'boy," says Mobile Bay baykeeper Casi Callaway. "I was raised on half-a-pound per person and then add a pound."

In addition, the FDA did not do any testing for groups of people who might be at higher risk, such as pregnant women and children. "Very few labs can do [analysis of seafood] and do it correctly," contends toxicologist Ralph Portier at Louisiana State University.

For its part, the U.S. Environmental Protection Agency (EPA) mandates a more stringent standard for acceptable contamination levels in water than the FDA's limits in seafood. That can lead to conundrums where the EPA declares a water body unsafe for fishing even though the fish in that water body will contain a given toxicant at levels below the maximum allowed by FDA. "The FDA approach allows higher mercury than the EPA approach," notes public health scientist Daniel Harrington, also of Louisiana State. "So don't eat fish from the lake per EPA, but the fish bought in a grocery store per FDA might be more contaminated."

The EPA and NOAA continue to test Gulf Coast waters for the presence of hydrocarbon contaminants and continue to find them, although they are highly diluted. More than 200 million gallons of oil from the Macondo blowout are now dispersed in the roughly 640 quadrillion gallons of water in the Gulf of Mexico.

In any case, "everybody down here is eating all the seafood they can get," says Terrebonne Parish President Michel Claudet. "I'm eating it all the time."

Your faithful correspondent, for one, ate a heaping portion of shrimp gumbo, adequately doused in hot sauce. As Portier says: "The joke is we put Tabasco on seafood to hide the oil."

Editor's Note: Reporting for this article took place as a result of a fellowship from the Metcalf Institute for Marine and Environmental Reporting at the University of Rhode Island.