Think you're eating grouper? Nope, it's Vietnamese catfish. What about cod? Could be escolar. Can you tell the difference between red snapper and rockfish? Farm-raised salmon and wild? Here's the thing with fish: once you fillet it and get rid of its characteristics, it can get kind of, sort of hard to tell what from what.
The Smithsonian Mag has a piece on LeeAnn Applewhite of Applied Food Technologies, a company that specialises in identifying fish through their DNA (and not just what they're labelled). She's our protector of the truth on seafood and she's found that some lobster bisques at restaurants are actually shrimp and the most common fish fakeout is poing Vietnamese catfish (which is cheaper) as cod.
How does AFT figure out what fish is what? According to the Smithsonian:
To identify a piece of fish, AFT staff slice off a tiny sample from a fillet, heat it up to break down the tissue and open up its cells, and spin it in a centrifuge to extract the DNA.
They then amplify a few genes to reveal the difference. In the pictures above and below, which are from a 2011 study at Oceana, you have (from top to bottom and left to right) escolar and atlantic cod, nile perch and grouper, swordfish and mako shark, red snapper and rockfish and farmed salmon and wild salmon. It's safer to just not eat than eat, I guess.