FISH FEEL PAIN
Fish do feel pain. The evidence we have to support sentience and pain perception is as good as anything we have for birds and mammal
. Fish have a capacity for self-awareness.
Fish are able to detect and respond to noxious stimuli, and the increasing scientific consensus
is that they experience pain.
Fish demonstrate pain-related changes in physiology and behaviour
that are reduced by painkillers, and that they exhibit higher brain activity when painfully stimulated.
The evidence of pain and fear system in fish is so similar to that in humans that it is logical to conclude that fish feel fear and pain. Fish should be accorded the same considerations as terrestrial vertebrates in regard to relief from pain
Fish detect what harms them
In an experiment, twenty-two pain receptors were found on the face of rainbow trout.
Neural activity was recorded from single cells in the face when either a mechanical probe, heat, or a weak acid was applied. The fish nerve endings on the face were actually more sensitive
, when pressure was applied, than those in humans, and were more sensitive than our eyes.
AVMA Guidelines for the Euthanasia of Animals (pdf)
Fish feel sharp and aching pain
In humans and other higher vertebrates, there are two types of nerve fibres used in pain transmission:
- A fibres that transmit pain signals quickly, involved in the fast, pricking sensation of pain.
- C fibres that transmit pain signals slowly, involved in aching pain.
In vertebrates, including fish and humans, the trigeminal nerve (the fifth cranial nerve), conveys sensory signal information from the head and mouth to the brain. In one experiment, in Scotland, rainbow trout were deeply anaesthetized. The head was operated on to expose the trigeminal nerve, which was stimulated by very fine wire, heat and chemicals. The research found both of these fibre types.
Fish feel pain in their eyes
The fish’s eye is similar to the human eye, with the trigeminal nerve ending in receptors in the outer layer of the eye. Twenty-seven pain receptors were found in the eye of fish, and were as sensitive as in the human eye.
Fish are sensitive to pain
Russian scientists recorded the responses of various fish to painful electrical shocks, which caused their tails to jerk. They were then given painkillers, followed by further shocks. Painkillers reduced the tail jerks by up to 89%.
The most sensitive areas to pain
were the tail and pectoral fins
, skin around the eye
, and olfactory sacs. Pain sensitivity was found to be comparable to humans.
In research carried out at Manchester University in England, the face of the trout was stimulated while responses in the trigeminal nerve in the brain were recorded. It was found that skin receptors of trout are more
sensitive to mechanical
stimulus than mammals and birds. It was conjectured that this is because fish are continuously exposed to water pressure, bacteria and fungi. Fish were also pain-sensitive to lower thresholds of heat
Simple fish feel pain
The Lamprey is one of the most primitive vertebrates. Nevertheless, nerve receptors responded to painful stimuli, such as puncturing or burning of the skin.
Fish use chemicals in pain transmission
Substance P is widely distributed in the brain, spinal cord, and peripheral nervous system in people. It is an important element in pain perception, and in the transmission of pain information into the central nervous system. Canadian researchers found the distribution of a P-like substance in the brain of brown ghost knifefish.
A fish’s brain is complex enough to feel pain
Fish pain specialist, and author of Do Fish Feel Pain, Dr Braithwaite, told the Los Angeles Times that, although simpler than our own, the brain of a fish has been found to be more similar than once thought. In the amygdala and hippocampus, our brains handle emotion, learning, and memory. The equivalent area has been found in the forebrains of fish. When this part is damaged, they lose their fear, forget how to navigate mazes, and are impaired emotionally.
Birds and amphibians lack the complex outer layer of the brain that humans do, the neocortex yet many studies have amply demonstrated pain and suffering in these groups.
Norwegian scientists applied electrical shocks to the tails of anaesthetised salmon.
They were able to trace this painful stimulation to the telencephalon in the brain.
The German magazine, Der Spiegel, in 2011, reported on research that showed that this area had not previously been hightlighted, because rather than being in the inside, as in the human brain, it moves to the outside, after embryonic development.
Fish and mammals may have inherited the same brain functions from an ancient ancestor.
The scientists at the University of Seville, in an experiment, damaged the telencephalon of goldfish. This resulted in the fish being unable to learn to avoid an electric shock. Similar damage to the amygdala and hippocampus of mammals produces the same effect.
The outer layer of the telencephalon is the grey matter of the pallium, involved in pain processing, and the equivalent of the cerebral cortex in humans.
Pain signals reach the fish's brain
In an experiment in Ireland, goldfish were pricked with a pin and a heat probe. Responses from this painful event were measured in the spinal cord, and in the brain, from the cerebellum through to the telencephalon (forebrain), where fish feel pain.
The research shows that there is a neural pain pathway from the peripheral nervous system to the central nervous system, including the brain.
Fish learn to remember painful situations
Fish have demonstrated that they remember the circumstances of painful experiences and will afterwards seek to avoid the same situation.
Dutch researchers used anglers for three days to fish for pike, which had never been fished before, with live bait or spinner hooks. The fish were then tagged and returned to the water.
It was then found that pike previously only hooked once by a spinner, rarely took it again, and avoided spinners for the remaining five days.
were fished again the next Summer, together with unfished carp. Individual fish, and then the whole population of the two pools learned, and then remembered, that the spinner was to be avoided
. This memory lasted for at least one year
Similar results were found with carp. This time, carp
were able to remember their terrifying experience one year later
, and, as with the pike, they were able to learn from the unfortunate experience of other previously hooked carp.
Sticklebacks receive some protection from predator fish through their sharp spines. In 1957 at Oxford University, researchers found pike and perch initially snapped up, but then rejected, sticklebacks. Within a few experiences, the pike and perch learned to avoid the sticklebacks and the pain from the spines
, altogether. It was found that when spines were removed from sticklebacks, their protection disappeared.
at Missouri University were placed in a tank, where they had to respond if light colours at each end were the same, by swimming to the other end. When they were wrong, they received an electric shock. Later, they had to respond if the colours of the lights were different. The goldfish learnt, and remembered, how to avoid the shocks
At Belfast University, goldfish and trout were also given mild electric shocks when they entered a particular part of their tank. The fish reacted with rapid breathing and an increase in their blood cortisol, an indication of stress. They remembered to keep to one end of the tank, thus avoiding pain.
In an experiment by other researchers, goldfish were conditioned to expect an electric shock
after a light was shown to them. Their fear produced a slowing of the heart when the light preceded the shock. However, when an analgesic, lidocaine, was injected into the brain (cerebellum) of other goldfish, it prevented the goldfish from feeling pain, and so they did not show fear when the light was shone.
Fish brains and behaviour indicate capacity for feeling pain
Cod and bream learn to associate sound of trawler with fishing gear
Cod and bream learn to associate the sound of trawler with later contact with the fishing gear, causing them to be frightened by the sound alone, leading to decreased catchability during repeated trawls.
Fish who escape gear may risk being unable to feed properly afterwards, or may be eaten, as they recover from stress or bodily harm.
A single traumatic encounter can be remembered, and avoided, for several months. Cod, in the lab, have also learnt to avoid baited hooks
Fish prefer a tank with painkiller over an enriched pain without pain relief
In an experimental tank, zebrafish chose to live where there were plants and gravel, instead of a barren environment. However, when they were injected with a pain-causing chemical, they preferred to spend time in the barren environment where there was painkiller in the water.
Fish show their pain
Not only can fish detect and perceive painful events, but they also show disturbed behaviour.
Fish, like mammals, have been reported to make sounds when pain is felt. Although normally silent, wounded European weather loach vocalize their pain using their swim bladders.
Scientists in the 1930s discovered that wounded minnows released a chemical that caused fear in other minnows. Scientists in Singapore isolated the chemical in zebrafish, who froze in place.
Fish will often not swim close to an object that they have not encountered before. Trout that had acetic acid (vinegar) injected under their skin, were less wary than fish who had not had the injection, when a lego tower was placed in their tank. The fish showed the normal cautious response if they were given morphine later.
The researchers said that the painful acid distracted the attention of the fish. For this to be happening, the fish must be consciously aware of the negative experience of the pain.
Researchers in Scotland injected into the lips of rainbow trout either bee venom or vinegar. A control group was handled but received no injection. The effects on the fish was a near doubling of respiration
, stopping of feeding for three hours
, and reduced swimming
. They rocked back and forth
, balancing on either pectoral fin, while resting on the gravel, and they also rubbed their lips into the gravel and tank walls
At Liverpool and Manchester Universities, England, trout once more had acid injected into their lips. After their painful experience, when individual fish were returned to a familiar social group they showed reduced aggression. When the fish were returned to an unfamiliar social group, they showed the usual level of aggression, suggesting that maintaining dominance status took priority over showing signs of pain. The scientists concluded that fish are considerably affected by pain, and that the perception is not just a simple reflex. The experiment showed that fish are able to perceive and manage the pain felt.
FISH SPECIES VARY IN THEIR RESPONSE TO PAIN
Report of the Panel of Enquiry into Shooting and Angling (pdf)
Fish respond to painkillers
It can be demonstrated that fish feel less, or no, pain, when blocked by analgesic drugs.
Fish also possess natural chemicals in their brains that reduce pain. If we were to believe that fish do not experience pain, why would these chemicals be present?
- Painkillers make fish less sensitive to heat, pressure, electricity, acid, and other harmful chemicals
- When painkillers are blocked by another drug, fish react normally to pain
Morphine is a strong analgesic in mammals. In Norway, scientists gradually applied heat to the skin of goldfish. The fish showed an escape response at a particular temperature threshold. Later, however, when morphine was injected, the temperature had to rise significantly higher to cause the same response.
In New Orleans, scientists applied electricity to fish to produce an "agitated swimming response." After fish were injected with morphine, the voltage had to be increased to achieve the same response. However, when naloxone was injected, the analgesic effect of morphine was blocked. Similar results were found by researchers in Portugal.
Researchers in Scotland injected acetic acid into the lips of rainbow trout. The fish rocked on either pectoral fin from side to side, and rubbed their lips into the gravel and against the sides of the tank. The respiration rate almost doubled.
They were then given morphine. This reduced the abnormal behaviour, and calmed down the respiration. The study said that these pain-related behaviours were not simple reflexes.
Pain in aquatic animals, The Journal of Experimental Biology
At only five days after being fertilized, zebrafish larvae were exposed to weak acid, causing them to slow their swimming. However, when painkillers - aspirin, lidocaine, or morphine - were dissolved in their tank, before the acid was later added, they swam normally.
Many animals have opiate receptors, which are involved in regulating pain in the body. Fish also have their own natural painkillers. Researchers in England found opiate receptors in the brains of goldfish, catfish, African lungfish, and rainbow trout.
Russian scientists held fish in a water chamber. They applied short bursts of electricity to the tail fin. which jerked in response. Painkillers were given to the fish, and this allowed higher voltages to be applied before the fish responded.
However, if a chemical known to block painkillers was then given, the fish once more jerked their tails in response to the lower voltages.
When the experiment ended, it was found that different species took varying times to recover. The sturgeon swam slowly and preferred to lie at the bottom of the tank - this lasted for five days. The scientists concluded that fish feel pain, which affects their physiology and behaviour.
Farm Animal Welfare Committee 2014 report, Opinion on the Welfare of Farmed Fish (pdf)
A fibres: nerves that transmit pain signals quickly, involved in the fast, pricking sensation
C fibres: nerves that transmit pain signals slowly, involved in aching pain
mechanoreceptors: pain nerve receptors that respond to pressure
naloxone: a drug that blocks the analgesic effect of opioids drugs
nociception: sensory nervous system’s response to harmful stimuli
nociceptor: pain receptor
olfactory sacs: specialized sensory organ for detection and identification of minute quantities of chemicals in the environment
pain: unpleasant sensory, emotional experience associated with tissue damage
pallium: grey matter involved in pain processing, on the outer layer of the telencephalon
polymodal: pain nerve receptors that respond to chemicals, heat, and pressure
telencephalon: part of the forebrain
trigeminal nerve: main sensory nerve of the face
Do Fishes Have Nociceptors? Evidence for the Evolution of a Vertebrate Sensory System
, V, Sneddon
, L, Gentle
Proc. R. Soc. Lond. B 270, 1115-1121,
1. Roslin Institute, Midlothian, 2. University of Edinburgh
"We located 58 receptors on the face and head
of the rainbow trout. Twenty-two of these receptors could be classified as nociceptors
"The polymodal nociceptors found here in the trout have similar properties to those found in amphibians, birds and mammals including humans
"This study demonstrates that the rainbow trout is capable of nociception
"Receptor diameter, thermal thresholds and mechanical responses are similar to those measured in higher vertebrate groups
"Mechanical thresholds were lower than those found in humans
: at least 0.6g is required for noxious stimulation in human skin but many of the nociceptors in the fish skin were stimulated by 0.1g. This may be a consequence of the more easily damaged nature of the fish skin requiring the nociceptors to have lower thresholds
. Similar thresholds were found in mammalian eye nociceptors
"Evoked activity demonstrated that there were fast adapting mechanoreceptors and polymodal slowly adapting mechanoreceptors that responded not only to mechanical stimulation but also to thermal and or chemical stimuli."
A and C fibres found in trout
"The present study on the trout has demonstrated that the trigeminal nerve has both C and A-delta fibres.
There were a large number of A-delta fibres in the trigeminal nerve of the rainbow trout, therefore, they have the potential for nociceptive capabilities."
Trigeminal nerve projects into the brain
"The trigeminal nerve projects to the relevant brain areas, the thalamus, cerebellum and medulla, which are involved in pain or nociceptive processing in higher vertebrates.
Fish are subject to noxious pollutants, unpalatable or poisonous food and also mouth injuries as a result of aggression, spiny prey and angling. It is conceivable that it would be adaptive for the animal to be able to perceive these potentially injurious stimuli and react appropriately."
Pain fibres similar to humans
"The size range of each fibre type was similar to those found in other animals. The cell bodies of the trigeminal ganglion were of a similar size to those found in man."
"This indicates that the basic components of the trigeminal nerve are similar in all vertebrates but only the relative number of fibre types that comprise the afferent nerves differ in fish."
"Nociceptors are slowly adapting polymodal mechanoreceptors and these receptor types were found on the head and face of the rainbow trout in this study."
"Therefore, the physiological recordings mirror the anatomical findings and show that there are fibres that could potentially act as nociceptive nerves."
Tail fin given electric shocks
"Optico-mechanical system was used to record the response to painful electrical stimulation before and after administration of analgetic agents."
Most sensitive areas
"It was found that the caudal, dorsal and pectoral fins, the skin surface around the eyes, and the epithelium of olfactory sacs were the most sensitive nociceptive zones."
Receptors present throughout the body
"Studies performed on Cyprinus carpio (carp), Parasalmo mykiss (trout), Gadus morhua (cod), and Acipenser ruthenus (sturgeon) indicated that the fishes possess a developed system of pain sensitivity with receptors (nociceptors) presented on the whole body."
Pain thresholds comparable to humans
"Nociceptive thresholds of fish under this condition was comparable with human’s one."
High density of pain receptors to cope with life under water
"The high density of nociceptors on fins is likely to be related to the fact, in particular, that fins are damaged in fish during their nest-building activity or agressive interactions."
Nociception in Fish: Stimulus–response Properties of Receptors on the Head of Trout Oncorhynchus Mykiss
Ashleya, P, Sneddon, McCrohan, C,
University of Manchester, University of Liverpool, UK
Trout skin sensitive to pressure
"Trout cutaneous receptors recorded here are more sensitive overall to mechanical stimuli than those of mammals and birds, with some responding to stimuli as low 0.001g."
Sensitivity to pain
"It is likely that fish, continuously exposed to external water pressure and to bacterial and fungal agents, require greater cutaneous sensitivity, including for nociception, than terrestrial animals."
Trout more sensitive to lower temperatures than mammals
"The heat thresholds recorded here from mechanothermal and polymodal receptors ranged from 20°C, though mean values were around 29 and 33°C, respectively. This range is below that considered noxious in mammalian nociception, but can be explained by the proposition that nociceptors in fish have evolved to match habitat temperatures."
"Birds and amphibians lack a neocortex; this suggests that they also do not experience pain. Yet many scientific studies have amply demonstrated pain and suffering in these groups (e.g. birds, Gentle, 1992; amphibians, Stevens, 1992)."
Gentle, M.J., 1992, Pain in birds Anim. Welfare 1, 235–247
Stevens, C.W., 1992. Alternatives to the use of mammals for pain research Life Sci. 50, 901–912.
Avoidance Response in Goldfish: Emotional and Temporal Involvement of Medial and Lateral Telencephalic Pallium
, M, Torres
, B, Salas
The Journal of Neuroscience, 24(9):2335–2342,
University of Seville, Spain
Mechanoreceptive and Nociceptive Responses in the Central Nervous System of Goldfish (Carassius auratus) and Trout (Oncorhynchus mykiss)
, R, Laming
The Journal of Pain Volume 6, Issue 9 , Pages 561-568,
University of Belfast, Belfast, Northern Ireland
Pain pathway through to forebrain
"This study has shown that there is neuronal activity in all brain areas including the telencephalon, suggesting a nociceptive pathway from the periphery to the higher central nervous system of fish."
Responses to pin prick, measured from spinal column to brain
"Nociceptive responses to a repetitive pin-prod stimulus in goldfish and trout were assessed. Single unit recordings were taken from the spinal cord, cerebellum, tectum, and telencephalon. Neuronal responses were elicited in all these regions of the central nervous system in both species of fish during brush (mechanoceptive) and pin-prod (nociceptive) stimuli."
Responses to heat probe
"In addition, in trout, a heated prod stimulus was used. Mechanoreceptive and nociceptive neuronal responses to various stimuli were elicited in all regions, and responses were detected as far as the telencephalon in both species. In goldfish, a noxious stimulus produced greater neuronal activity than a mechanoreceptive stimulus. This was not found in trout. The accurate setting of timed prods allowed the latency of the response to be calculated in all regions. From these data, conduction velocities suggested that A delta and C fibers were activated; both fiber groups previously have been shown to be involved in nociception in fish."
Acquired Hook-Avoidance in the Pike Esox Lucius L. Fished with Artificial and Natural Baits
Journal of Fish Biology, 2, 155-160,
1. Organization for the Improvement of Inland Fisheries, Utrecht, and Zoological Laboratory, 2. University of Groningen, The Netherlands
"The pike used, therefore, had no experience of angling."
"After an initial day of nearly equal catchability with either spinner or live bait of the previously unfished pike, catchability by spinner dropped quickly to very low levels during the succeeding days. This low catchability held during a week at least."
"It was difficult to capture pike more than once by spinning."
"The most likely cause of their lowered catchability is their experience of spinner-fishing."
"Evidently a pike captured once by spinning rarely takes a spinner again (at least during the first few days)."
"Any experience with a spinner hook, whether or not an actual landing follows, is sufficient for learning to avoid further capture by spinner."
"Analysis of the data in three ways yield results which consistently point to quick learning to avoid an artificial lure"
 Angling Experiments With Carp Decreasing Catchability Through One-Trial Learning
Beukema, et al,
Netherlands Journal of Zoology 20(1): 81-92,
Groningen University, Netherlands
"When high proportions of the fished carp were hooked and subsequently lost, the whole populations fished for (not only the captured parts) became nearly uncatchable."
Catchability of carp one year after being either captured or hooked and lost was some three times lower."
"Sticklebacks are rejected when, after being snapped up, their spines hurt the predator’s mouth.
After very few experiences both Perch and Pike become negatively conditioned to the sight of sticklebacks and avoid them before they have made contact"
On a number of occasions a completely swallowed stickleback was regurgitated alive after some minutes.
Usually it was rejected immediately after it had been snapped up, and the Pike would make violent coiling movements, and would "cough" intensely several times.
After eating, the Perch frequently gulps, gapes, or belches, and we have even seen the whole body vibrate with rapid jerks after a stickleback had been swallowed. Sometimes a half-swallowed stickleback that has become lodged in the throat of the Perch may be dislodged by a combination of gaping movements and vigorous sideways shaking of the head."
 Fish Cognition and Behavior,
 Pain Perception in Fish Evidence and Implications for the Use of Fish,
Journal of Consciousness Studies, 18, No. 9-10, pp. 209-29,
University of Liverpool, England
"One chamber was barren
and the other chamber was enriched
with gravel, a plant, and through the transparent rear of the chamber a group of zebrafish provided social enrichment. Zebrafish displayed a clear preference for the enriched chamber for at least six consecutive tests."
"At this point the zebrafish were injected with
saline, a non-noxious control, or a noxious chemical
and placed in the maze. In half of these trials, an analgesic was placed into the barren
, unfavourable chamber."
"Those fish experiencing pain lost their preference for the enriched chamber and were willing to spend more time in the barren, unfavourable area only if analgesia was present.
"This demonstrates that the fish were willing to pay the cost of being in an unpreferred environment to obtain analgesia
, and thus it can be inferred that these fish must have obtained some reward possibly in terms of pain relief such that the pain was reduced."
Pain Sensitivity of Fishes and Analgesia Induced by Opioid and Nonopioid Agents
, L, Lapshin, D,
Moscow, with reference from Nikolskii ID, Protasov VR, Romanenko EV, Shishkova EV (1968) Zvuki ryb. Atlas (Sounds of fishes. Atlas), Moscow: Nauka
"Special sounds that are emitted at wounding
were also recorded in Misgurnus fossilis [European weatherfish or European weather loach], these sounds were generated by the swimming bladder and were characterized by a frequency spectrum of 0 to 4000 Hz with maxima of 500, 1500, and 3000 Hz and continued for 490 ms, on average."
"According to these parameters, this pain ‘shout’ differed from sounds that accompanied food seizure, feeding, or spawning behavior (Nikolskii et al., 1968)."
"When one fish is injured, others nearby may dart, freeze, huddle, swim to the bottom or leap from the water.
"At low concentrations, the fish were "mildly perturbed," Dr. Jesuthasan said. At high concentrations, they stopped darting altogether and froze in place for an hour or longer."
"It has also been demonstrated that the trout has a prolonged negative behavioural and physiologic response to a noxious event, and this was also seen in the present study. Other fish species can learn to avoid noxious stimuli such as electric shock.
The administration of an analgesic reduces these responses to almost normal. Together with the results of this study, the criteria for animal pain have been fulfilled for the trout."
Do Fishes Have Nociceptors? Evidence For The Evolution Of A Vertebrate Sensory System
Sneddon, L.U , Braithwaite, V,A, Gentle, M.J
Proceeding of the Royal Society London 270, 1115-1121,
Roslin Institute and University of Edinburgh
"Our findings provide new evidence that fish are considerably affected by pain ... and that the perception is not just a simple nociceptive reflex."
"These findings may reflect an ability to prioritize motivational drivers in fish, and as such provide evidence for central processing of pain rather than merely showing a nociceptive reflex."
Reilly, 2008, Behavioural analysis of a nociceptive event in fish: Comparisons between three species demonstrate specific responses, Applied Animal Behaviour Science, 114, pp. 248-259
Roques, 2010, Tailfin clipping, a painful procedure: Studies on Nile tilapia and common carp, Physiology & Behavior, 101, pp. 533-540.
Sneedon, 2011, Pain Perception in Fish Evidence and Implications for the Use of Fish, Journal of Consciousness Studies, 18, No. 9-10, pp. 209-29
Thermonociception in Fish: Effects of Two Different Doses of Morphine on Thermal Threshold and Post-test Behaviour in Goldfish (carassius Auratus)
Applied Animal Behaviour Science, Volume 119, Issues 1–2, June 2009, Pages 101-107,
1. The Norwegian School of Veterinary Science, Oslo, Norway, 2. Purdue University, USA
"The findings supported the current hypothesis, and the earlier observations by Bardach (1956), that goldfish, like birds, reptiles and mammals, perceive heat as noxious, and that the avoidance responses observed were caused by activation of heat sensitive nociceptors, and not by thermal receptors."
[26b] European Food Safety Authority, General approach to fish welfare and to the concept of sentience in fish Scientific Opinion of the Panel on Animal Health and Welfare,
The EFSA Journal 954, 1-27,
"There is scientific evidence to support the assumption that some fish species have brain structures potentially capable of experiencing pain and fear. The balance of evidence indicates that some fish species have the capacity to experience pain."
"Responses of fish, of some species and under certain situations, suggest that they are able to experience fear."
"there is some evidence for the neural components of sentience¹ in some species of fish"
Similar Antagonism of Morphine Analgesia by Mif-1 and Naloxone in Carassius Auratus
Ehrensing, R and Michell, G
Pharmacology Biochemistry A Behavior, Vol. 17, pp. 757-761,
1. Department of Psychiatry, Ochsner Medical Institutions, New Orleans, 2. Tulane University School of Medicine, New Orleans
"fish possess the necessary sensory components to detect a noxious stimulus and also have brain areas similar to those of higher vertebrates that respond to painful stimuli."
"there is convincing evidence that fish possess nociceptors that react in a fashion similar to those in mammals."
"Opiate receptors have been found in fish as well as enkephalin like substances in various brain areas of goldfish, catfish, African lungfish and rainbow trout."
Natural analgesics in trout, cod, carp, sturgeon
Chervova, L,S, Lapshin, D, N
Proceedings of The Fourth International Iran & Russia Conference,
Moscow State University
"In mammals, the analgesic effects are primarily mediated through mu opioid receptors. Our results indicate that in fish, the same receptors are responsible for increasing the pain threshold. The decrease in pain sensitivity under the action of nonopioid preparations analginum and sidnophenum as well as analgesy caused by stress, illustrates the presence in fishes of other endogenous analgesic systems in addition to the opioid system."