There’s a new answer to that question, thanks to research from Purdue University and the Norwegian School of Veterinary Science. And the answer may surprise you.
Researchers attached small heaters with a built-in safety cut-off to 16 goldfish.
Half of the fish were injected with saline, the other half received morphine, a powerful pain medication.
All of the fish tried to get away from the heat at about the same temperature; researchers had thought the anesthetized fish would be able to withstand higher temperatures before having such a reaction.
The real surprise came two hours later. The fish who had been anesthetized were behaving normally. The fish who had only received saline were being wary and anxious as exhibited by defensive behaviors.
This means that those fish not only experienced pain, but they processed the experience in their brains and became afraid that the pain would return again.
This makes it easier to understand how experiences which cause stress to a fish can lead to illness. If you have pet fish, you should do everything you can to minimize their discomfort and avoid causing them stress. Pay special attention when introducing a new fish into your aquarium or fish tank.