Fear, like pain, serves a function that is fundamental to survival in protecting animals against dangerous environmental threats.
Behavioural responses to potentially threatening stimuli have been described for fish. These include escape responses, such as fast starts or erratic movement, freezing and sinking in the water. In a number of studies these behaviours were shown in response to conditioning, i.e. learnt. Learned avoidance behaviour, as discussed on the page fish feel pain, gives further evidence that the displayed behaviour is not merely a reflex response.
A study of rainbow trout showed that they can learn to avoid threatening stimuli, indicating that they experience fear. Rainbow trout were placed individually into a tank comprising 2 chambers connected by a doorway. When subjected to the frightening stimulus of a plunging dip net in the chamber containing the fish, the fish escaped through the doorway to the other chamber. Each fish was then presented with a neutral stimulus of a light that went on 10 s before the net plunged into the water. Over a 5-day period, all fish learned to avoid the plunging net by swimming through the doorway when the light was illuminated. All fish showed evidence of longer-term memory by performing this response on the first occasion they were tested after 7 days of no testing.
Learning is thought to involve receptors in the brain that are activated by a substance called NMDA. Chemicals that stop the effect of these NMDA receptors, (antagonists of NMDA receptors) have been shown to impair learning and fear conditioning in mammals. Experiments have shown that administering an NMDA receptor antagonist into the brain of a goldfish likewise impairs the fishes’ fear conditioning.