After being vegetarian for a few years, I made the decision to begin eating fish again, as science has overwhelmingly told me to do so in order to lead a healthier life. My problem is that fisheries are possibly the most unsustainable resources on the earth (a huge reason why I am/was vegetarian in the first place is because I hate how unsustainable meat production is). Anyway, I just wanted to say to you that I think it is very important that you read up on what you are consuming. Most countries have a guide (NZ one is below, and more information for NZ is HERE) that you can take with you in your wallet to help you know what to buy, and it is not very difficult to choose a more sustainable choice when faced with the option at a supermarket or restaurant if you have an idea of what the better choice is.
There is also a "red list" of fish which you should try not to eat under any circumstances (because of rapid depletion of the species, destructive fishing practices etc), and that is HERE.
Just to make it easier for you in other countries, I've found your guides, links to them are here:
USA (heaps of info here)
Europe/Asia/Africa (various guides from World Wildlife Fund)
One last thing... recently I've found it quite difficult to know what to say on my blog, as I've become increasingly aware of having opinions (that I would like to share) on things which are really important to me, (such as the topic above, subjects relating to rational thinking, and other such slightly more volatile topics than the usual "I made a pretty thing" posts) and I'm not sure if I should include them or not. Maybe it's just that I feel like finally, after 23 years I can come out of my shell a bit more and add to conversation instead of my usual sit back and listen, but this may not be the right forum for it. I guess I'm just trailing off now, but let me think about this and get back to you.