The December 2011 edition of ConsumerReports includes a distressing expose of mislabled fish. Researchers bought 14 different kinds of fresh and frozen fish sold at Metropolitan New York City retail stores and restaurants and sent 190 DNA samples to an independent laboratory to see how the samples matched with the labels under which the fish was sold or served. They discovered that one-fifth of the samples were mislabeled as a different species of fish.

More specifically, only four of the fourteen types of fish purchased – Chilean Sea Bass, coho salmon, bluefin and ahi tuna – were always identified correctly. All of the “lemon sole” samples were identified incorrectly and twelve of the 22 “red snappers,” that is, 60%, were not the claimed species. Worse still, none of “red snapper” samples sold at eighteen separate retail markets (as opposed to restaurants) could be positively identified as red snapper. In contrast, 24 of the 28 salmon samples were indeed confirmed to be salmon.

Examining the reasons for the mislabeling, ConsumerReports concludes that it is less a matter of fraud and more a matter of the way commercial fish are processed. Fish pass through so many different hands from the time they are caught and each level of processing may have different terminology. In addition, when processed at sea, the guts and head of fish are removed making it much harder to properly identify.

Nevertheless, even unintentional mislabeling poses a health concern. Tilefish is often confused with grouper. And tilefish has detectable levels of mercury averaging three times more than grouper, making it particularly dangerous to children and women of childbearing age.

Whether the same problem persists for fish sold in kosher restaurants and kosher fishmongers is unclear but bears investigation.