A new investigation provides fresh evidence that restaurants and markets continue to dupe seafood lovers into paying top dollar for low-grade fish.

As part of a special “Fishy Business” series, the Boston Globe spent five months buying fish from dozens of establishments throughout Massachusetts and sending the samples off to a lab in Canada. DNA tests found 48 percent of the fish had been mislabeled as a more expensive type of fish.

Fish samples were gathered from 134 restaurants, grocery stores and seafood markets, and the results were staggering.  Every one of 23 white tuna samples tested turned out to be something other than tuna. In most cases the fish labeled tuna was escolar, which the Globe said was “nicknamed the Ex-Lax of fish by some in the industry for the digestion problems it can cause.”

All but two of the 26 red snapper samples were another kind of fish, the Globe reported. That came as no surprise to Cape Cod fisherman Eric Hesse, who was quoted in the report.

“Mislabeling fish is at a ridiculous level,” said Hesse. “The dealers and restaurants have a vested interest in keeping the illusion going. Every time they can say they are selling fresh, local fish and get away with selling [Pacific] frozen, they don’t have to buy it from us. It kills us.”

The problem extends far beyond Boston and affects consumers nationwide. Earlier this year, ABC News correspondent Elisabeth Leamy reported that seafood may be mislabeled as often as 70 percent of the time.

“According to Food and Drug Administration port inspections,” Leamys said. “A third of seafood sold in the U.S. is mislabeled as one type when it’s actually something else, even something cheaper.”

The environmental group Oceana said part of the problem is the FDA’s lax enforcement of laws that make it a crime to mislabel seafood.

“American consumers would be outraged if they ordered roast beef and they got horse meat or God forbid, whale meat,” Michael Hirshfield, a chief scientist at Oceana, told Leamy. “They should be outraged if they order snapper and they get tilapia or some endangered species.”

FDA officials acknowledged they could do more to police against fish fraud. The agency has nine new seafood testing machines and is training inspectors in how  to use them. Field testing is expected to begin early next year.

For now, not all the Globe’s results were so disheartening. Every sample tested from Walmart, Trader Joes and  BJ’s Wholesale was correctly labeled, as was every sample of mahi mahi and swordfish.