Thermonociception in fish: Effects of two different doses of morphine on thermal threshold and post-test behaviour in goldfish (Carassius auratus)

The question of whether fish can perceive pain is controversial, and pain is a potentially grave threat to fish welfare. To be able to study pain in a species, knowledge of its nociceptive system is necessary. There is therefore a need for standardised, repeatable and quantifiable measures of nociception and pain in fish. Sensitivity to noxious heat is readily quantifiable. We developed an apparatus to expose goldfish to controlled, localised heat stimulation, and tested the hypothesis that goldfish perceive heat as aversive. We predicted that they would respond to increasing heat with an escape response, that morphine would decrease their heat sensitivity and that the heat stimulation would affect post-test behaviour. A safety cut-off temperature of 50 °C was built into the test apparatus. All 16 fish responded to the heat with an escape response, with a mean baseline of 38 °C. However, morphine at 40 and 50 mg kg−1 could not be demonstrated to have a biologically relevant analgesic effect, but did significantly decrease the impact of heat stimulation on behaviour in the home tank. To our knowledge, this study is the first to systematically investigate thermonociception in unanaesthetised fish.