“Finding Nemo” fans, you’d better sit down. I’ve
got some bad news.
First off, fish can’t talk. Matter of fact, fish
can’t really think, either. But here’s the kicker.
Fish apparently can’t even feel pain.
That’s the latest from the world of piscine
anatomy, and it’s not without controversy. In fact,
it’s turning into a big case of international
scientific “he said, she said.” At the heart of it
all is a question about how different human animals
are from the rest of the natural world, and how we
think about the animals we share a planet with.
Last year, Lynne Sneddon, a professor of animal
biology at the University of Liverpool, in England,
published a study in which she tried to provoke pain
in fish. Not just an “owwie,” mind you, but actually
“pain” — a sensation of equal parts physical
discomfort and emotional suffering usually reserved
for creatures with big brains.
Sneddon divided her captive rainbow trout (Oncorhynchus
mykiss) into four groups. One was injected in the
snout with bee venom, and another with acetic acid.
For you fish-and-chips fans with a taste for irony,
that’s the acid in malt vinegar. Both are chemicals
commonly used to test pain in laboratory research. A
third group was injected with saline as a control
group, to determine if the needle poke was the
source of the reaction. The fourth was handled by
researchers, but not injected, to rule out the
stress of the experiment being the cause.
At the heart of Sneddon’s research was the
importance of a group of neurological sensors around
the fishes’ mouth called nociceptors. In her
research, Sneddon identified 58 of them in the
fish’s face and head that were triggered by a
chemical, mechanical, or temperature stimulation.
These sensors, designed to warn their owner about
“noxious stimulation,” are the frontline defenses
against repeatedly impaling oneself on sharp
objects. They cause an unconscious pulling away from
things that damage the body. They’re hard-wired into
the hind brain, the central processor of life that
controls such things as breathing, circulation,
movement, eating, drinking, and involuntary
reflexes. Humans have a system very much like this.
“Anomalous behaviors were exhibited by trout
subjected to bee venom and acetic acid,” Sneddon
says. “Fish demonstrated ‘rocking’ motion,
strikingly similar to the kind of motion seen in
stressed higher vertebrates like mammals, and the
trout injected with acetic acid were also observed
to rub their lips onto the gravel in their tank and
on the tank walls. These do not appear to be reflex
That reaction fulfills a set of criteria for
animal suffering, Sneddon says. To make sure, she
gave fish morphine to see if they “felt better”
after treatment. Sure enough, their respirations
slowed and they stop swaying. So that settles it.
Fish feel pain.
Not so fast. James Rose of the University of
Wyoming Department of Psychology and Department of
Zoology and Physiology disagrees. Studying the
neurological structure of a fish brain, Rose
concludes fish can’t possibly feel pain, even if
they display a few suspicious-looking behaviors,
because they don’t have the brains for it.
According to a medical definition, pain is an
unpleasant sensory and emotional experience
associated with tissue damage. It’s so subjective
that everyone feels it differently, and you can feel
it even if you haven’t actually mangled yourself.
The key is the emotional component. In order to
suffer, your brain needs wiring that lets it feel
both sensation and emotion. That’s the problem, Rose
says. In fish, there’s not a snowball’s chance
because they don’t have the hardware to have a
Sneddon uses a looser definition of pain that’s
widely used in animal-based research. If the animal
has basic neurological structure, including
nociceptors connected to a central nervous system,
the ability to naturally create painkillers called
neuropeptide opiates, a proven reaction to
analgesics, and humanlike reactions to pain, it’s a
Not all fish pass the test, she says. Sharks and
rays, both members of the family elasmobranchs
because they have cartilaginous skeletons, don’t
have nociceptors. Trout, a member of the teleost
family of fish with bony skeletons, do have the
collection of pain processors, and they do seem to
react like humans would. So she votes yea.
But just because it looks like pain, and it acts
like pain, doesn’t mean it really is pain, Rose
says. And he’s got a pretty good argument, grounded
in the fundamental rule of neurobiology: If the
apparatus isn’t there — which it’s not in fish — the
sensation isn’t either. He votes no.
Beyond nociceptors, human brains and fish brains
don’t have much in common, according to Rose. A
trout brain looks like a short, thin rope with a
knot tied in it. The midbrain knot is mostly the
optic lobe, but also includes the hippocampus for
memory and pituitary gland for growth and
reproduction. Aft of the knot is the cerebellum and
brainstem. On the short end in front of the knot are
the cerebral hemispheres, most of which are occupied
by the olfactory lobes.
Like all mammals — and unlike all fish — the
cerebral hemispheres of humans are bigger than the
brain stem. The frontal, temporal, and parietal
lobes of the brain work with its outer layer, called
the neocortex, to provide “primary consciousness” —
the ability to know what’s going on at any given
time, the ability to follow commands, and the
ability to use verbal or nonverbal communication.
This part of the brain allows a lab rat with
arthritis to choose to drink from bad-tasting water
containing painkillers instead of sweetened water,
which is preferred by rats without the condition. In
animals where the neocortex is particularly
well-developed — humans and some other primates —
it’s also home to “high-order consciousness,” which
includes language, creativity, autobiography, and
the desire to ask abstract questions, such as this
Remove the cerebral hemispheres, and a fish
still pretty much acts like a fish, albeit one with
no sense of smell. Do it in a human and you get a
person in a constant vegetative state. Take a little
less brain, specifically just parts of the frontal
lobe — a lobotomy – and the patient may shout at a
“painful” jolt, but they don’t report the experience
as being particularly unpleasant.
anatomically speaking, fish can’t feel pain, why do
they act like they can? When hooked by fishermen,
trout and sharks react in much the same way — by
thrashing, darting, and pulling. Sharks don’t have
pain nociceptors, and they don’t have
pain-transmitting nerves like humans or trout. Rose
suggests the animals are simply trying to escape, an
unconscious flight reaction to being pulled in a
direction they don’t want to go. And even though
trout produce their own internal chemical analgesic,
those hormones probably have additional roles
besides numbing pain, such as speeding up healing,
as they do in humans.
Snow suggests the rocking of Sneddon’s injected
fish may actually be a neurological reaction to the
high dose of toxin in bee venom — the dosage was
equivalent to a 3-ounce injection into the lip of a
normal adult human. That dosage may be so high that
it’s causing neurological problems for the fish,
causing it difficulty in staying upright. In fact,
he suggests Sneddon’s research may indicate fish
actually have an amazing resilience to pain, because
they returned to feeding in only three hours after
the traumatic toxin poisoning.
He’s also not buying the idea that fish may feel
pain in their own way. It takes a lot of brain power
for primates to have even primary consciousness —
entry level on the pain scale. The odds are slim
that fish can do that with a smaller setup. “To
propose that fishes have conscious awareness of pain
with vastly simpler cerebral hemispheres amounts to
saying that the operations performed on the modern
computer could also have been done by the 1982 model
without additional hardware and software,” he says.
Ultimately, “if it looks like pain, it is pain”
isn’t a good definition, he says. Humans born
without cerebral hemispheres will still show facial
signs of “pain” when exposed to noxious stimuli,
even though they’re permanently unconscious.
Likewise, people with spinal cord injuries may
reflexively pull away from a pin prick, even though
they can’t feel it. Both are nociceptor reactions.
As it is, no one’s really sure what fish feel,
and they aren’t talking. Certainly, they don’t
experience the world as primates do, but,
apparently, trout don’t experience it like sharks,
lungfish, electric rays, or tuna, either, because
those fish have significantly different brain
structures, suited to the niche each has evolved to
A big clue to answering the pain question would
be if fish would seek analgesic relief from pain if
given the chance — if they would self-medicate, like
the arthritic rats. No one’s tried that yet. So
would observations that differentiate stress from
pain behavior. Some birds and reptiles will show
guarding behavior until they’re spooked, apparently
unable to process both at the same time. More
research into fish brain structures may lead to more
definitive answers — even about how all brains work.
New research from the University of Manchester seems
to indicate fish can regrow injured brain tissue.
Human brains can’t do that — but finding out how it
works in them may help us figure out how it might
work in us.
A Fine Kettle of Fish
But from here, a mildly intriguing academic
argument gets messy.
Sneddon and her colleagues began their study out
of concern for the welfare of animals in commercial
fish farms. With that motivation, and those results,
their research was quickly cited as “good news” for
animal-rights activists opposed to recreational
fishing. In fact, one group wrote the president of
the University of Wyoming to protest Rose, asking
him to rescind the professor’s research because it
was so clearly biased by Rose’s self-professed
status as a fisherman. That’s not what university
presidents do, the president reportedly replied.
For his part, Rose thinks a whole lot of energy
is wasted discussing fish pain. Anthropomorphizing
fish doesn’t solve their biggest problems, which
really don’t have anything to do with whether they
have a consciousness. This much effort should be
made to answer bigger questions of conservation,
including water quality, water temperatures, and
habitat protection, he says.
“[My] conclusion in no way devalues fish or
diminishes our responsibility for respectful and
responsible stewardship of them. Fishes constitute a
highly evolved, diverse, and complex life for whose
history on the Earth vastly eclipses the brief
existence of humans,” Rose says. “Our increasingly
deleterious impacts on fishes at the population and
ecological levels require us to use our best
scientific knowledge and understanding to foster
their health and viability.”
In other words, the issue isn’t whether they
feel pain. It’s this: If we want them around, we’re
the ones who have some thinking to do.
Just because it looks like pain, and it acts like pain, doesn’t mean it really is
pain — at least according to one researcher.
For More Information
Rose, J.D. “The Neurobehavioral Nature of Fishes
and the Question of Awareness of Pain.” Reviews in
Fisheries Sciences, 10 (1): pp. 1-38. 2002.
Sneddon, L.U., Braithwaite, V.A., and Gentle,
M.J. “Do Fish Have Nociceptors: Evidence for the
Evolution of a Vertebrate Sensory System.”
Proceedings of the Royal Society, series B. 2003.