A man sorts shrimp in Thailand, one of five countries that supply more than three-quarters of the shrimp imported to the United States. The other top importers are India, Indonesia, Ecuador and Vietnam. (Barbara Walton/European Pressphoto Agency)

When Washingtonians dig into boiled shrimp with drawn butter at a local restaurant, the odds are nearly 50-50 they won’t be eating the species of shellfish promoted on the menu.

Seven of 15 shrimp samples obtained last year from Washington-area restaurants, or 47 percent, were misrepresented to consumers, according to advocacy group Oceana, which released a report Thursday asserting widespread deception behind America’s favorite seafood. Overall, the group found that 43 of 143 shrimp samples obtained from restaurants and grocery stores in four regions — Washington; New York City; Portland, Ore.; and the Gulf Coast — were misrepresented.

The most prevalent problem was mislabeled shrimp, said Kimberly Warner, senior scientist with Oceana and the study’s lead author. “Then there were things that were unusual,” she added.

Among the “unusual” findings were three shrimp species that had never previously been described genetically, as well as banded coral shrimp, typically sold as an aquarium pet, found in a bag of frozen salad shrimp in the gulf area. The most common mislabeling found was farmed whiteleg shrimp sold as more expensive and flavorful wild or gulf shrimp.

Oceana didn’t name the stores and restaurants where investigators bought the shrimp because the group doesn’t know where along the supply chain the fraud took place. But Oceana’s Warner noted that one D.C. restaurant offered a dish of rock shrimp, a relatively rare deep-water species known for its lobster-like flavor. It was actually brown shrimp, a common species found in the gulf and the southeast Atlantic.

When Oceana bought shrimp at D.C. grocery stores, the results were somewhat better. Of 15 shrimp products purchased, three, or 20 percent, were found to be misrepresented.

But District grocers topped all four regions in the number of products — 48 percent — that offered no country-of-origin label, which the Agriculture Department mandates of retailers .

Overall, with 10 of 30 surveyed products misrepresented — 33 percent — Washington had the second-highest level of shrimp mislabeling, behind New York’s 43 percent. Portland scored the best of the surveyed regions, with only 5 percent of shrimp misrepresented.

More than 1.6 billion pounds of shrimp were imported into the United States in 2013, representing about 90 percent of the shrimp consumed in the country, according to a report recently released by the National Oceanic and Atmospheric Administration’s National Marine Fisheries Service. More than three-quarters of the imported shrimp comes from five countries: India, Thailand, Indonesia, Ecuador and Vietnam.

The Food and Drug Administration is responsible for ensuring the safety of all imported seafood. According to a July report by the Food & Environment Reporting Network, the agency inspects only 1 to 2 percent of seafood imports and rejected only about 0.33 percent of the seafood inspected in 2012. But FERN noted that imports are also overseen by officials with both the U.S. Customs Service and NOAA Fisheries, who inspect as much as 40 percent of the seafood arriving on our shores.

Regardless, according to Oceana, the combination of widespread mislabeling and a patchwork of governmental oversight could create health, or at least ethical, problems for consumers. If, for example, they’re unwittingly dining on farmed shrimp from Thailand rather than wild-caught gulf shrimp, they might be supporting an operation that relies on forced labor to catch the fish that are fed to the shrimp, based on a recent State Department report. Or they might be eating shrimp that comes from farms that rely on antibiotics, fungicides and other chemicals that negatively affect the environment.

When it comes to mislabeled restaurant shrimp, “this is not CSI,” said Gavin Gibbons, a spokesman for the National Fisheries Institute, which represents seafood companies and restaurants. To determine the source of fraud, he said, an auditor compares the supplier’s invoice, the product’s DNA test and the menu.

“If the invoice and the DNA test don’t match the menu, then the restaurant is at fault. If the DNA test and the menu don’t match the invoice, then the wholesaler is at fault,” said Gibbons. “The investigation process is not a difficult one. You just have to see it to its completion.”