"If you can't trust your restaurant ... it's kind of off-putting," said Hem Borromeo, an academic adviser at Excelsior College who was not affiliated with the Oceana report. He said a New York Times article last year about fish mislabeling in New York City enlightened him about how pervasive the practice is.
(More From NBC: Flu Hit American Workers Hard in January)
At a sushi eatery he visited earlier this year, Borromeo said he encountered a piece of yellowtail he suspected was not quite as advertised. "I had ... a suspicion," he said. He ate the mystery fish anyway, but added, "I didn't go back there again."
The most common mislabeled fish is snapper; only seven of Oceana's 120 "snapper" samples were the real deal. Snapper is an easy target because it looks like other, cheaper fish. Once it's skinned, filleted and cooked, it's hard for even discerning foodies — or cooks, for that matter — to tell whether it's actually tilapia on the plate.
There's a tremendous financial incentive for restaurants to pass off cheaper fish to unsuspecting diners, said Marcus Guiliano, owner of Aroma Thyme Bistro in Ellenville, N.Y., a restaurant certified by the Green Restaurant Association. Guiliano video-blogs about mislabeled restaurant food at TruthinMenu.com.
(More From NBC: Younger Americans Have No House, No Car, Less Debt)
"If you bought cheap tilapia today, it would be maybe $3.50 a pound on the high side," he said. The wholesale price of snapper, meanwhile, could be as much as $11 per pound. And it's not just snapper. Farmed Atlantic salmon wholesales for about $6 per pound, Guiliano said, while authentic wild salmon easily costs twice as much. Fake fish can boost profits, and customers are usually never the wiser.
For a restaurant owner to know what they're actually getting, Guiliano said, "you buy from a person who works directly with the boats. I always ask where, when and how was it caught, and what was the name of the vessel."
From a business perspective, it's hard for restaurateurs to budget food costs accurately if the price of a particular type of fish fluctuates; from a customer service perspective, it can be risky to tell diners an item on the menu isn't available.
And marketing plays a powerful role, too.
"People just get stuck on names. That's the big problem with a lot of items in our culinary field," said Paul Rother, a culinary school teacher and restaurant industry veteran.
"Those are things that help us ... market food," Guiliano said. "Certain things in a menu, when you write it, makes it more appealing."
For instance, the name "white tuna" has a certain cachet, yet 84 percent of the samples in Oceana's study turned out instead to be escolar, a fatty, deep sea-dwelling fish that can give diners gastrointestinal distress.
(More From NBC: Construction Resumes on Nation's Biggest Home)
"When I was younger I worked at a Greek [restaurant] and we used to get whole sides of escolar," Rother said. Having broken down the fish into fillets, he said he knew escolar when he saw it. On one occasion, Rother confronted the owner of a sushi restaurant about the mislabeling. The owner angrily denied it, he said.
Falsified fish is endemic, in part, because the market for seafood has become globalized and more complex, said Beth Lowell, Oceana's campaign director for Stop Seafood Fraud. More than 90 percent of the seafood we eat is imported.
"The more hands, the more processing your seafood goes through, the more opportunities for seafood fraud. The further you get away from the whole fish, the more likely that seafood fraud can occur," she said. Mislabeling, whether unintentional or deliberate, can happen when the fish is packed, shipped or sold by wholesalers or retailers.
For instance, as New England's traditional cod-fishing industry dwindles — beyond the point of recovery, some worry — Americans eat cod that was caught in Norway and packed in China, fisheries economist Jenny Sun of the Gulf of Maine Research Institute told The Associated Press.
Less than 1 percent of seafood imports are screened for fraud, Lowell said. Oceana is campaigning for a traceability standard and more stepped-up enforcement. "There's not a lot of enforcement on the labeling of seafood," she said. Importers and other middlemen who knowingly pass on this counterfeit fish are gambling that they won't get caught, Lowell said. "And they're winning."