My argument is that you should examine what your intended outcome is. If your goal is (1) to feel better about yourself by not contributing to the extinction of fish, then by all means continue what you're doing. If, however, your goal is (2) to halt or significantly slow the extinction of fish, then simply not eating fish is completely insufficient, and you need to figure out how to do something that has a larger effect. I think you're arguing as though your goal is (1).

"All I can do is change my behavior. I can't change the behavior of hundreds of other people as much as I would want to."

Well, millions, I assume you mean.

"make the cognitive effort to recognize the wrongness."

I think you're assuming my conclusion. :)