Trout, in this experiment, had painful substances injected into their lips. This caused them to rock from side to side, rub their lips in gravel and against the sides of their tank. Their breathing rate almost doubled. When morphine was given, these responses were reduced.
The researchers concluded that the behaviours were not merely reflexes, but signs that pain was being suffered.
"The present study examined the acute effects of administering a noxious chemical to the lips of rainbow trout to assess what changes occurred in behaviour and physiology."
"The noxiously treated individuals performed anomalous behaviours where they rocked on either pectoral fin from side to side and they also rubbed their lips into the gravel and against the sides of the tank. Opercular beat rate (gill or ventilation rate) increased almost double fold after the noxious treatment whereas the controls only showed a 30% increase."
"Administering morphine significantly reduced the pain-related behaviours and opercular beat rate and thus morphine appears to act as an analgesic in the rainbow trout. It is concluded that these pain-related behaviours are not simple reflexes and therefore there is the potential for pain perception in fish."