It's pretty hard to pass off certain seafood as something else; lobster is one that comes to mind. Yet it's relatively easy when you're looking at less-identifiable types of fish. Here's a list of some of the more commonly mislabeled fish, along with their often-used substitutes [source: Oceana]:
Alaskan/Pacific cod: Asian catfish, Atlantic cod, threadfin slickhead, tilapia
Alaskan/Pacific halibut: Atlantic halibut, blueline tilefish
Atlantic cod: Pacific cod, white hake
Chilean seabass: Antarctic toothfish
Grouper: Asian catfish, king mackerel, whitefin weakfish
Lemon sole: Blackback flounder, summer flounder, flathead sole, yellowfin sole
Red snapper: Caribbean red snapper, crimson snapper, Pacific ocean perch, yellowtail rockfish, madai, tilapia, white bass
Salmon (wild, king, sockeye): Farmed Atlantic salmon
Sea bass: Antarctic toothfish, Patagonian toothfish
Snapper: Giltheaded seabream, madai, tilapia, Pacific ocean perch, widow rockfish, yellowtail rockfish
White tuna: Escolar
Some of the fish most often mislabeled from the list above are grouper, snapper and white tuna. Grouper is an obvious choice for fraudulent substitutions, as swapping it out for Asian catfish, for example, nets the seller four times more money. But another problem with grouper is that the Food and Drug Administration (FDA) recognizes 66 species of fish as "grouper," making it difficult for distributors and restaurateurs to know what's what [source: Lou].
In the Oceana study, an astonishing 87 percent of snapper samples were mislabeled, making it the most mislabeled type of fish. Tuna came in second, with a 59 percent mislabeling rate. White tuna is especially problematic. It's not even a real fish species. Albacore tuna is, and that's considered white meat, which is how the term "white tuna" came into existence. Escolar is almost always slipped in as white tuna's substitute, though escolar is banned by the FDA because it acts as a laxative [sources: Goetz, Oceana].