Oceana is a great NGO focusing on ocean conservation. Over the past few years, they've done detective work using DNA testing to figure out if fish sold in grocery stores, restaurants, and sushi venues was properly labeled or if certain species were being passed off as other, more commercially desirable ones. Well, they didn't do all this work for nothing...
It turns out that out of the 1200 seafood samples Oceana collected from 674 retail outlets in 21 states, 33% were mislabeled. The graph below shows that sushi places are the worst offender, with 3/4 of samples tested not what they claimed to be!
Another shocking finding is that 59% of the fish labeled 'tuna' sold at restaurants and grocery stores in the U.S. is not actually tuna, and 87% of 'snapper' tested is not actually snapper ("In fact, only seven of the 120 samples of red snapper purchased nationwide were actually red snapper. The other 113 samples were another fish.").
Precision: When a type of fish is mislabelled, it's not necessarily as a completely different species. One type of tuna could be labeled as another type of tuna, for example. You can find more details on page 12 and 16 here (pdf).
Oceana has been doing this kind of research into seafood fraud for years. Check out our previous post about it: Bait and Switch Report Reveals Rampant Seafood Fraud.