In cases of blatant mislabeling, a chef or supplier often takes the bet that a local or federal agency charged with stopping deceptive practices is not likely to walk in the door. “This has been going on for as long as I’ve been cooking,” said Tom Colicchio, a New York chef and television personality. “When you start really getting into this stuff, there’s so many things people mislabel.”

At Mr. Colicchio’s New York restaurants, all but about 5 percent of the meat he serves is from animals raised without antibiotics, he said. It costs him about 30 percent more, so he charges more. “Yet I have a restaurant down the street that says they have organic chicken when they don’t, and they charge less money for it,” he said. “It’s all part of mislabeling and duping the public.”

Consumers are misled most frequently when they buy fish, investigators say, because there are so many fish in the sea and such limited knowledge among diners. The Food and Drug Administration lists 519 acceptable market names for fish, but more than 1,700 species are sold, said Morgan Liscinsky, a spokesman with the agency.

Marketing thousands of species in the ocean to a dining public who often has to be coaxed to move beyond the top five — shrimp, tuna, salmon, pollock and tilapia — is not an exact science.

The line between marketing something like Patagonian toothfish as Chilean sea bass or serving langostino and calling it lobster is a fine one.