Scientists from Norway and the USA have offered further evidence that fish are able to feel pain.
In a study published in the most recent issue of the journal Applied Animal Behaviour Science, Janicke Nordgreen and coauthors developed an apparatus to expose goldfish (Carassius auratus) to controlled, localised heat in order to test the hypothesis that goldfish perceive heat as aversive.
The authors also injected half of the experimental group with the painkiller morphine and the other half with saline as a further test of this hypothesis, predicting that the morphine-injected fish would be able to tolerate higher temperatures before showing an aversive response.
The apparatus consisted of small foil heaters placed in contact with fish skin, and the heat exposure was safely controlled (with a safety cut-off at 50C in the event of equipment failure) to prevent physical damage to tissues.
The authors observed the goldfishes upon treatment, counting the number of times the goldfish displayed an escape reaction, defined as C-starts (movement of the head and tail towards the same side of the body forming a ~~C ), swimming (movement of the body to form an S-shape) or tail-flicking (flicking the tail without sideways movements of the head or trunk region), on exposure to heat.
The scientists found that goldfish displayed an escape reaction to heat being applied on their bodies, implying that they found it to be aversive.
They also found that morphine had no effect on the fishes, contrary to their expectation that the morphine-treated fishes would be able to withstand higher temperatures before reacting to the pain of the heat. However, the authors did notice that the morphine did have an effect on the fishes a few hours after treatment.
According to co-author Joseph Garner, The fish given the morphine acted like they always had: swimming and being fish. The fish that had gotten saline - even though they responded the same in the test - later acted different, though. They acted with defensive behaviours, indicating wariness, or fear and anxiety.
The experiment shows that fish do not only respond to painful stimuli with reflexes, but change their behaviour also after the event, said Nordgreen
Together with what we know from experiments carried out by other groups, this indicates that the fish consciously perceive the test situation as painful and switch to behaviours indicative of having been through an aversive experience.
For more information, see the paper: Nordgreen, J, JP Garner, AM Janczak, B Ranheim, WM Muir and TE Horsberg (2009) Thermonociception in fish: Effects of two different doses of morphine on thermal threshold and post-test behaviour in goldfish (Carassius auratus). Applied Animal Behaviour Science 119, pp. 101"117.