[w00t]

It's a fish, it barely has feelings at all, let alone actual pain or fear.
They run on instinct, which isn't emotion, and let's be honest, the fish would probably be hunted sooner or later anyways. It's just cutting off some time.

They can be compared to a log of wood that fucks underwater and makes for good food.

Cows are also pretty phenomimally stupid, why is it less ethical to kill them? We've domesticated them to the point that they likely wouldn't survive without our care.