Purdue University Assistant Professor Joseph Garner and Janicke Nordgreen, a doctoral student in the Norwegian School of Veterinary Science, attached small foil heaters to goldfish and slowly increased the temperature.
Half of the fish were injected with morphine, and the others received saline. The researchers believed those with the morphine would be able to withstand higher temperatures before reacting if they actually felt pain. However, both groups of fish showed a response at about the same temperature.
But later observed in their home tanks, the researchers noticed fish from each group were exhibiting different behaviors.
"The fish given the morphine acted like they always had: swimming and being fish," Garner said. "The fish that had gotten saline -- even though they responded the same in the test -- later acted different, though. They acted with defensive behaviors, indicating wariness, or fear and anxiety."
Nordgreen said those behavioral differences showed fish can feel both reflexive and cognitive pain.
The scientists said their findings could raise questions about slaughter methods and standards of care could be revisited to ensure fish are being treated humanely.
A paper detailing the finding appears in the early online issue of the journal Applied Animal Behavior Science.