The fish you buy in a supermarket or restaurant might not be what you think it is. (Image credit: Getty Images)

Nearly 40 percent of seafood sold in New York City is mislabeled, according to a conservation group's new report on a fishy practice that spells trouble for people with food allergies.

"Recent testing has revealed that dishonest labeling and fraudulent seafood substitution for certain species is rampant and widespread," researchers from the ocean conservation group Oceana wrote in their report, which they said was based on DNA testing of 142 seafood samples collected from unidentified New York City grocery stores, restaurants and sushi bars.

Oceana previously reported fish mislabeling rates as high as 48 percent in Boston and 55 percent in Los Angeles.

Oceana said the findings are particularly troubling given that seafood ranks among the top eight food allergens. And since fish allergies are often species-specific, experts say the bait-and-switch opens the door to dangerous exposures.

"If [a person] is not allergic to the fish they think they are getting, and that fish is substituted with one to which they are allergic, they obviously could have a serious allergic reaction," said Dr. David Fleischer, an associate professor of pediatric allergy and immunology at National Jewish Health in Denver, Colo. "Patients need to be able to trust the people they purchase fish from."

Here's a table of commonly substituted fish from the report:

Among the "most troubling substitutions," according to the report, was fish labeled as white tuna that turned out to be escolar, a type of snake mackerel linked to gastrointestinal problems. Also, fish sold as red snapper and halibut turned out to be tilefish, which has mercury levels that land it on the U.S. Food and Drug Administration's "do-not-eat" list for pregnant or nursing women and young children.

"Without accurate, honest labels that show exactly what fish you are eating and where it was harvested, those who need this critical advice about specific fish will be left unprotected," the report authors wrote.

Previous reports on fish fraud have sparked outrage from politicians, who argue the FDA should do more to curb seafood mislabeling.

"Seafood fraud is not only deceptive marketing, but it can also pose serious health concerns, particularly for pregnant women seeking to limit exposure to heavy metals or individuals with serious allergies to certain types of fish," Senator Barbara Boxer, D-Calif. wrote in an October 2012 letter to the FDA. "Consumers should not have to question the safety of their seafood."

But it's unclear where along the chain "from bait to plate" the mislabeling is taking place. Citing a 2009 Government Accountability Office report, Boxer said 86 percent of seafood consumed in the U.S. originates overseas but only 2 percent of it is inspected by the FDA and only 0.01 percent is "explicitly inspected for fraud or mislabeling."

"Seafood can follow a complex path from the point when it is caught to the point when it is sold to a consumer, making it difficult to isolate the point where fraud occurs," she wrote. "To effectively address this problem, we need better traceability and enforcement throughout the entire chain of sale, from bait to plate."

A spokeswoman for the FDA said the agency had not yet reviewed the Oceana report and "therefore cannot comment on the report at this time." But, she added, "All seafood is required to be labeled in a manner that is truthful and not misleading and in accordance with federal regulations. It is not acceptable to misrepresent the identity of a product, including at the final point of sale to a consumer."

Buying fish from reputable dealers and being wary about unusually low prices can help protect consumers from fish fraud, according to the FDA. The agency also has a list of commonly substituted seafood products and photos of whole fish and fillets.