Hi Mia,
The Environmental Defense Fund has a very nice series of reguarly updated webpages on this topic. I suggest you take a look. In particular they have a nice "substitutions list" you can print and take to the supermarket with you to make healthy (and environmentally sound) decisions about which fish species to enjoy, and which to substitute in recipes when the ones called for are not so safe to eat.
http://www.edf.org/page.cfm?tagID=1541