Many Europeans are fretting these days over horse meat, and whether it might have adulterated their shepherd’s pie. Over here, it’s all about the red snapper.

That tempting seafood delight glistening on the ice at the market, or sizzling at the restaurant table in its aromatic jacket of garlic and ginger? It may not be at all what you think, or indeed even close, according to a big new study of fish bought and genetically tested in 12 parts of the country in restaurants, markets and sushi bars by a nonprofit ocean protection group, Oceana.

In the 120 samples labeled red snapper and bought for testing nationwide, for example, 28 different species of fish were found, including 17 that were not even in the snapper family, according to the study, which was released Thursday.

The study also contained surprises about where consumers were most likely to be misled sushi bars topped the list in every city studied while grocery stores were most likely to be selling fish honestly. Restaurants ranked in the middle.

Part of the problem, said the study’s chief author, Kimberly Warner, is that there are quite simply a lot of fish in the sea, and many of them look alike. Over all, the study found that about one-third of the 1,215 fish samples bought, from 2010 to 2012, were mislabeled.

“Even a relatively educated consumer couldn’t look at a whole fish and say, ‘I’m sure that’s a red snapper and not lane snapper,’ she said.

Geographically, the study had bad news for residents of Southern California, land of the midnight sushi. They were more likely than residents in any of the other sampled regions to be eating something other than what they had been told. About 52 percent of the samples bought and sent for testing turned out to be something else.

Salmon-crazed Seattle, by contrast, was tied with cod-country Boston as having the lowest deception rates. Still, almost one in five fish samples were wrongly labeled.

Some types of fish fared worse in different places. In New York, for example, fish that was not really tuna was being passed off as tuna in 94 percent of the samples taken, the study said.

What is in a name, anyway, when it comes to fish? Shakespeare might not have pondered the question exactly, but the seafood industry certainly has.

In the real world of perception and marketing, a fish called “slimehead a real name, by the way is probably not going to fly off the menu. Far better to call it “orange roughy, a distinction allowed by the Federal Food and Drug Administration. The government also allows Patagonian toothfish, real name, to be called Chilean sea bass, invented marketing name.

The Oceana study did not declare a fish mislabeled if the seller was following the federal guidelines.

But what the study found pervasive was mislabeling beyond what is allowed by federal food regulators by retail outlets using a name that consumers are more likely to want to buy. Almost two-thirds of the “wild salmon samples, for example, were found actually to be farmed Atlantic salmon, which is considered less healthy and environmentally sustainable.

The National Fisheries Institute, a trade group representing the industry, did not respond to a request for comment on the study. But the institute has specifically criticized Oceana’s work in the past as being more agenda-driven than scientific and rigorous.

Dr. Warner said that the study did raise some red flags about health. Fish known to accumulate mercury in their flesh, in particular, should be avoided, especially by pregnant women. But in what the study called “one of the most egregious swaps, in New York, tilefish known for its mercury content and on federal advisory lists for sensitive populations to avoid was sold as red snapper. Tilefish was also found substituted for halibut.

She also cautioned that the study did not aim to produce a real scientific, top-to-bottom sampling of the seafood system. In Boston, for example, only grocery store fish were tested, leaving out sushi bars that might have lowered the city’s honesty rate (although, in a separate effort in 2011 and 2012, The Boston Globe found widespread mislabeling in stores and restaurants).

And the Oceana study’s authors also could not determine where in the food chain the mislabeling arose the wholesaler, the retailer or at the fishing dock itself or whether it came about through misunderstanding or deliberate concealment.

A common pattern, though, Dr. Warner said, was economic gain at the consumer’s expense, with understudy fish tilapia, in particular often substituting for a menu’s star, without a word to the audience.

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