In 2007 a Chicago woman bought some monkfish from an Asian market, cooked it up in a soup, ate it, and shortly thereafter began vomiting. Yet when the Food and Drug Administration investigated the incident, they found that this wasn’t a case of bad monkfish. In fact, it wasn’t monkfish at all.

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The FDA learned this thanks to Bob Hanner, an associate professor at the University of Guelph’s Biodiversity Institute of Ontario. Using DNA extracted from the victim’s soup, Hanner identified the mystery creature as pufferfish—a species that is highly regulated by American and Canadian governments due to its neurotoxic properties. The Chicago woman suffered pins and needles for a day, as well as weakness in her legs requiring weeks of rehabilitative care. But given the amount of toxin she had consumed, she was lucky not to have died.

The 2007 episode was important, because it represented one of the first prominent applications of a Canadian-pioneered technique called DNA bar-coding. The genetic equivalent of the black and white UPCs printed on retail products, DNA bar codes allow scientists to identify species by comparing the sequence of a specific gene to a reference database. And it’s not just the stray faux monkfish that betrays a fraudulent DNA signature: according to Hanner, one in four fish samples tested in his lab is mislabelled.

While most mislabelling incidents won’t land people in hospital, Hanner’s data is nonetheless unsettling. Labelling laws are in place to ensure that consumers know exactly what they are buying at the supermarket. Yet we are often duped when it comes to our fish—which is ironic, given that seafood is frequently treated as the health-conscious foodie’s lean protein of choice.

Take escolar, a deep-sea fish. Though no one asks for this low-value species by name at the supermarket, it sometimes gets sold as white tuna or Atlantic cod—intentionally mislabelled for economic gain. For escolar themselves, the oil that pervades their skin and muscles provides a source of buoyancy and energy. However, for some buyers unfortunate enough to consume it in significant quantities, these indigestible lipids can generate days of discomfort. “It’s probably not going to kill you, but nobody wants to pay for the experience,” Hanner says. (The same year Hanner was analysing soup remnants, 600 people in Hong Kong ended up in hospital after eating escolar labelled as Atlantic cod. The fish is banned in a number of countries, though Canada is not one of them.)

The issue of mislabelling originates in part with the global nature of the fish supply chain: A species caught in one corner of the planet will often be shipped to another corner where labour costs are lower. In the process, all outwardly recognizable features of the fish are removed—because consumers want no-fuss meals free of heads and tails. The products are then relabelled and exported. As the seafood crosses borders, original names may become lost in translation. While we might like to believe that this is simply a matter of human error, the mislabelling almost invariably results in a cheaper fish being relabelled as a more expensive variety.

The number of species that regulators must contend with is immense: The FDA and Canadian Food Inspection Agency lists that provide legally acceptable market names for fish and seafood each contain more than 1,800 entries. Even between the two countries, there are discrepancies in naming conventions. In Canada, for example, “kingfish” is the mackerel species Scomberomorus cavalla. In the United States, “kingfish” is an acceptable market name for four species, none of which is S. cavalla. (Europe recently resolved this by requiring species names to be included on all fish labels.)

Despite its prevalence, mislabelling has not been a priority among Canadian regulators. Hanner believes this must change. “One of the more insidious cases of mislabelling is farmed fish being sold as wild-caught,” he says. Aquaculture products are required to undergo random toxicity screening because of the antibiotics and pesticides used by the industry, as well as their possible exposure to carcinogenic substances. That fish-farm products might be mislabelled as wild therefore “creates a loophole.”

Until recently, most of Hanner’s work had been with the FDA, which has adapted the diagnostic methods developed at Guelph for its own purposes. Though Canadian agencies have been involved with the bar-coding network since its inception, our inspectors still lack on-the-spot, easy-to-use test kits that allow for the rapid screening of all seafood species.

The result: for Canadian consumers who want to be sure that their monkfish is monkfish, the only surefire method is to buy the creature whole—head, tail, and all. With their enormous skull and fearsome jaws, these members of the genus Lophius hardly qualify as beauties of the sea. But at least you’ll know what’s swimming in your soup.

This appeared in the December 2015 issue.

Nicola Temple Nicola Temple ( nicolatemple.com ) wrote Sorting the Beef from the Bull, a book about food forensics. Katie Carey Katie Carey ( katie-carey.com ) is in artist. She has contributed to the New York Times, the Boston Globe, and the Village Voice.