The AHAW report, discussed above, lists some of the criteria used to indicate whether an animal, including fish, might be capable of experiencing pain. It goes on to gives some examples of evidence supporting each of these in fish species:

1. the existence of functional nociceptors
2. the presence and action of endogenous opioids and opioid receptors
3. the activation of brain structures involved in pain processing
4. the existence of pathways leading to higher brain structures
5. the action of analgesics in reducing nociceptive responses
6. the occurrence of avoidance learning
7. the suspension of normal behaviour associated with a noxious stimulus

All of the above show a pain system in fish that is similar to that of mammals and birds.

(i) The existence of functional nociceptors (pain receptors)

Fish have pain receptors. Pain receptors are called “nociceptors” because they detect noxious, i.e. harmful, things such as high temperatures or harmful chemicals. Nociceptors have been found on the face of rainbow trout. According to Professor Donald Broom:

“most vertebrate animals which have been investigated seem to have very similar pain receptors and associated central nervous pathways”.

(ii) The presence and action of endogenous opioids and opioid receptors

Endogenous opioids are substances produced in the brain in order to reduce pain. In mammals, these natural pain killers work through 3 distinct types of opioid receptor, and these have also been identified in zebrafish. Other evidence that fish produce these substances is given by the fact that:

“when goldfish are subjected to stressful conditions, there is an elevation of pro-opiomelanocortin, the precursor to enkephalins and endorphins, just as there would be in humans”

and that:

“The distribution of enkephalins in the fish brain shows a similar pattern to that seen in higher vertebrates”.

The Fisheries Society of the British Isles (FSBI) reports that the response of goldfish to analgesics (pain killers) is “similar to that of a rat” and says of these natural pain killers:

“one has to ask why they are needed in fish if these animals do not experience pain”.

(iii) The activation of brain structures involved in pain processing

The processing of pain in fish involves the brain. Electrical activity has been measured in the forebrain of trout and goldfish. Measurements differed for fish subjected to something noxious (e.g. heated prods) than for something harmless (e.g. being stoked with a paint brush).

The fact that activity occurs in the brain when they experience harmful stimuli, suggests that responses seen in fish are more than just reflexes, and that a feeling of pain is involved.

(iv) The existence of pathways leading to higher brain structures

The pain receptors of fish connect, via sensory pathways, to the brain.

In fish, as in other vertebrates, information received by nociceptors (pain receptors) in the skin is relayed to the brain via two major routes. Information from the head is sent via the trigeminal tract, while information from the rest of the body is sent via the spinothalamic tract. In fish, the trigeminal tract has been shown to project to the thalamus (part of the diencephalon in the forebrain) as it does in other vertebrates.

(v) The action of analgesics in reducing nociceptive responses

Analgesics (i.e. pain killers) work on fish, which gives further evidence of a fish pain system similar to that of other vertebrates, such as mammals.

Analgesics reduce the adverse behaviour seen in response to noxious (harmful) stimuli, and this indicates that a feeling of pain is involved.

Some of the research by Sneddon et al into pain perception in rainbow trout is discussed in (vii) below. Rainbow trout that had been injected in the lips with acetic acid (a noxious substance) showed adverse behaviour. They were observed rubbing their lips on the substrate of the tank and displayed a rocking behaviour, as well as a faster breathing rate. In a separate study, Sneddon et al showed that when morphine was administered to the fish, these effects were reduced.

AHAW goes on to cite research in which goldfish subjected to electric shocks show agitated swimming, but if injected with morphine, the threshold for this response increased.

(vi) The occurrence of avoidance learning

There is evidence that fish can learn to avoid noxious (harmful) stimuli, such as common carp and pike avoiding hooks in angling trials and goldfish that have learnt to avoid electric shocks.

In one experiment, paradise fish were given an electric shock when they entered a black compartment. They subsequently avoided the black compartment and learned to activate an escape hatch to avoid further shocks.

Avoidance learning has also been seen in rainbow trout, as discussed on the page fish feel fear.

(vii) The suspension of normal behaviour associated with a noxious stimulus.

It is sometimes argued by critics of fish sentience that, although fish can detect noxious (harmful) stimuli through nociceptors (pain receptors), that their response is a reflex behaviour without feeling. However, the behavioural responses seen in fish, to stimuli known to be painful in humans (e.g. bee venom), appear to be more than just reflexes.

AHAW argues that where a noxious stimulus has adverse effects on an animal’s normal behaviour beyond a simple reflex, then this may indicate that the animal is perceiving pain, stating that:

“Reflex responses occur instantaneously and within a few seconds but some of the responses of fish may be prolonged to 3 to 6 hours”.

AHAW goes on to describe a study by Sneddon et al to investigate the behavioural response of rainbow trout to noxious substances (acetic acid and bee venom) injected into their lips. Changes in behaviour over a prolonged period of time appeared to result from experiencing pain:

“These fish showed an enhanced respiration rate for approximately 3 hours, did not feed within this period, and showed anomalous behaviours such as rubbing of the affected area on the aquarium substratum and glass and rocking from side to side on either pectoral fin”.

In a separate study by Sneddon et al, fish injected with acid also failed to show their normal fear response to a novel challenge. Rainbow trout are fearful of novel objects and try to keep a distance from them, at least for a period of time. The failure of these fish to avoid new objects indicates that the painful stimulus dominated their attention.

This research into pain perception in fish was widely reported in the media in 2003 (see BBC report fish do feel pain).

A study published in 2009 after the AHAW panel’s report, found that goldfish subjected to an aversive, but non harmful, heat were displaying signs of fear 2 hours later. The Telegraph online (see Telegraph reports goldfish can feel pain) reported this, quoting one of the researchers Dr. Joseph Garner of Purdue University, Indiana:

“Morphine had some effect on their behaviour in the test, but the major effect was this response two hours later. That was really key…Those fish not given morphine showed hovering behaviour and were less active. These are defence and fear behaviours.

“We believe this hovering and inactivity are indicators of a general increase in fearfulness, wariness, and a generalisation of a bad experience. It is extremely difficult to explain this two hours later as a reflex”.

Fish are sentient subpages

• fish feel pain
• fish feel fear
• implications of sentience
• Do fish feel pain? by Victoria Braithwaite
• pain in crustaceans