The idea that fish can’t feel pain goes deep in history. The common belief was that the biology of a fish was far too simple to process pain.





But anatomically, fish are certainly not “too simple” to feel pain.





There are two key criteria animals must fill to be considered capable of experiencing pain, according to this review on assessing animal pain. First, they must respond to a potentially painful event differently than they would to a pain-free stimulus. Second, the animal should change their normal behaviors based on the painful event.





Numerous studies have shown fish to meet both of these criteria.





In one experiment, zebrafish were put in a tank with two different chambers; one was stimulating, with vegetation and objects to explore, and the other was barren. A group of zebrafish were injected with saline water, and another group with painful acetic acid. Both groups chose the stimulating chamber. This was to be expected: fish typically prefer chambers with stimulation.





However, when a painkiller was dissolved in the barren chamber, the fish injected with acid chose to swim in the less preferred yet pain-relieving barren chamber. The fish injected with saline stayed in the preferred chamber.





Jonathan Balcombe, author of What a Fish Knows , stressed the experiment’s significance in proving that fish can feel pain. He told SeafoodSource that the experiment shows fish are willing to pay a cost, or go into a chamber they normally avoid, to get relief from their pain.





"What a Fish Knows" by Jonathan Balcombe

Lynne Sneddon, a leading researcher on fish pain, explained that when humans are in pain, we become distracted and do other tasks less well. And fish experiencing pain are no different. “Fish consumed by pain do not respond to fear-causing situations and do not show normal anti-predator behaviour,” Sneddon told The Guardian .





Yet when Sneddon’s team administered drugs such as aspirin, lidocaine, and morphine, the drugs made the pain symptoms disappear. “If fish did not experience pain, then analgesic drugs would have no effect." Another study showed that fish learn to avoid electric shocks usually in one or a few trials, showing their capacity to shift their typical behaviors in response to experienced pain.





When considering all of the empirical evidence together, these studies show fish fulfill the criteria for animal pain: they respond to potentially painful events differently than they would an unpainful one, and they change their normal behaviors based on the painful event.





If fish can suffer – why don’t they receive the same welfare protections as other animals?





Jeremy Bentham, who developed the theory of utilitarianism, wrote that the most important question to ask is not, “Can they reason? nor, Can they talk? but, Can they suffer?”





In a 2014 interview on BBC Newsnight, the late Penn State University biologist Victoria Braithwaite and Bertie Armstrong, head of the Scottish Fishermen’s Federation, discussed fish pain and welfare.





Armstrong argued that “the balance of scientific evidence is that fish do not feel pain as we do.”





Braithwaite said that’s beside the point. We cannot determine whether dogs, cats, chickens, cattle or lab animals feel the same pain as humans, yet these animals have been given increasingly humane treatment and legal protections because of their proven ability to suffer.





As more research shows that fish can feel pain , Braithwaite believed that better standards for fish welfare will be increasingly important, too.





“More and more people are willing to accept the facts,” Braithwaite said. “Fish do feel pain. It’s likely different from what humans feel, but it is still a kind of pain.”





The evidence that fish are capable of feeling pain is enough reason to treat them humanely. As the public around the world is increasingly concerned about the well-being of other farm animals like pigs and chickens, fish should not be forgotten.