Fish is frequently misidentified on menus and grocery store counters in New York City, even at expensive restaurants and specialty shops, DNA testing for a new study found. National supermarket chains had the best record for accuracy in seafood labeling, the researchers reported.

The researchers, from the conservation group Oceana, said that genetic analyses showed that 39 percent of nearly 150 samples of fresh seafood collected from 81 establishments in the city this summer were mislabeled. The study did not identify any of the restaurants or stores, although it noted that most were in Manhattan.

In some cases, cheaper types of fish were substituted for expensive species. In others, fish that consumers have been urged to avoid because stocks are depleted, putting the species or a fishery at risk, was identified as a type of fish that is not threatened. Although such mislabeling violates laws protecting consumers, it is hard to detect.

Some of the findings present public health concerns. Thirteen types of fish, including tilapia and tilefish, were falsely identified as red snapper. Tilefish contains such high mercury levels that the federal Food and Drug Administration advises women who are pregnant or nursing and young children not to eat it.