Something fishy was going on at a famous Mississippi seafood joint.

Mary Mahoney’s Old French House Restaurant, a well-known institution in the heart of Biloxi’s fishing capital, passed off foreign, frozen seafood as local Gulf Coast catches for years, federal prosecutors charged.

Mary Mahoney’s pleaded guilty on Thursday to misbranding seafood and wire fraud, according to the US Attorney’s Office for the Southern District of Mississippi.

Co-owner Anthony Charles Cvitanovich, 55, also pleaded guilty to misbranding 17,190 pounds of seafood during 2018 and 2019.

The mainstay, which was founded in 1962, admitted that between 2013 and 2019, it worked with co-conspirators — including its wholesale supplier — to fraudulently sell over 29 tons of fish that was frozen and imported from Africa, India and South America.

It then catfished customers into paying more for lake perch, tripletail, triggerfish and unicorn filefish that they thought were premium Gulf red snapper and redfish, according to prosecutors.

“The conspirators … benefited from the sale of seafood that would not have been as marketable nor as profitably sold if its actual species and origin had been known,” prosecutors said.

They also said the scheme started as early as 2002.

“When people spend their hard-earned dollars to enjoy the incredible local seafood on the Mississippi Gulf Coast, they should get what they paid for, not frozen fish from overseas,” US Attorney Todd Gee said in a statement.

“Mislabeling food and defrauding customers are serious crimes, and this case will help convince restaurants and seafood suppliers that it is not worth lying to customers about what is on the menu,” he added.

The bass-lighting seemingly ended in 2019, when a federal raid went down and briefly shuttered the eatery.

The restaurant later issued a press release saying the FDA was there “in conjunction with an investigation of mislabeled fish,” according to the Biloxi Sun Herald.

Co-owner Bobby Mahoney was not charged with a crime — and his lawyer said it was all a fluke.

“They made a mistake,” Michael Cavanaugh told the Sun Herald, and said the misbranding of fish was “a glitch.”

“It was resolved,” he added. “They’re moving on.”

The restaurant and Cvitanovich are set to be sentenced in September. Mary Mahoney’s faces a maximum penalty of five years’ probation and a $500,000 fine. Cvitanovich, who was released on bond, faces up to three years in prison and a $10,000 fine.

Cvitanovich’s lawyer also contended the issue was corrected in 2019.

“Over the past five years, we have had extensive discussions with the federal government over inaccurate entrée descriptions of a certain item on our menu,” Tim Holleman said.

Genetic testing of fish by the Food and Drug Administration confirmed the fraudulent scheme that customers fell for hook, line and sinker, according to the US Attorney’s Office for the Southern District of Mississippi.

The restaurant owners are on the hook for $1.35 million that must be forfeited to the government, the Sun Herald reported.

The family spot — known for its gumbo — was named one of the 50 most beautiful restaurants in the country earlier this year. It sits on Highway 90 across from casinos and a Hard Rock Hotel.