Somsak, what I wrote here was just my personal idea. I am aware of some theories (not all) related to the vegetarian/environmentalist/animal-rights movements. The one you mentioned is actually quite good. Not only cereals that are "wasted" in farming could feed more people, but also avoid deforestation and therefore slow down global warming. There are also interesting theories about the gasses produced by farms' animals (their droppings apparently produce emissions which are probably more dangerous in size than the ones produced by cars - but that's just a theory).
I am not sure whether pescatarians always choose not-farmed fish. As a matter of fact it's not even easy to make sure that the fish you buy is not from a farm.
BTW I found a very interesting website
http://transitiontowns.org/
They use a concept that I had used in one of my researches (I thought I had finally invented something, but as usual, I was wrong :( ): the concept of "resilience" applied to economy (I had applied that to a concept of "sustainable company").