Professor Victoria Braithwaite, in her book, Do Fish Feel Pain?, concludes that fish do feel pain and they do suffer.
She said that the evidence for this is as good as any evidence that we have for birds and mammals.
"Almost all of the characteristics of the mammalian pain system are also described for fish. Emotions, feelings and learning from these are controlled in the fish brain in areas anatomically different but functionally very similar to those in mammals.
The evidence of pain and fear system function in fish is so similar to that in humans and other mammals that it is logical to conclude that fish feel fear and pain. Fish are sentient beings."
Dr Lynne U. SneddonPain in aquatic animals, The Journal of Experimental Biology (2015) 218, 967-976 doi:10.1242/jeb.088823
"Contemporary studies over the last 10 years have demonstrated that bony fish possess nociceptors that are similar to those in mammals; that they demonstrate pain-related changes in physiology and behaviour that are reduced by painkillers; that they exhibit higher brain activity when painfully stimulated."
“Fish also possess many of the substances associated with nociception such as substance P, N-methyl-Daspartate (NMDA), opioids, and endogenous opioids. Studies have shown that fish are capable of learning to avoid noxious stimuli and that morphine blocks learning. Finally, adverse behavioral and physiological responses have been observed in fish enduring a noxious event that are ameliorated by the administration of painkillers or analgesics.”
Farm Animal Welfare Committee 2014 report, Opinion on the Welfare of Farmed Fish
"Fish are able to detect and respond to noxious stimuli, and FAWC supports the increasing scientific consensus that they experience pain."
"Suggestions that finfish* responses to pain merely represent simple reflexes have been refuted by studies demonstrating forebrain and midbrain electrical activity in response to stimulation and differing with type of nociceptor stimulation.
Learning and memory consolidation in trials where finfish are taught to avoid noxious stimuli have moved the issue of finfish cognition and sentience forward to the point where the preponderance of accumulated evidence supports the position that finfish should be accorded the same considerations as terrestrial vertebrates in regard to relief from pain."
The European Food Safety Authority in 2009 said that the balance of evidence indicates that fish have the capacity to experience pain and fear.
The European Union’s Standing Committee of the European Convention for the Protection of Animals Kept for Farming Purposes (EU Recommendation Concerning Farmed Fish, 2005) said that the skin of fish contains sensory receptors for touch, pressure and pain.
"Evidence that the term pain is applicable to fish comes from anatomical, physiological and behavioural studies whose results are very similar to those of studies on birds and mammals. The fact that fish are cold-blooded does not prevent them from having a pain system and, indeed, such a system is valuable in preserving life and maximising the biological fitness of individuals. The receptor cells, neuronal pathways and specialised transmitter substances in the pain system are very similar in fish to those in mammals."