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Special report | Fishy business

How the Globe tested fish

Fish samples were collected from restaurants and grocery stores across Massachusetts. Most of the testing was focused on certain species — including Atlantic cod and tuna — because they have been identified by regulators as more likely to be mislabeled.

 Each sample included the receipt and was labeled with the date collected, restaurant or store name, and price. Samples were stored in industrial freezers in a locked room at the Globe.

 Using a sanitized scalpel, reporters placed small pieces of fish — roughly the size of a Q-tip head — in separate ethanol-filled test tubes supplied by officials at the Biodiversity Institute of Ontario at the University of Guelph, which is home to a 26-nation consortium called the International Barcode of Life.

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 Test tubes were sent by express mail to the Biodiversity Institute.

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 Barcode of Life researchers successfully conducted DNA analysis from most of the samples, matching them with a species in their database. Two samples were discarded because the DNA could not be read. The Globe also sent a select group of specimens to a second lab for DNA testing.

RESULTS: Of 76 fish samples collected from stores and restaurants that sold mislabeled fish in 2011, more than three-quarters of them were again mislabeled in 2012.