I'm not sure I understand what your argument is. 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. If your argument is that individual effort isn't worth the benefit, I'm not sure what to tell you other than this again seems nothing more than an apologist rationalization to defend existing destructive behavior.
It's easier to rationalize your existing behavior than it is to change it. But instead of changing it, maybe just make the cognitive effort to recognize the wrongness. This requires no behavioral change, just an attitude change. If you do this, after enough time behavioral change comes easier. In other words detach reasoning about the ethics of the behavior from the work involved in optimizing your life.
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."
It's easier to rationalize your existing behavior than it is to change it. But instead of changing it, maybe just make the cognitive effort to recognize the wrongness. This requires no behavioral change, just an attitude change. If you do this, after enough time behavioral change comes easier. In other words detach reasoning about the ethics of the behavior from the work involved in optimizing your life.