I read an interesting story this morning about fish, yet another piece of evidence in the case against eating too much (if you hadn't already come in with a guilty verdict). Seems that farmed fish grow much more efficiently when they're young, but as they get older -- and bigger -- they get more sluggish about converting their food to mass.
In other words, the bigger the fish, the more wasteful its use of resources.
So if you buy farmed fish, and you care about sustainability, and you want to put your mouth where your mouth is, go for the smaller fillets, not the big slab. And as a nice side benefit, you eat less protein (which we rich Americans get way more of than we need) and have more room for vegetables. Here's a nice plate of food that I think would get me good marks on that pop quiz, even if it is an all white/brown meal.
By contrast, if you are going for wild-caught fish, you should avoid the smaller ones, which often have not reached full reproductive capability. Eating them would be shooting yourself in the foot, threatening future generations of fish sandwiches.
More to think about, if you weren't already ethically fraught by visiting the grocery store.