>>51343029They have nerves. Nerves send sensations to the brain, and pain is a type of sensation that tells the brain "STOP LETTING THIS HAPPEN YOU DUMB MOTHERFUCKER". Fish struggle when they're agitated and confused, and they most certainly feel the pain, which adds to the agitation and confusion they feel already. Just like us, fish know when shit hurts. They just don't scream like we do.