Where fish feel pain

Authors: G√ľnther Stockinger
Journal: Spiegel Online
Year: 11 March 2011

Researchers discovered that the limbic system in the human brain has its counterpart in the fish’s telencephalon. Both are involved in pain perception.

der spiegel where senses are interpreted


Bad news for angling enthusiasts: Fish might be much more than the mere reflex machines previously thought. New research shows that they are apparently conscious of and can suffer from pain, and scientists are calling for them to be treated the same as mammals and birds.

Could millions of hobby anglers and sports fishermen really be wrong? They believe that getting a nasty-looking hook stuck in the mouth doesn’t hurt fish. Fishing enthusiasts insist the nervous system of the aquatic creatures is far too primitive for them to feel real pain.

Until now, angling aficionados have seldom been accused of cruelty since fish are regarded as lower life forms. Indeed, hardly anyone believes that they have feelings like mammals and birds, and most people only have emotional feelings for warm-blooded animals.

But now the image of these robot-like creatures with their purported three-second memory is starting to crack. The latest findings from fish biologists, neuroanatomists and behavioral scientists show that these evolutionarily ancient vertebrates are far more than mere reflex machines.

Researchers from Queen’s University, in Belfast, have proven that when fish are subjected to pain stimuli, the signals by no means simply ebb away in the spinal cord. Scientists have discovered sensitive skin areas directly behind the gill covers of goldfish and trout. Using implanted electrodes, they have been able to show that the nerve cells located there send signals directly to the fish’s brain.

When researchers poked the animals with needles, a flurry of neuron messages were transmitted to the endbrain - the very region of the brain where pain signals are also processed by birds and mammals.

Not a Simple Reflex

Similar results have now been achieved with Atlantic salmon, carp and cod. "These studies demonstrate that higher brain areas are implicated in the fish response to potentially painful events and that their response is not a simple reflex," explains Lynne Sneddon, a fish expert at England’s University of Chester.

A Spanish research¹ team was even able to identify an area of the goldfish brain that appears to serve a function similar to the limbic system, the region of the human brain that becomes highly active when people experience fear or pain. As with mammals, these cerebral receptors in fish consist of a number of anatomical structures: Incoming signals to the amygdala are processed by an emotional filter, while the hippocampus is for memory, but also plays a key role in spatial orientation.

Researchers have long searched in vain for these two regions - apparently because they were looking in the wrong place. It turns out that as a fish matures from an embryo to a full-grown adult, its brain architecture is turned inside out: While the human amygdala and hippocampus lie deep below the cerebral hemispheres of humans, the comparable structures of a fully developed fish are located directly on the surface of the endbrain.

Behavioral tests have confirmed these findings: Goldfish whose hippocampus-like structures in the endbrain have been surgically disabled suddenly lose their sense of orientation - as do mammals whose corresponding cerebral regions have been disabled.

Furthermore, when researchers put the amygdala-like sections of the endbrain out of action, the fish were no longer capable of learning from electric shocks.

This proves that these supposedly insensitive aquatic animals have the necessary hardware in their heads to feel fear and pain. "Even though the structure and function of the fish equivalent is very much simpler than our own limbic system, the fact that scientists have discovered the presence of similar structures is impressive," explains Victoria Braithwaite, a zoologists at Pennsylvania State University.

A number of years ago, Braithwaite caused a stir with another discovery that she made about fish physiology. She found more than 20 pain receptors around the mouth and head of rainbow trout - ironically located precisely where the barbed hooks of anglers penetrate the fish’s flesh.

These front receptors of the nervous system react not only to pinpricks, but also to heat and noxious chemicals. Combined with the specialized nerve fibers that transmit pain impulses, the receptors don’t work any differently than they do among higher vertebrates.

But are fish also capable of converting their perception of these complex signals into a conscious awareness of pain? A wide range of behavioral tests at least suggest this is the case.

Rainbow trout whose lips were injected with bee venom or acetic acid ventilated vigorously with their gill covers for nearly three-and-a-half hours, stopped feeding, rocked back and forth on the floor of the tank or rubbed their lips on the glass walls. They displayed far more than just three-second reactions.

Trout that had been subjected to noxious chemicals paid little attention to a brightly-colored Lego tower introduced into their tank despite the fact that they normally avoid new objects, suggesting that their attention was dominated by pain. However, fish that simultaneously received painkillers and chemicals displayed the usual degree of caution with regard to the foreign objects - because the morphine had apparently eliminated the pain.

The experts not only believe them capable of fear and pain, but also sensations of pleasure. For example, oxytocin - often referred to as the "love hormone" - has also been documented in fish.

For many experts, however, the lack of a cerebral cortex no longer appears to be sufficient reason to rule out conscious awareness. Remarkable medical cases have cast doubt on the old school of thought: Neurologists occasionally report people who have only half a cerebrum. Where others have synapses chattering away, these individuals only have brain fluid swishing about - and yet they are often highly intelligent and socially well adapted.

¹Rodriguez, F., C. Broglio, E. Dtiran, A. Gomez, and C. Salas C, ’Neural Mechanisms of Learning in Teleost Fish’, in C. Brown, K. Laland, and J. Krause (eds). Fish Cognition and Behaviour (Oxford: Blackwell, 2006), 243-77.

Fish Pain