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When you’re paying for food by the pound, it’d be nice if the weight you’re being charged for was accurate. Apparently, according to a new investigation into frozen seafood at supermarkets, quite often this isn’t the case. And no matter if the flawed measurements represent honest mistakes or purposeful deception, the result is the same: Consumers are paying for more than they’re getting, and there’s ample reason to believe there’s something fishy about the seafood business.

In fishing contests, you’re generally disqualified if you freeze fish before they’re weighed. That’s because a fish that’s full of ice weighs more than one that’s fresh.

A new investigation by the Boston Globe explains that this basic concept causes havoc for shoppers hoping to get what they pay for at the grocery store:

Typically, frozen seafood is coated with ice to keep it fresh and minimize freezer burn. Some businesses in the supply chain add extra ice and include it in the weight declared on the label. Retailers end up charging for the water, and shoppers pay more money for less fish.

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For the investigation, the Globe hired an independent lab to weigh 43 different seafood items purchased at various supermarkets in Massachusetts. The results showed that it’s commonplace for seafoods to weigh less than what the grocery store label indicates. Two-thirds of the samples that originated from one Rhode Island fish supplier, Henry Gonsalves Co., weighed less than the labels stated at the Market Basket and Save-A-Lot grocer chains. Underweight frozen food was also found in major food retailers such as Walmart, Target, and Stop & Shop—for example, a package of scallops listed at 16 ounces actually weighed 13 ounces.

What makes it fairly easy for false measurements to wind up on labels is in the industry practice of “glazing” seafood with ice. The official weight is supposed to exclude glazing and packaging, but few, if any, grocery shoppers bother to remove the packaging, spray and drain the item, and weigh it to see if the measurement matches up with the label.

If they did, there’s a decent chance they’d discover they’re paying for less than they received. In a 2010 study by state investigators in Connecticut, half of seafood samples were underweight. The seafood inspection program of the National Oceanic and Atmospheric Administration found fraud in more than 40% of products submitted to the agency for testing, and at least 80% of those cases involved inaccurate weights.

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The inaccuracies mean that consumers, and perhaps supermarkets as well, are getting charged a bit extra regularly. This is money that adds up over time, as Lisa Weddig of the National Fisheries Institute told the Globe:

“Rather than looking at this as 30 cents here and 30 cents there, we should be looking at this as a $69 billion seafood industry and these practices could be costing the industry and consumers tens, if not hundreds, of millions of dollars in the end,” Weddig said.

The Globe, by the way, is on quite a roll in terms of snagging big fish-fraud stories. Last fall, an investigation revealed that supermarkets and restaurants often mislabeled seafood, so that, for example, consumers who thought they were getting cod actually wound up eating cheaper hake.

Brad Tuttle is a reporter at TIME. Find him on Twitter at @bradrtuttle. You can also continue the discussion on TIME’s Facebook page and on Twitter at @TIME.