Researchers from Purdue University and the Norwegian School of Veterinary Science say they have proven that fish feel pain in what they say is the first study to systematically investigate thermonociception in unanaesthetized fish.
The findings could raise questions about slaughter methods and how fish are handled in research, said Joseph Garner, D.Phil, an assistant professor of animal sciences at Purdue.
The study, published online April 18 in the journal Applied Animal Behavior Science, has prompted fresh calls for angling to be banned in some countries, according to U.K. publisher MailOnline.
To test the hypothesis that goldfish perceive heat as aversive, the researchers attached small foil heaters to 16 goldfish and slowly increased the temperature. A safety cut-off temperature of 50 degrees Celsius was built into the test apparatus.
Half of the fish were injected with morphine beforehand while the other half received saline. The researchers believed that those with the morphine would withstand higher temperatures before reacting if they felt pain. Instead, both groups responded to the heat with an escape response at about the same temperature, with a mean baseline of 38 degrees Celsius.
Two hours later, however, the researchers noticed that the fish from each group exhibited different behaviors. The fish that received morphine acted normally while the fish that had not received the painkiller “acted with defense behaviors, indicating wariness, or fear and anxiety.”
These behavioral differences indicate that fish can feel both reflexive and cognitive pain, said Janicke Nordgreen, a doctoral student in the Norwegian School of Veterinary Science.
Nordgreen noted that this study, along with other experiments carried out by other groups, “indicates that the fish consciously perceive the test situation as painful and switch to behaviors indicative of having been through an aversive experience.”
The fish in the saline group not only experienced pain in the test and responded to it, they could cognitively process that pain, which caused the later fear and anxiety, Dr. Garner said. That's much like how humans experience pain, he added.