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
Ikejime, it basically stabs the brain of the fish instead of suffocating it slowly, and then quickly guts and bleeds it.  It also provides better meat quality at the cost of having to do each fish manually