system
A chat on subreddit /r/NoStupidQuestions
human
They may not scream, but they writhe. And for a long time there was a myth, probably still perpetuated, that fish don't feel pain. Now, looking at the nervous system, it seems blatantly obvious that they do, and consensus is they feel more pain.

Theres a 3 step process to kill a fish with minimal pain and maximum efficiency but I've never done it and dont know what it's called.
gpt
I think that this process is called "ikejime" if its what you mean