At a Bertucci’s restaurant in Boston’s Longwood Medical Area, the baby cod advertised on the menu was hake, a less expensive and mushier fish found off the coast of southern Africa.

All 23 white tuna samples tested as some other type of fish, usually escolar, which is nicknamed the “ex-lax’’ fish by some in the industry because of the digestion problems it can cause.

The Globe-sponsored DNA testing found 24 of the 26 red snapper samples were in fact other, less prized species, including fish collected at Minado restaurant in Natick, Teriyaki House in South Boston, and the now closed Big Papi’s Grille in Framingham, owned in part by Red Sox slugger David Ortiz.

It happens for a range of reasons, from outright fraud to a chef’s ignorance to the sometimes real difficulty of discerning one fillet from another. But industry specialists say money is commonly the motivator: It’s a way to increase profits - a cheaper fish sold as something more pricey - on the assumption that customers will not detect the difference.

The results underscore the dramatic lack of oversight in the seafood business compared with other food industries such as meat and poultry. Nationally, mislabeled fish is estimated to cost diners and the industry up to hundreds of millions of dollars annually, according to the National Fisheries Institute, a trade group.

The Globe collected fish from 134 restaurants, grocery stores, and seafood markets from Leominster to Provincetown, and hired a laboratory in Canada to conduct DNA testing on the samples. Analyses by the DNA lab and other scientists showed that 87 of 183 were sold with the wrong species name - 48 percent.

Those were among the findings of a five-month Globe investigation into the mislabeling of fish. It showed that Massachusetts consumers routinely and unwittingly overpay for less desirable, sometimes undesirable, species - or buy seafood that is simply not what it is advertised to be. In many cases, the fish was caught thousands of miles away and frozen, not hauled in by local fishermen, as the menu claimed. It may be perfectly palatable - just not what the customer ordered. But sometimes mislabeled seafood can cause allergic reactions, violate dietary restrictions, or contain chemicals banned in the United States.

At Chau Chow Seafood Restaurant in Dorchester, the $23 flounder fillet turned out to be a Vietnamese catfish known as swai - nutritionally inferior and often priced under $4 a pound.

The sliver of raw fish sold as white tuna at Skipjack’s in Foxborough was actually escolar, an oily, cheaper species banned in Japan because it can make people sick. The Alaskan butterfish at celebrity chef Ming Tsai’s Blue Ginger in Wellesley was really sablefish, traditionally a staple at Jewish delicatessens, not upscale dining establishments.

Recent studies indicate that between 20 percent and 70 percent or more of snapper, cod, grouper, and wild salmon are mislabeled at restaurants and stores. Oceana, a nonprofit based in Washington, D.C., that is campaigning against seafood fraud, also discovered mislabeling this year when it conducted DNA tests of fresh and frozen fish at 15 Boston-area supermarkets. Nearly one in five fillets tested had the wrong species name, according to Oceana. Atlantic cod was the most frequently misrepresented fish.

But after being questioned by a Globe reporter about the substitution, Garcia recently revised the menu at his small Cuban restaurant to list ocean perch.

“The flavor is pretty good,’’ he said. “I have never received any complaints about it in the last couple of years.’’

“They are completely different fish. I’m not going to lie to you,’’ he said. The switch, Garcia said, began when red snapper was hard to find and more expensive - he could buy ocean perch for about $4 pound, compared with roughly $8 a pound for red snapper fillets.

For instance, Nobel Garcia, owner of El Oriental de Cuba restaurant in Jamaica Plain, admitted serving ocean perch instead of the $14 red snapper in garlic sauce promised on his menu.

Some restaurant owners whose fish was found to be misrepresented said they, too, were victims, misled by distributors. Others acknowledged that they swap species when supplies are low or fresh fish isn’t available, or to trim costs.

Seafood substitution can take place anywhere along the international route most wild and farmed fish take to a diner’s plate in the United States. The practice is carried out by fishermen, importers, wholesalers, restaurants, and stores.

The Globe chose to focus most of its testing on certain species, such as red snapper and tuna, because they have been identified by regulators as more likely to be mislabeled, so the findings do not represent all types of fish sold.

Frozen fish at grocery stores was far less frequently misidentified, with some sellers - including Walmart, Trader Joe’s, and BJ’s Wholesale Club - passing muster in all instances. At restaurants, mahi mahi and swordfish were correctly labeled in all samples tested.

“Mislabeling fish is at a ridiculous level,’’ said Eric Hesse, a Cape Cod commercial fisherman. “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 from us. It kills us.’’

Previously frozen chunks of Pacific cod took the place of fresh New England cod or haddock at popular restaurants such as Ken’s Steak House in Framingham, Doyle’s Cafe in Jamaica Plain, and Cy’s Nantucket Bar & Grill.

Seafood substitution makes it harder for consumers to accept that some species, including red snapper, are overfished, since it regularly appears on restaurant menus.

“If people see something on the menu all the time, they may have no idea it is disappearing from the ocean,’’ said Beth Lowell, an Oceana campaign director.

A fish transformed

Throughout much of the last century, the cold waters off New England supplied fresh fish that was delivered daily to Massachusetts restaurants and other businesses.

But as overfishing began to deplete populations, the region’s vast network of fresh-fish houses and processing plants increasingly turned to seafood caught thousands of miles away. Restaurants and grocery stores that once exclusively bought just-caught fish began to accept frozen fillets.

Imported fish - most of it frozen - makes up 86 percent of the seafood Americans eat, compared with less than 50 percent in 1980. Last year, the United States imported about 5.5 billion pounds of fish worth almost $15 billion, much of it from China.

As consumption of foreign seafood has increased, so has mislabeling, according to industry specialists. Because the sea-to-plate process has become so long and complicated, there are more opportunities for fraud and mistakes to take place. And the absence of regular testing by federal and state agencies has allowed fish substitution to thrive.

The US Food and Drug Administration, which oversees the labeling of imported and domestically shipped fish, maintains a list of acceptable market names for a particular species.

The agency said it is working to develop a better program to identify mislabeled fish using the same DNA method employed by the lab the Globe hired. Officials said they expect to start testing at six field laboratories by early next year.

Without such safeguards in place, consumers often can’t tell what kind of fish they are eating. Some seafood importers, for instance, have repeatedly misnamed Vietnamese catfish by calling it grouper, most likely to avoid high tariffs on this Asian species.

Substitutions can also take place at processing plants where whole fish are transformed into standardized fillets that are often indistinguishable from one another. At some restaurants, chefs call fish anything their imagination conjures, disguising the identity with sauces and spices.

“Once you fillet a fish, it can be very difficult to tell what it is, if not impossible,’’ said John Sackton, publisher of Seafoodnews.com, an online industry newsletter based in Lexington. “Even the best chefs can have difficulty.’’