The scientific consensus is that fish feel pain.

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 have feelings, fish feel emotion, fish experience pain, fish demonstrate pain, and yes, do fish suffer.
Fish show pain-related changes in physiology and behaviour that are reduced by painkillers, and that they exhibit higher brain activity when painfully stimulated.

The cruel sport of fishing causes considerable distress to fish.

Fish have nerves, as we do, to detect the elements that cause pain - heat, chemicals, and pressure.

  • Fish have nerve receptors that detect painful events
  • Pain signals are sent to higher brain areas
  • Pain receptors are similar to amphibians, birds, and mammals
  • The three types of pain receptors in people are also in fish
  • Fish pain sensitivity is comparable to humans

Fish detect what harms them

In an experiment, twenty-two pain receptors were found on the face of rainbow trout.

22 pain receptors
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. (Braithwaite, 2003).
american veterinary medical association fish pain AVMA Guidelines for the Euthanasia of Animals (pdf)
The European Union’s Standing Committee of the European Convention for the Protection of Animals Kept for Farming Purposes said that the skin of fish contains sensory receptors for touch, pressure and pain.

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. (Sneddon, 2006)

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. (Ashley, 2006).

pain receptors in fish 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 (Lilia).
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 than mammals (Ashley, 2007).

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 (Matthews, 1978).

simple fish feel pain

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 (Weld, 1992).

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.

Fish pain specialist, and author, Dr Braithwaite evidence

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, (Cooke, 2007).

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 (Nordgreen, 2007). 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 (Portavella, 2004).

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.

fish and human brain compared

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, (Dunlop, 2005).

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 (Beukema, 1970).

spinner used to catch fish
The pike 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, (Beukema, 1970).

pike and perch learned to avoid fish hooks
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, (Hoogland, 1957).
fish avoid stickback spines
Goldfish 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 (Zerbolio, 1983).


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 (Dunlop, 2006).

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 (Masayuki, 2006).

Professor Donald Broom, Cambridge University quote 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 (Brown, 2011).

cod learn 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 (Sneddon, 2011).

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 (Chervova, 1968).

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 (Jesuthasan, 2012).

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 (Sneddon, 2011).

in pain, rainbow trout rubbed their lips into the gravel and tank walls
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 (Sneddon, 2003).

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 (Sneddon, 2009).

FISH SPECIES VARY IN THEIR RESPONSE TO PAIN(Pain)

Fish Species
Swimming
Ventilation Rate
Feeding
Abnormal Behaviour
Rainbow Trout reduced increased reduced yes
Carp same same reduced yes
Zebrafish reduced increased reduced yes
Report of the Panel of Enquiry into Shooting and Angling quote
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, (Nordgreen, 2009).
european food safety authority quote

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 (Ehrensing, 1981). Similar results were found by researchers in Portugal (Correia, 2011).

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 (Sneddon, 2003).

Dr Lynne Sneddon fish pain quote
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 (Sneedon, 2017).

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 (Sneedon, 2004).

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 (Chervova).

Farm Animal Welfare Committee quote Farm Animal Welfare Committee 2014 report, Opinion on the Welfare of Farmed Fish (pdf)

References
Terms
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
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Fish Pain