Rainbow trout were anaesthetised. Small amounts of salt solution or vinegar was then injected into their snouts.
A brightly coloured Lego brick tower was introduced into the tank. Fish injected with the innocuous salt solution were wary of the tower and avoided it. However, the fish injected with vinegar moved close to the tower. The researchers said that the pain of the vinegar impaired the fishes' attention, and so they did not show the normal avoidance response.
The whole experiment was repeated again, but this time, the fish given the vinegar also received morphine, a pain killer. These fish then showed the normal wary response. Thus, the fish had shown the earlier unusual behaviour because they were able to feel pain.
"It has also been demonstrated that the trout has a prolonged negative behavioural and physiologic response to a noxious event, and this was also seen in the present study. Other fish species can learn to avoid noxious stimuli such as electric shock. The administration of an analgesic reduces these responses to almost normal. Together with the results of this study, the criteria for animal pain have been fulfilled for the trout."