Thinking of some fish for dinner? Maybe a nice red snapper filet? Beware, because the fish you buy may not be snapper at all.

According to a study by Oceana, a nonprofit ocean conservation group, one-third of the more than 1,200 seafood items purchased by researchers nationwide between 2010 and 2012 was mislabeled. DNA samples showed the fish was something other than what the label claimed. The most commonly mislabeled fish was snapper. Researchers found 87 percent of the snapper they purchased from stores and restaurants was improperly labeled. Among the most common substitutes for snapper: seabream, tilapia and rockfish.

Tony Maltese, who has spent 50 years in the seafood industry as a commercial fisherman and a retailer — he's a former vice president at the Fairway Market grocery chain in New York — says faking fish is all too easy for a crooked merchant.

"The average person looking at it, they can't tell. But you can sell four or five different fish that look like a red snapper and they're not a red snapper," he told CNBC's "American Greed."

The Oceana study blames the widespread fraud on an "increasingly complex and obscure seafood supply chain," making it difficult to determine whether the fraud starts on the boat, at the wholesaler, at the store or a combination of all three.

Maltese says that while most people in the industry are honest, the incentives for some to cheat keep growing.

"Right now, the state of the fishing industry in this country is tough," he said. "I've been doing it my whole life and it becomes harder and harder every year to make a living."

He blames government regulations aimed at controlling overfishing, as well as unscrupulous merchants who can, for example, purchase swai — an Asian catfish — for $3 or $4 per pound, then sell it in the U.S. as grouper for more than five times that amount. The more people cheat, the more difficult it becomes for honest fishermen to make a living.

One of the most egregious cheaters of all time was Carlos Rafael, who operated a commercial fishing fleet in New Bedford, Massachusetts. Known locally as "The Codfather," he ruled the waters with an iron fist.

"I control the f------ flounder market in New Bedford," Rafael said in a conversation recorded by undercover federal investigators, obtained by "American Greed." IRS agents posed as Russian mobsters in the sting.

He did so by squeezing out smaller competitors, falsifying government reports and mislabeling his catch to make it look like he was complying with the regulations. Rafael is serving a four-year prison sentence after pleading guilty last year to 28 criminal counts including falsifying federal records, falsely labeling fish and illegal smuggling of cash, in a scheme that threatened the livelihood and the reputations of honest people in the industry.

"The vast majority of fishermen in this port, New Bedford, all up and down the East Coast, are honest, hardworking guys," said Rodman Sykes, a third-generation commercial fisherman.

The Oceana study says crimes like Rafael's — and other rampant fraud in the industry — pose risks to the environment by endangering sensitive fish populations. The study says some of the fraud could even be hazardous to our health.

"You'd like to know what you're buying," Maltese said. "Who knows what you're eating, you know? It's a very serious problem, mislabeling fish. It's been that way for the past 20 years."

How can you tell if the fish that you are buying is what the label says it is? Maltese says when it comes to certain fish such as snapper and grouper, beauty truly is skin deep.

"Flip it over. Tell the man you want to see the skin on it," he said. "If it's skinned, don't buy it as a red snapper or a grouper."

Red snapper skin should be a bright pink, almost red. Grouper skin is speckled gray. Without the skin, it is nearly impossible to tell what fish you are looking at.

"When you take the skin off, it looks like several different fish," Maltese said.

Another frequent substitution is farm-raised salmon sold as wild or sockeye salmon. Maltese says wild salmon tends to be less brightly colored than the farm-raised variety, which are often raised on specially formulated food to make them more orange. Farm-raised salmon may also have white lines running through the meat. Those lines are fat, which is a telltale sign the fish was not caught in the wild.

"The wild salmon very rarely has any fat lines in it because they have to swim, and they stay in shape trying to catch their food, whereas the farmed fish sits around and eats pellets," he said.

Maltese says the law requires fish markets to display a sign showing where the fish is from and whether it is fresh or frozen. But there is no substitute for knowing your fish seller.

"They should be around a while," he said. "You have to pick a place that has volume, a lot of customers, and if you see a lot of customers in the store, you know they're turning the fish over and if they're around 30, 35 years, 20 years, you know that there's a reason for that."

The Oceana report advises consumers to ask questions, such as what kind of fish it is, whether it was caught in the wild or farm-raised, and when and how it was caught.

The organization says to remember the old saying. If the price is too good to be true, it probably is.

"You are likely purchasing a completely different species than what is on the label," the report says.

And if possible, buy the whole fish instead of a filet. That is much more difficult to fake.

To make sure the fish you are buying is fresh, Maltese says to give it a good whiff.

"If you pick up a piece of fish and it smells like fish, it's not fresh," he said.

The colors should be bright, the eyes clear and rounded. If you detect a sheen or film on the fish, stay away. Chances are it has been soaked in brine to make it appear fresher than it is.

"It's not a natural thing to have a sheen on fresh fish," Maltese said.

Above all, when you are shopping for that red snapper for tonight — or anytime you are in the market for seafood — it is important to keep in mind that you are navigating a market that is relentlessly buffeted by basic economics.

"There's more people now eating fish than there ever was in history, and they realize how healthy it is worldwide," Maltese said. "We used to think the ocean was a bottomless pit. But we found out it's not so. It's supply and demand."

See how "Codfather" Carlos Rafael earned his nickname — and why fish fraud is so hard to police — on an ALL NEW episode of "American Greed," Monday, Sept. 24 at 10pm ET/PT only on CNBC...