To sort of answer the original question, if I were to start to eat vegetarian, I might be more inclined to give up shellfish first, only because it isn't kosher and because I can justify the health benefits of continuing to eat fish. I don't keep kosher at all but if I were to give something up, I'd look at the kosher way of thinking.

I have had numerous people ask me if I am vegetarian because of my food preferences. I say,"No, but I eat a lot of vegetarian style foods" or I say,"No, I just don't eat a lot of meat."