Commercial fishing kills approximately 1-2 trillion fish annually; the welfare of these individuals is almost never considered.
The scale of commercial fishing makes it arguably the largest source of animal suffering on Earth. Fish caught in nets suffocate over 20-40 minutes. Trawled fish experience pressure changes, physical trauma, and prolonged dying. The welfare case for faster killing methods is identical to that for terrestrial livestock: if fish can suffer, they deserve the fastest available death. The technology exists; the will to require it does not.