I am vegan, have been vegetarian for 13 years now. I think it's sad when people choose to be vegetarian for ethical reasons but exclude fish from their abstinence. I know several "pescatarians" who are vegetarian except for seafood.
I don't like guilt tactics but I think people really are misinformed about how fish.
Fish feel pain. They do not have 3-second memories. In fact,
they can be taught tricks, which takes long term memory. They are, quite frankly, the most exploited animals on the planet. They are hauled on mass in nets onto boats and then allowed to slowly suffocate to death (tossed about without a thought), or hooked with sharp barbs through their lips (where they are more sensitive to pain than humans) and then allowed to suffocate or are bashed over the head.
Because fishing is such a huge industry, and catching and killing fish humanely would be so very cost prohibitive, there are very few laws protecting fish from mistreatment. Many states and countries don't even have fish included in animal cruelty laws. After all, if they said pet fish had to be treated humanely, what about all the wild fish that are routinely killed in very inhumane ways?
Animals want to live and they want to do so with as little distress as possible. Because of the quantities needed to supply demand, the harvesting of all kinds of seafood is very inhumane. It is also extremely damaging to the environment (overfishing and harvesting has hurt wild populations immensely, plus the damage done to habitats, etc.).
If someone eats seafood, there are steps they can take to ensure they are choosing
sustainably harvested species. From an environmental, economic, and compassionate angle, abstaining from fish entirely is a better choice.