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. cited in Carbone 2004, p. 150 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"

Reptiles and amphibians Veterinary articles have been published stating both reptiles and amphibians experience pain in a way analogous to humans, and that analgesics are effective in these two classes of vertebrates.

Argument by analogy In 2012 the American philosopher Gary Varner reviewed the research literature on pain in animals. His findings are summarised in the following table. The table in the article is based on table 5.2, page 113.

Argument by analogy Property Fish Amphibians Reptiles Birds Mammals Has nociceptors Has brain Nociceptors and brain linked ? / ? / ? / Has endogenous opioids Analgesics affect responses ? ? Response to damaging stimuli similar to humans

Notes

Arguing by analogy, Varner claims that any animal which exhibits the properties listed in the table could be said to experience pain. On that basis, he concludes that all vertebrates, including fish, probably experience pain, but invertebrates apart from cephalopods probably do not experience pain.

Crustaceans

Some studies however find crustaceans do show responses consistent with signs of pain and distress.

Experiencing pain Although there are numerous definitions of pain, almost all involve two key components.

First, nociception is required. This is the ability to detect noxious stimuli which evoke a reflex response that rapidly moves the entire animal, or the affected part of its body, away from the source of the stimulus. The concept of nociception does not imply any adverse, subjective "feeling" – it is a reflex action. An example in humans would be the rapid withdrawal of a finger that has touched something hot – the withdrawal occurs before any sensation of pain is actually experienced.

The second component is the experience of "pain" itself, or suffering – the internal, emotional interpretation of the nociceptive experience. Again in humans, this is when the withdrawn finger begins to hurt, moments after the withdrawal. Pain is therefore a private, emotional experience. Pain cannot be directly measured in other animals, including other humans; responses to putatively painful stimuli can be measured, but not the experience itself. To address this problem when assessing the capacity of other species to experience pain, argument-by-analogy is used. This is based on the principle that if an animal responds to a stimulus in a similar way to humans, it is likely to have had an analogous experience.

Nociception

Nociception usually involves the transmission of a signal along a chain of nerve fibers from the site of a noxious stimulus at the periphery to the spinal cord and brain. This process evokes a reflex arc response generated at the spinal cord and not involving the brain, such as flinching or withdrawal of a limb. Nociception is found, in one form or another, across all major animal taxa. Nociception can be observed using modern imaging techniques; and a physiological and behavioural response to nociception can often be detected. However, nociceptive responses can be so subtle in prey animals that trained (human) observers cannot perceive them, whereas natural predators can and subsequently target injured individuals.

Emotional pain

Sometimes a distinction is made between "physical pain" and "emotional" or "psychological pain". Emotional pain is the pain experienced in the absence of physical trauma, for example, the pain experienced by humans after the loss of a loved one, or the break-up of a relationship. It has been argued that only mammals can feel "emotional pain", because they are the only animals that have a neocortexa part of the brain's cortex considered to be the "thinking area". However, research has provided evidence that in addition to monkeys, dogs and cats, birds can also show signs of emotional pain and display behaviours associated with depression during or after a painful experience, specifically, a lack of motivation, lethargy, anorexia and unresponsiveness to other animals.

Physical pain

The nerve impulses of the nociception response may be conducted to the brain thereby registering the location, intensity, quality and unpleasantness of the stimulus. This subjective component of pain involves conscious awareness of both the sensation and the unpleasantness (the aversive, negative affect). The brain processes underlying conscious awareness of the unpleasantness (suffering), are not well understood.

There have been several published lists of criteria for establishing whether non-human animals experience pain, e.g. Some criteria that may indicate the potential of another species, including fishes, to feel pain include: Has a suitable nervous system and sensory receptors Has opioid receptors and shows reduced responses to noxious stimuli when given analgesics and local anaesthetics Physiological changes to noxious stimuli Displays protective motor reactions that might include reduced use of an affected area such as limping, rubbing, holding or autotomy Shows avoidance learning Shows trade-offs between noxious stimulus avoidance and other motivational requirements High cognitive ability and sentience

Adaptive value