Just from the bit in the article, he seems to be arguing that since fish (or chickens) can't be conciously "aware" of pain they can't feel it. That seems to me to be an unneccessarily restrictive definiton of pain. Almost a silly definition. And certainly more of a philosophical distinction then a neurological one. Pain and fear are, beyond a certain point, inarticulate. We don't think pain, we feel it. I don't think we can ever figure out exactly what the experience is like for fish, but I don't think he's saying (or can say) anything more than "Its different from ours".