Do fish feel pain?
V. Braithwaite 2010.
Dr Victoria Braithwaite makes the science behind the debate around pain in fish accessible to non-scientists. She describes the many different pieces of evidence that together build up a picture of fish as animals that, she concludes, “have the mental capacity to feel pain”:
Dr Braithwaite begins by discussing pain in humans. Pain is a process which is considered as having two distinct parts. The first part is the detection of potentially harmful stimuli (eg high temperature) by pain receptors in the skin. These pain receptors are called “nociceptors” because they detect noxious stimuli, and the process is called “nociception”. This results in messages being sent via nerves to the brain. The second part is the emotional response generated in consequence, whereby the person actually feels pain, and this is much less well understood. The crucial question for animal welfare is whether fish also experience an unpleasant emotional response to painful events.
The fish biologist describes how she and her colleagues found nociceptors in the face of trout, and that these nociceptors were active when exposed to harmful stimuli (high temperature, high pressure and acetic acid). Nociception must be involved if an animal feels pain. However nociception is not, in itself, considered sufficient evidence for the ability to feel pain, since primitive animals which don’t have central nervous systems, such as anemones, show nociceptive-like responses.
The behaviour of animals in response to events has often been studied as an indication of the animal’s emotional state. Dr Braithwaite’s team devised some experiments to see how a fish’s behaviour would be affected by a painful experience. They found that trout injected in the lip with bee venom or vinegar displayed abnormal behaviours. Their breathing rates dramatically increased. Some rubbed their lips on the tank walls and substrate while some rocked from side to side on their pectoral fins. These may have been their way of trying to relieve the irritation. The fish failed to feed for several hours, despite having gone without food that day. This research was published in the Proceedings of the Royal Society of London in May 2003 and attained much media interest. At this point, Dr Braithwaite’s response to the “do fish feel pain” question was “well possibly…”.
Could they be sure that the abnormal behaviour seen in response to painful injections was evidence of a negative emotional experience? The team wanted to see if higher order behaviour was affected and conducted some experiments to see how a normal fear response was affected by pain. Trout are naturally afraid of novel objects and will keep their distance for some time. When a Lego tower was introduced into a tank containing trout, the fish displayed this fear response. However, for fish that had been injected with vinegar, the fish failed to keep their distance. Their fear response had been diminished by the effect of the injection, suggesting that their attention was dominated by pain.
The book then considers the related question of whether fish are conscious. Many people have argued that an animal must be conscious to experience pain. The author first gives some impressive examples of evidence that fish can construct internal representations of their environment. Goldfish can learn to navigate mazes using memory. Gobies that have become confined in a rockpool because the tide has gone out, can accurately jump to neighbouring rockpools they cannot see because they learned a map of the depressions in the locality when the tide was in. She also demonstrates that cichlid fish are capable of logical deduction.
Recent scientific research has found evidence of a limbic system in the fish brain. The limbic system in the human brain is believed to be responsible for emotions. Scientists have shown that the brain is active when fish (goldfish, trout, salmon) experience painful stimuli. Dr Braithwaite describes an experiment in which a trout can choose between two areas of a tank, in one of which it will receive electric shocks while in the other it does not. Normally the trout avoids the former. However, if by being in the area where it will receive electric shocks it is also nearer to another fish, then the trout chooses to remain in this area because it is a social animal. The trout’s response to electric shocks is therefore context-dependent, suggesting that conscious emotional decision-making is involved.
The book’s author then gives a surprising example of fish behaviour, involving cooperation between groupers and eels, which suggests that fish also have self-consciousness. In this context, self-consciousness means the ability to think about your own actions, to consider different possible scenarios, and to modify your decisions on how to act as necessary. Think of all the examples you know of animals that hunt together. With the exception of those involving humans, they are all between animals of the same species. This example of cooperative hunting between two different species of fish, groupers and eels, is striking.
Groupers are large predatory fish that hunt smaller reef fish in open water. Moray eels, on the other hand, slither through crevices in coral reefs to corner their prey in holes. Fish avoid eel predation by swimming into open water, and avoid grouper predation by hiding in crevices. Imagine what a formidable hunting team these two species would make together. As Dr Braithwaite explains, a recent report shows these two fish have developed a way of communicating with one another to do just that.
When a grouper chases a prey fish, its quarry may seek refuge in a small hole on the reef. It then becomes a sit-and-wait game because the grouper can’t follow the fish into reef crevices. Groupers have learnt that when this happens, they can go and get help. The grouper moves off in search of an eel and, when a potential hunting partner is found, signals to it by vigorously shaking its head. The eel can choose to ignore the signal but often it responds by leaving its crevice to follow the grouper, and is led to where the prey fish was last seen. The eel then starts to explore the reef around that area. Sometimes the grouper shows the eel where the fish went in by performing a headstand accompanied with more head shaking. In response, the eel swims to the location and enters the reef.
On roughly half the occasions, the eel flushes the prey fish from the reef and the waiting grouper darts in and snatches its meal. On many other occasions, the eel corners the prey within the reef and gains a meal itself. While we don’t have clear footage of the whole story, aspects of this cooperative behaviour may been seen in the youtube clips below.
The moray eel and the grouper represent an example of a “sophisticated, complex behaviour that requires the hunting partners to communicate and recognise each other’s intentions”. If we consider cooperative hunting as an example of self-consciousness, argues Dr Braithwaite, then surely this is evidence of self-consciouness in fish?
If we accept that birds and mammals can feel suffer, the author says, then that there is now sufficient evidence that fish can too. This book is written in everyday language in a narrative style. We recommend this book to all with a curiosity for what animals think and feel.
Victoria Braithwaite is Professor of Fisheries and Biology at Penn State University, USA and a Visiting Professor of Biology, University of Bergen, Norway.
grouper and eel hunting cooperation on youtube
In this clip we see a grouper approach an eel resting in its crevice and signal, with headshaking movements, close to the eels head:
In this clip, a grouper leads an eel off to hunt:
In this clip a grouper shows an eel (out of view) where the prey fish was last seen by performing a headstand accompanied by head shaking:
In this clip, an eel responds to the headstand with head shaking signal by exploring the area: