This post is a little outside what I usually write about, but it's an area that's been important to me for years. Kim O'Donnel, who writes about food for washingtonpost.com, has been covering the environmental sustainability of fish populations, in conjunction with the health benefits and risks of eating fish, in her blog this week; she's not done, so I can't give away the story, but one message is that our current fish consumption, and in particular where it would go if we were all to follow current health recommendations (two 3-oz. portions per person per week), is not sustainable, at least if we keep gobbling up the big three, salmon, tuna, and shrimp. So Kim proposes exploring other fish: sardines, tilapia, mussels, carp, and squid, for example.
I have to say I'm frustrated at how many restaurants still serve cod, which is particularly threatened. If you go up to New England, where I'm from, folks seem to think it's their birthright to eat cod until it's gone, and the Boston Globe regularly prints cod recipes without any acknowledgment of overfishing.
Here's a 2-page PDF that folds into a wallet-sized guide to better (and worse) seafood choices.