Personally, I am a vegitarian. Now I do not eat fish, because I see the purpose as the meat intake of people now-a-days is wayyyyyyyy too much. You don't need three meals of meat as the main course. You only need about two servings of the size of the palm of your hand. People who are all "ohh I need my bacon and sausage for breakfast a hamburger at lunch and a huge steak for dinner" first of all, that is extremely unhealthy for you. Then if you prepare the food at home, often too much is cooked and thrown away. So we kill animals to throw them away because we over analyze how much meat intake we really need? That is inhumane. Then by buying, cooking and throwning away meat that is not needed, the need for more for another meal is obviously needed. So what do we do? Put hormones in chickens and cows to have them grow faster or get meatier. So they can be killed off quicker and sold? Just imagine how unnatural these hormones are and how they could affect you.
Also, the way the animals are treated in the slaughter houses is terrible. But see for yourself. Go to the Peta site and watch their video's and it will open your eyes on how they find sick things ammusing. But, you would have to be a pretty sick person if you were willing to kill and skin down animals everyday and be able to stand it.
But to answer the question, some vegitarians see it from the bible about how Jesus served fish and bread that it is okay to eat fish. And it is a good source of protein, which is one of the things pretty hard to get when cutting out meat. But I seem to make it just fine without any meat.
If you're going to get anything out of this, I'm not trying to convert you or anything. You should just think about the meat intake you personally do daily. It would be healthier for you to stop eating extra amounts of meat and hopefully it could start to make a difference in the number of animals slaughtered and how they do it.