The capacity of fish to experience pain has been extensively studied; the evidence strongly supports fish as sentient beings deserving of welfare consideration in aquaculture.
The scientific case for fish sentience is now robust. Fish have the neural hardware for nociception, produce opioid peptides in response to painful stimuli, and modify behaviour to avoid pain and seek relief — the hallmarks of subjective pain experience. This means the billions of farmed fish subjected to crowding, handling, disease, and slaughter deserve welfare consideration analogous to that given to mammals and birds. Regulatory frameworks are beginning to catch up, but implementation lags far behind the science.