This post from Eric Loomis is astounding.  Cod is dead.  North Atlantic cod anyway.  Check out that graph.  Look at how quickly the catch drops off.  This is what happens when you over fish.

Sustainable fishing does exist, though.  And you can do your part to support sustainable fishing.  The Monterey Bay Aquarium has a handy dandy Seafood Watch Program that allows you to check to see if your favorite fish is sustainably caught.  Only buy fish that is sustainably caught.