Pain in fish

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.

Fish fulfill several criteria proposed as indicating that non-human animals may 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.

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, Cambridge University, England, 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.

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 may 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 our 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 U.S. 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 U.S. 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.

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.

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 ourselves, 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 behavioral 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 primates and humans 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 monkeys, dogs, cats and birds can 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

The adaptive value of nociception is obvious; an organism detecting a noxious stimulus immediately withdraws the limb, appendage or entire body from the noxious stimulus and thereby avoids further (potential) injury. However, a characteristic of pain (in mammals at least) is that pain can result in hyperalgesia (a heightened sensitivity to noxious stimuli) and allodynia (a heightened sensitivity to non-noxious stimuli). When this heightened sensitisation occurs, the adaptive value is less clear. First, the pain arising from the heightened sensitisation can be disproportionate to the actual tissue damage caused. Second, the heightened sensitisation may also become chronic, persisting well beyond the tissues healing. This can mean that rather than the actual tissue damage causing pain, it is the pain due to the heightened sensitisation that becomes the concern. This means the sensitisation process is sometimes termed maladaptive. It is often suggested hyperalgesia and allodynia assist organisms to protect themselves during healing, but experimental evidence to support this has been lacking.

In 2014, the adaptive value of sensitisation due to injury was tested using the predatory interactions between longfin inshore squid (''Doryteuthis pealeii'') and black sea bass (''Centropristis striata'') which are natural predators of this squid. If injured squid are targeted by a bass, they began their defensive behaviours sooner (indicated by greater alert distances and longer flight initiation distances) than uninjured squid. If anaesthetic (1% ethanol and MgCl₂) is administered prior to the injury, this prevents the sensitisation and blocks the behavioural effect. The authors claim this study is the first experimental evidence to support the argument that nociceptive sensitisation is actually an adaptive response to injuries.

The question has been asked, "If fish cannot feel pain, why do stingrays have purely defensive tail spines that deliver venom? Stingrays' ancestral predators are fish. And why do many fishes possess defensive fin spines, some also with venom that produces pain in humans?"

Research findings

Peripheral nervous system

Receptors

Primitive fish such as lampreys (''Petromyzon marinus'') have free nerve endings in the skin that respond to heat and mechanical pressure. However, behavioural reactions associated with nociception have not been recorded, and it is also difficult to determine whether the mechanoreceptors in lamprey are truly nociceptive-specific or simply pressure-specific.

Nociceptors in fish were first identified in 2002. The study was designed to determine whether nociceptors were present in the trigeminal nerve on the head of the trout and to observe the physiological and behavioural consequences of prolonged noxious stimulation. Rainbow trout lips were injected with acetic acid, while another group were injected with bee venom. These substances were chosen because protons of the acid stimulate nociceptive nerves in mammals and frogs, while venom has an inflammatory effect in mammals and both are known to be painful in humans. The fish exhibited abnormal behaviours such as side-to-side rocking and rubbing of their lips along the sides and floors of the tanks. Their respiration rate increased, and they reduced the amount of swimming. The acid group also rubbed their lips on the gravel. Rubbing an injured area to ameliorate pain has been demonstrated in humans and in mammals. Fifty-eight receptors were located on the face and head of the rainbow trout. Twenty-two of these receptors could be classified as nociceptors, as they responded to mechanical pressure and heat (more than 40 °C). Eighteen also reacted to acetic acid. The response of the receptors to mechanical, noxious thermal and chemical stimulation clearly characterised them as polymodal nociceptors. They had similar properties to those found in amphibians, birds and mammals, including humans. Trout that were injected with venom or acid took approximately 3 hours to resume eating, whereas the saline and control groups took approximately 1 hour. This may be guarding behaviour, where animals avoid using a painful limb, preventing continuing pain and harm being caused to the area.

Rainbow trout (''Oncorhynchus mykiss'') have polymodal nociceptors on the face and snout that respond to mechanical pressure, temperatures in the noxious range (> 40 °C), and 1% acetic acid (a chemical irritant). Cutaneous receptors overall were found to be more sensitive to mechanical stimuli than those in mammals and birds, with some responding to stimuli as low 0.001g. In humans at least 0.6 g is required. This may be because fish skin is more easily damaged, necessitating nociceptors to have lower thresholds. Further studies found nociceptors to be more widely distributed over the bodies of rainbow trout, as well as those of cod and carp. The most sensitive areas of the body are around the eyes, nostrils, fleshy parts of the tail, and pectoral and dorsal fins.

Rainbow trout also have corneal nociceptors. Out of 27 receptors investigated in one study, seven were polymodal nociceptors and six were mechanothermal nociceptors. Mechanical and thermal thresholds were lower than those of cutaneous receptors, indicating greater sensitivity in the cornea.

Bony fish possess nociceptors that are similar in function to those in mammals.

Nerve fibres

There are two types of nerve fibre relevant to pain in fish. Group C nerve fibres are a type of sensory nerve fibre which lack a myelin sheath and have a small diameter, meaning they have a low nerve conduction velocity. The suffering that humans associate with burns, toothaches, or crushing injury are caused by C fibre activity. A typical human cutaneous nerve contains 83% Group C nerve fibres. A-delta fibres are another type of sensory nerve fibre, however, these are myelinated and therefore transmit impulses faster than non-myelinated C fibres. A-delta fibres carry cold, pressure and some pain signals, and are associated with acute pain that results in "pulling away" from noxious stimuli.

Bony fish possess both Group C and A-delta fibres representing 38.7% (combined) of the fibres in the tail nerves of common carp and 36% of the trigeminal nerve of rainbow trout. However, only 5% and 4% of these are C fibres in the carp and rainbow trout, respectively.

Some species of cartilagenous fish possess A-delta fibres, however, C fibres are either absent or
found in very low numbers. The Agnatha (hagfishes and lamprey) primarily have Group C fibres.

Central nervous system

The central nervous system (CNS) of fish contains a spinal cord, medulla oblongata, and the brain, divided into telencephalon, diencephalon, mesencephalon and cerebellum.

In fish, similar to other vertebrates, nociception travels from the peripheral nerves along the spinal nerves and is relayed through the spinal cord to the thalamus. The thalamus is connected to the telencephalon by multiple connections through the grey matter pallium, which has been demonstrated to receive nerve relays for noxious and mechanical stimuli.

The major tracts that convey pain information from the periphery to the brain are the spinothalamic tract (body) and the trigeminal tract (head). Both have been studied in agnathans, teleost, and elasmobranch fish (trigeminal in the common carp,
spinothalamic tract in the sea robin, '' Prionotus carolinus'').

Brain

If sensory responses in fish are limited to the spinal cord and hindbrain, they might be considered as simply reflexive. However, recordings from the spinal cord, cerebellum, tectum and telencephalon in both trout and goldfish (''Carassius auratus'') show these all respond to noxious stimuli. This indicates a nociceptive pathway from the periphery to the higher CNS of fish.

Microarray analysis of gene expression shows the brain is active at the molecular level in the forebrain, midbrain and hindbrain of common carp and rainbow trout. Several genes involved in mammalian nociception, such as brain-derived neurotrophic factor (BDNF) and the cannabinoid CB1 receptor are regulated in the fish brain after a nociceptive event.

Somatosensory evoked potentials (SEPs) are weak electric responses in the CNS following stimulation of peripheral sensory nerves. These further indicate there is a pathway from the peripheral nociceptors to higher brain regions. In goldfish, rainbow trout, Atlantic salmon (''Salmo salar'') and Atlantic cod

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