DO FISH FEEL PAIN?
This is not an easy question to answer. Reasonable arguments have been made both to
support and refute the claim that fish are capable of sensing and experiencing pain.
Summaries of two such arguments follow.
The Case Against the Experience of Pain in the Fish
James Rose of the University of Wyoming has put forth a strong argument for the inability of
fish to experience pain that relies on analogy between human and fish neuroanatomy. Rose
emphasizes the distinction between reaction to injury and psychological experience of pain and
emphasizes that the existence of the former does not evince the existence of the latter. Indeed,
human experiments have proven that pain is experienced in the brain and that sensation of and
reaction to noxious, or potentially harmful, stimuli can occur without the experience of pain. The
concept of nociception makes this possible.
Nociception:
The term nociception refers to the detection of noxious stimuli by the nervous system. The
peripheral nervous receptors we call nociceptors sense stimuli and report to the central nervous
system where motor responses are initiated and the sensation of pain is perceived. Some fish
species certainly do have nociceptive neurones analogous to those found in the human (see
Neuro-physiology of Pain in Fish). However, this means only that these animals are capable of
sensing noxious stimuli; it provides no evidence for the psychological experience of pain.
Pain Related Chemicals:
Teleost nervous systems also produce opiate-related compounds and proteins similar to the
GABA/benzodiazepen receptors that play a role in the sensation of human pain. However, these
compounds are not exclusively related to pain in humans and may play other roles in the
physiology of fish. For greater evidence of the psychological experience of pain it seems
appropriate to examine the brain.
The Brain:
According to Bermond (1997) the highly developed neocortex of the human cerebral
hemispheres is responsible for our ability to experience emotions and sensations such as pain.
The existence of this feature in the fish brain would strengthen an argument for the ability of fish
to experience pain. However, the fish brain is dominated by brainstem components and features
very primitive cerebral hemispheres that lack neocortex. Humans require this neocortex for basic
sensory functions as it is thought to be responsible for interpreting the sensory information
received and processed by our brainstem and spinal cord. In fish, a higher level of cortical
sensory interpretation appears nonexistent, since fish behaviour is unaffected by cortical damage.
For example, cortical damage in a human may cause blindness whereas the complete removal of
a fish’s cerebral hemispheres causes no apparent change in sensory-dependant behaviour.
If we assume, as Rose and Bermond do, that the neocortex is necessary for pain sensation, then
we must admit that sensation of pain in any animal lacking an analogous structure is unlikely.
Fish would therefore lack the neurological capability to experience the negative psychological
sensation of pain.
Figure 1. Comparison of human brain with a trout brain. A. Diagram of a midline view of the human brain. The cerebral hemisphere is shaded in
darker gray and the brainstem is in lighter gray. B. Diagram of a midline view of a rainbow trout brain. The cerebral hemisphere (darker gray) is very small relative to the size of the brainstem (lighter
gray). The white region at the left of the cerebral hemisphere is the olfactory bulb, which processes odor information. The olfactory bulb of a trout and many other fishes is large compared to the size
of the brain as a whole, but the olfactory bulb in humans is relatively small. C. Diagram of the brain of a 12 inch rainbow trout shown at the same scale as the human brain diagram.
Figure 2. The diagram below shows the basic regions of the human central nervous system, the
large cerebral hemispheres, the brainstem and the spinal cord.
The Case Against the Experience of Pain in the Fish:
How convincing is the argument as stated above? Is the lack of certain so-called higher brain
structures evidence enough to deny that fish feel pain? The following argument may refute such
a claim quite effectively.
Accessing Subjective Experience:
It cannot be denied that psychological states are entirely private experiences. This fact alone
requires us to make inferences about subjective experiences of non-humans in one of two
manners:
1. by analogy between the behaviours and physiological states of humans and of animals
2. by making the case that the existence of a subjective state is evolutionarily significant and
a precondition for the species’ evolution
Argument by Analogy:
The first method is employed routinely by animal welfare scientists when assessing the
experience of mammals and birds and has even been used to argue convincingly for the capacity
of invertebrates to suffer (Sherwin 2001). In the argument outlined above, Rose makes a case by
analogy to cast doubt upon pain sensation in fish by showing that fish neuroanatomy is
sufficiently different from that of humans. The behavioural evidence provided on this site (kk) is
similar to that used to explain painful experiences of mammals and birds and even human infants
in other research (Dubner & Ren 1999, Sanford et al 1986, Anand & Craig 1996). If we use such
indicators to describe pain in so-called higher vertebrates, then why not use them to describe pain
in fish?
Argument by Evolutionary Necessity:
The second method is used quite eloquently by Dawkins when she says:

Pain evolved because, by being unpleasant, it keeps us away from the larger evolutionary
disaster of death. Pain is part of a mechanism for helping us to avoid immediate sources
of injury, and also to refrain from repeating actions that have resulted in damage (1998).
This argument is highly persuasive. Clearly any animal could not be successful unless it featured
both a mechanism for detecting potentially harmful stimuli and a kind of negative or unpleasant
psychological or subjective state or experience with which it could associate such stimuli. Fish, it
appears, may have remarkably different systems of nociception and brain function from
mammals and therefore may not experience the precise sensation of pain that humans do but this
does not mean that fish are incapable of experiencing a negative psychological state analogous to
human pain in response to noxious stimuli.
This reasoning is undoubtedly compelling. Though what is quite apparent from both of the
arguments outlined above is that our current body of knowledge about the neuro-physiology of
fish is inadequate for either argument to be entirely convincing. Therefore it is particularly
relevant in this case to remember that "absence of evidence is not evidence of absence"
(Sherwin, 2001). We must remain open-minded and recognize that our ability to answer the
question "do fish feel pain?" with confidence is limited by the constraints of our own perception.
References:
Anand, KJS and Craig, KD. 1996. New perspectives on the definition of pain. Pain. 67: 3-6.
Bermond, B. 1997. The myth of animal suffering. In Dol, M, Kasamoentalib, S Lijmbach, S,
Rivas, E and vandenBos, R (eds). Animal Consciousness and
Animal Ethics: Perspectives from the Netherlands. pp 125-143. Van Gorcum: Assen, The
Netherlands.
Dubner, R and Ren, K. 1999. Assessing transient and persistent pain in animals. In: Wall PD and
Melzack (eds) Textbook of Pain, 4th ed.. pp 359-369. Churchill Livingstone: Edinburgh, UK.
Sanford, J, Ewbank, R, Molony, V, Tavernor, WD, Uvarov, O. 1986. Guidelines for the
recognition and assessment of pain in animals. Veterinary Record. 118: 334-338.
Sherwin, C. 2001. Can invertebrates suffer? Or, how robust is argument–by analogy? Animal
Welfare. 10: S103-118.