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Do Fish Feel Pain?
By Dr. James D. Rose, University of Wyoming
Do fish, like humans, experience pain and suffering? People hold
very differing beliefs about this question. Some would believe that
if fish react to stimuli that would cause a person to feel pain that
the fish must also be feeling pain. Others assume that fish are too
different from humans for the matter to be of concern. Many people
don?t know quite what to think about the issue. Neuroscience
research has clarified the neurological and psychological processes
that cause the experience of pain, so we can address this question
from a large base of factual information.
PAIN IS A PSYCHOLOGICAL EXPERIENCE THAT IS SEPARATE FROM BEHAVIOURAL
REACTIONS TO INJURIOUS STIMULI
It has become very clear that pain is a psychological experience
with both a perceptual aspect and an emotional aspect. The
perceptual aspect tells us that we have been injured, like the first
sensation when you hit your thumb with a hammer. The emotional
aspect is separate as in the suffering that follows after we are
first aware of hitting our thumb. But, injurious stimuli do not
always lead to the experience of pain. Think of a trip to the
dentist. When a dentist injects a local anesthetic into your jaw to
block nerve conduction, some of your teeth and a part of your mouth
feel numb. When a tooth is then drilled, the sensory nerve cells in
the tooth that would normally trigger pain are still excited, but
the nerve block prevents activity in these receptors from being sent
to the brain, so pain is not felt. In addition, a person?s
behavioral reaction to pain is separate from pain experience. We see
this separation when a person endures pain without showing any
discomfort. On the other hand, people sometimes react behaviorally
to injury without any feeling any experience of pain or suffering.
This kind of separation between behavioral and psychological
responses to injury results from certain forms of damage of the
brain or spinal cord. Because the experience of pain is separate
from the behavioral response to injury, the term nociception is used
to refer to detection of injury by the nervous system (which may or
may not lead to pain). Injurious stimuli that usually lead to pain
experience are called nociceptive stimuli. The term pain should be
used only to refer to the unpleasant psychological experience that
can result from a nociceptive stimulus.
REACTIONS TO INJURY ARE PRESENT IN ALL FORMS OF ANIMAL LIFE BUT
THESE REACTIONS DO NOT MEAN THAT PAIN IS EXPERIENCED-IT IS NOT
NECESSARY FOR A NOCICEPTIVE STIMULUS TO BE CONSCIOUSLY EXEPERIENCED
FOR A BEHAVIORAL REACTION TO OCCUR
In humans, reactions to nociceptive stimuli are usually associated
with feelings of pain. Consequently, humans often assume that
reactions by animals to nociceptive stimuli mean that these animals
experience similar pain. In reality, reactions to nociceptive
stimuli are protective responses that can occur in forms of life
that are incapable of perceiving pain. The ability to detect and
react to nociceptive stimuli is a widespread characteristic of
animal life. Single-celled creatures such as an ameba will move away
from irritating chemical or mechanical stimuli. These reactions are
automatic and because the ameba doesn?t have a nervous system, it
has no ability to actually sense the stimulus that causes its
reaction or to feel pain. There are many other invertebrate
organisms (animals without backbones) that also react to nociceptive
stimuli, but with somewhat more complex patterns of escape than an
ameba. For example, starfish have a primitive nervous system that
interconnects sensory receptors detecting injurious stimuli with
muscle cells that cause movements, enabling the starfish to slowly
move away from a nociceptive stimulus. The starfish?s nervous system
has only a small number of nerve cells. It has no brain, so like the
ameba, its reactions are not very precise or complex and it can?t
experience, in the way of humans, the stimuli that trigger its
reactions. Thus, protective reactions don?t require very complex
nervous systems and can occur in animals incapable of perceiving,
that is being aware of, the stimuli that cause such reactions.
IN VERTEBRATES, REACTIONS TO INJURIOUS STIMULI ARE CONTROLLED BY THE
SPINAL CORD AND BRAINSTEM
Vertebrates generally have more complex nervous systems than
invertebrates and vertebrates have a clearly developed brain. This
brain receives information from the spinal cord about nociceptive
stimuli that contact the body surface. Working together with the
spinal cord, the brain generates rapid, coordinated responses that
cause the organism to escape these stimuli. These automatically
generated responses include withdrawal of the stimulated body part,
struggling, locomotion and in some animals, vocalizations. All of
these responses are generated by the lower levels of the nervous
system, including the brainstem and spinal cord.
HUMAN EXISTENCE IS CEREBRALLY-DOMINATED- A FISH?S EXISTENCE IS
BRAINSTEM DOMINATED
Human existence is dominated by functions of the massively developed
cerebral hemispheres. Fishes have only primitive cerebral
hemispheres and their existence is dominated by brainstem functions.
The brains of vertebrate animals differ greatly in structural and
functional complexity. Cold-blooded animals, such as fish, frogs,
salamanders, lizards and snakes, have simpler brains than
warm-blooded vertebrates, the birds and mammals. Fish have the
simplest types of brains, of any vertebrates, while humans, have the
most complex brains of any species. All mammals have enlarged
cerebral hemispheres that are mainly an outer layer of neocortex.
Conscious awareness of sensations, emotions and pain in humans
depend on our massively-developed neocortex and other specialized
brain regions in the cerebral hemispheres. If the cerebral
hemispheres of a human are destroyed, a comatose, vegetative state
results. Fish, in contrast, have very small cerebral hemispheres
that lack neocortex. If the cerebral hemispheres of a fish are
destroyed, the fish?s behavior is quite normal, because the simple
behaviors of which a fish is capable (including all of its reactions
to nociceptive stimuli) depend mainly on the brainstem and spinal
cord. Thus, a human?s existence is dominated by the cerebral
hemispheres, but a fish is a brainstem-dominated organism.
The capacity to perceive and be aware of sensory stimuli, rather
than just react to such stimuli requires a complex brain. In humans,
the cerebral hemispheres, especially the neocortex, is the
functional system that allows us to be aware of sensory stimuli. If
the cortex of the human brain is damaged or made dysfunctional, we
lose our awareness of sensations. For example, damage of the visual
part of the cortex causes blindness, even though vision-related
sensory activity is still occurring in subcortical parts of the
brain. If the neocortex is widely damaged we lose our capacity to be
aware of our existence in general. This loss of awareness occurs in
spite of the fact that the levels of our nervous system below the
cerebral hemispheres, the brainstem and spinal cord, can still be
functioning and processing signals from sensory stimuli, including
injurious stimuli. In a fish, ?seeing? is performed by the brainstem
and occurs automatically without awareness. Consequently, a fish?s
visual behavior is quite normal if the small cerebral hemispheres
are removed, but a human is blind if the visual cortex region of the
cerebral hemispheres is destroyed. This is because our visual
behavior depends greatly on conscious awareness of visual
sensations.
In spite of our unawareness of brainstem functions, the brainstem
and spinal cord contain programs that control our more automatic
behavioral functions. Smiling and laughter, vocalizations, keeping
our balance, breathing, swallowing and sleeping are all processes
that are generated by these lower, brainstem and spinal cord
programs.
FISH DO NOT HAVE THE BRAIN DEVELOPMENT THAT IS NECESSARY FOR THE
PSYCHOLOGICAL EXPERIENCE OF PAIN OR ANY OTHER TYPE OF AWARENESS
The experience of pain depends on functions of our complex, enlarged
cerebral hemispheres. The unpleasant emotional aspect of pain is
generated by specific regions of the human cerebral hemispheres,
especially the frontal lobes. The functional activity of these
frontal lobe regions is closely tied to the emotional aspect of pain
in humans and damage of these brain regions in people eliminates the
unpleasantness of pain. These regions do not exist in a fish brain.
Therefore, a fish doesn?t appear to have the neurological capacity
to experience the unpleasant psychological aspect of pain. This
point is especially important, because some opponents of fishing
have argued that fish are capable of feeling pain because some of
the lower, subcortical nervous system pathways important for
nociception are present in fish. Obviously this argument has no
validity because without the special frontal lobe regions that are
essential for pain experiences, lower pathways alone can?t produce
this experience. The rapid, well-coordinated escape responses of a
fish to nociceptive stimuli are generated automatically at brainstem
and spinal cord levels but, if a fish?s brainstem and spinal cord
work like a humans (and it is very likely that they do) there is no
awareness of neural activity occurring at these levels.
It might be argued that fish have the capacity to generate the
psychological experience of pain by a different process than that
occurring in the frontal lobes of the human brain, but such an
argument is insupportable. The capacity to experience pain, as we
know it, has required the massive expansion of our cerebral
hemispheres, thus allocating large numbers of brain cells to the
task of conscious experience, including the emotional reaction of
pain. The small, relatively simple fish brain is fully devoted to
regulating just the functions of which a fish is capable. A fish
brain is simple and efficient, and capable of only a limited number
of operations, much like a 1949 Volkswagen automobile. By
comparison, the human brain is built on the same basic plan as that
of a fish, but with massive expansions and additional capacities.
The human brain is more like a modern luxury car with all-wheel
drive, climate control, emission controls, electronic fuel
injection, anti-theft devices and computerized systems monitoring.
These refinements and additional functions couldn?t exist without
massive additional hardware. The massive additional neurological
hardware of the human cerebral hemispheres makes possible the
psychological dimension of our existence, including pain experience.
There are also huge differences between mammals in the degree of
complexity of cerebral hemisphere development, especially within the
frontal lobes. The brains of predatory mammals are typically larger
and more complex than brains of their prey. For example, the brains
of sheep and deer have a tiny fraction of the frontal lobe mass that
is present in humans, making it probable that the kinds of
psychological experience of these animals, including pain, is quite
different from human experience.
THE REACTIONS OF FISH TO NOCICEPTIVE STIMULI ARE SIMILAR TO THEIR
REACTIONS TO PREDATORS AND OTHER NON-NOCICEPTIVE STIMULI
When a fish is hooked by an angler, it typically responds with rapid
swimming behavior that appears to be a flight response. Human
observers sometimes interpret this flight response to be a reaction
to pain, as if the fish was capable of the same kind of pain
experience as a human. From the previous explanation, it should be
clear that fish behavior is a result of brainstem and spinal
patterns of activity that are automatically elicited by the
stimulation of being hooked, but that fish don?t have the brain
systems necessary to experience pain. It is very important to note
that the flight responses of a hooked fish are essentially no
different from responses of a fish being pursued by a visible
predator or a fish that has been startled by a vibration in the
water. These visual and vibratory stimuli do not activate
nociceptive types of sensory neurons so the flight responses can?t
be due to activation of pain-triggering neural systems. Instead,
these flight responses of fish are a general reaction to many types
of potentially threatening stimuli and can?t be taken to represent a
response to pain. Also, these flight responses are unlikely to
reflect fear because the brain regions known to be responsible for
the experience of fear, which include some of the same regions
necessary for the emotional aspect of pain, are not present in a
fish brain. Instead, these responses are simply protective reactions
to a wide range of stimuli associated with predators or other
threats, to which a fish automatically and rapidly responds.
Although fish don?t have the capacity to experience human-like pain
or suffering, their reactions to nociceptive stimuli or capture are
still important because these reactions include the secretion of
stress hormones. These stress hormones can have undesirable health
effects on fish if they are secreted in large amounts over a long
period of time. So, it?s important when practicing catch-and-release
fishing to observe the usually recommended procedures of landing a
fish before it is exhausted and returning it to the water quickly.
The facts about the neurological processes that generate pain make
it highly unlikely that fish experience the emotional distress and
suffering of pain. Thus, the struggles of a fish don?t signify
suffering when the fish is seized in the talons of an osprey, when
it is devoured while still alive by a Kodiak bear, or when it is
caught by an angler.
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