Oceana, a NGO focusing on ocean conservation, has DNA tested 1,200 seafood samples sold in 674 retail outlets in 21 states in U.S.A. to find out whether they were properly labeled or if certain species were being passed off as other, more commercially desirable ones.
It turns out that out of the 1,200 samples tested 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.").