The ethical part of my choice is not about the killing of animals, although I am sympathetic. The ethical issue for me is a humanitarian one. I view meat, as a luxury which to consume requires an incredible amount of natural resources (feed: other crops, and water) to sustain. I feel that those resources should be used in other ways to help those suffering on earth. I do not need to eat a steak that takes as much natural resources to make, as it would feed dozens of starving people.
Fish is (depending on it's source) a more sustainable resource. It is still a luxury item for me, and one that I reserve for special occasions only. Fish and other aquaculture are certainly not perfect ecological or economical markets, and as such I am very mindful of my consumption.
An additional part of this is cultural. For me, many traditional meals in my home were fish. While I was able to let go of red meat, poultry, dairy and eggs with little second thought, I do not think I am currently strong enough to detach without a certain level of ceremonial-ism. It may be that in some day in the future, I may retire fish as well. Certainly the arguments that apply to the other things I have given up apply to it as well.
My relationship with vegetarianism can be described best by a metaphor with recycling.
I may recycle aluminum (red meat), plastic (poultry), and glass (eggs & dairy) every time, but perhaps only recycle paper (fish & aquaculture) most of the time. The fact that I do not always recycle paper, does not render my other recycling efforts useless, nor does it make those efforts less sincere.
I am in a very exciting time in my life and this is only one part of how I am changing. My goal, in short, is to live more intentionally. Even in still eating fish, there are ways to be intentional.
I don't think there is only one reason for making choices like these. I'm fairly positive that most people assume (understandably) that all people who adopt a vegetarian (or similar) diet and lifestyle do it for the animal rights issue. Like I said, I'm sympathetic, and that IS an issue for me. It was not the issue that motivated me to actually make this lifestyle change.
It's my hope that either (1) the fishing industry will be able to achieve ecological sustainability, and/or (2) that I will be able to mourn and release the cultural ties I have to fish consumption. On this matter in particular, it is a very emotional issue.
Did I answer your question sufficiently?