Fish fulfill several criteria proposed as indicating that non-human animals experience pain. These fulfilled criteria include a suitable nervous system and sensory receptors, opioid receptors and reduced responses to noxious stimuli when given analgesics and local anaesthetics, physiological changes to noxious stimuli, displaying protective motor reactions, exhibiting avoidance learning and making trade-offs between noxious stimulus avoidance and other motivational requirements. Whether fish feel pain similar to humans or differently is a contentious issue. Pain is a complex mental state, with a distinct perceptual quality but also associated with suffering, which is an emotional state. Because of this complexity, the presence of pain in an animal, or another human for that matter, cannot be determined unambiguously using observational methods, but the conclusion that animals experience pain is often inferred on the basis of likely presence of phenomenal consciousness which is deduced from comparative brain physiology as well as physical and behavioural reactions. If fish feel pain, there are ethical and animal welfare implications including the consequences of exposure to pollutants, and practices involving commercial and recreational fishing, aquaculture, in ornamental fish and genetically modified fish and for fish used in scientific research.
Background The possibility that fish and other non-human animals experience pain has a long history. Initially, this was based around theoretical and philosophical argument, but more recently has turned to scientific investigation.
Philosophy
The idea that non-human animals might not feel pain goes back to the 17th-century French philosopher René Descartes, who argued that animals do not experience pain and suffering because they lack consciousness. In 1789 the British philosopher and social reformist Jeremy Bentham addressed in his book An Introduction to the Principles of Morals and Legislation the issue of the treatment of animals with the following often quoted words: "The question is not, Can they reason? nor, Can they talk? but, Can they suffer?" Charles Darwin said that "The lower animals, like man, manifestly feel pleasure and pain, happiness and misery." Peter Singer, a bioethicist and author of Animal Liberation published in 1975, suggested that consciousness is not necessarily the key issue: just because animals have smaller brains, or are 'less conscious' than humans, does not mean that they are not capable of feeling pain. He goes on further to argue that we do not assume newborn infants, people suffering from neurodegenerative brain diseases or people with learning disabilities experience less pain than we would. Bernard Rollin, the principal author of two United States federal laws regulating pain relief for animals, writes that researchers remained unsure into the 1980s as to whether animals experience pain, and veterinarians trained in the US before 1989 were taught to simply ignore animal pain. In his interactions with scientists and other veterinarians, Rollin was regularly asked to "prove" that animals are conscious, and to provide "scientifically acceptable" grounds for claiming that they feel pain. Continuing into the 1990s, discussions were further developed on the roles that philosophy and science had in understanding animal cognition and mentality. In subsequent years, it was argued there was strong support for the suggestion that some animals (most likely amniotes) have at least simple conscious thoughts and feelings and that the view animals feel pain differently to humans is now a minority view.
Scientific investigation
In the 20th and 21st centuries there were many scientific investigations of pain in non-human animals. Dr Lynne Sneddon, with her colleagues, Braithwaite, and Gentle, were the first to discover nociceptors (pain receptors) in fish. She stated that fish demonstrate pain-related changes in physiology and behaviour, that are reduced by painkillers, and they show higher brain activity when painfully stimulated. Professor Victoria Braithwaite, in her book Do Fish Feel Pain?, wrote that, fish, like birds and mammals, have a capacity for self-awareness, and can feel pain. Donald Broom, Professor of Animal Welfare at the University of Cambridge, said that most mammalian pain systems are also found in fish, who can feel fear and have emotions which are controlled in the fish brain in areas anatomically different but functionally very similar to those in mammals. The American Veterinary Medical Association accepts that fish feel pain saying that the evidence supports the position that fish should be accorded the same considerations as terrestrial vertebrates concerning relief from pain. The Royal Society for the Prevention of Cruelty to Animals, in Britain, commissioned in 1980 an independent panel of experts. They concluded that it was reasonable to believe that all vertebrates are capable of suffering to some degree or another. RSPCA Australia more recently added that evidence that fish are capable of experiencing pain and suffering has been growing for some years. The European Union Panel on Animal Health and Welfare European Food Safety Authority said that the balance of evidence indicates that some fish species can experience pain. The British Farm Animal Welfare Committee 2014's report, Opinion on the Welfare of Farmed Fish, said that the scientific consensus is that fish can detect and respond to noxious stimuli, and experience pain.
Mammals In 2001 studies were published showing that arthritic rats self-select analgesic opiates. In 2014 the veterinary Journal of Small Animal Practice published an article on the recognition of pain which started, "The ability to experience pain is universally shared by all mammals..." and in 2015 it was reported in the science journal Pain, that several mammalian species (rat, mouse, rabbit, cat and horse) adopt a facial expression in response to a noxious stimulus that is consistent with the expression of pain in humans.
Birds At the same time as the investigations using arthritic rats, studies were published showing that birds with gait abnormalities self-select for a diet that contains carprofen, a human analgesic. In 2005 it was written "Avian pain is likely analogous to pain experienced by most mammals" and in 2014, "...it is accepted that birds perceive and respond to noxious stimuli and that birds feel pain"
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