Be aware of what you eat! A resent research project showed that most of the fish which restaurants put on your plate are not what you ordered, but a cheap variety...
Well, cheating is as old as the world itself. As long as humans exist, some of them have been trying to sell lesser wares for the true thing - and rid you of a lot of money.
So, when you order tuna in a restaurant, there is a big chance you'll be getting the cheap yellowfin tuna instead of the bluefin one. Or you get pangasus filet instead of haddock.
There is only one thing you can do: only eat in a reputed fish restaurant, where the chef buys his fish straight from the fishermen - and allows you to see the catch. It may cost some more, but then you'll be sure not be tricked.