Do fish feel pain? The scientists on both sides

Many researchers think so, but scientists genuinely disagree, and the disagreement is about what pain is. Fish meet several criteria proposed as indicating pain, including a suitable nervous system and receptors, opioid receptors, reduced responses to noxious stimuli under painkillers, protective reactions and avoidance learning. Others argue that reacting to harm is not the same as feeling it.

The question is old. Descartes argued that animals do not experience pain and suffering because they lack consciousness. In 1789 Jeremy Bentham wrote that the question is not whether animals can reason or talk, but whether they can suffer. As late as the 1980s, Bernard Rollin writes, researchers remained unsure whether animals feel pain, and US veterinarians trained before 1989 were taught to simply ignore animal pain.

For fish, the lab evidence builds. The nerve endings that detect harm, called nociceptors, were first identified in fish in 2002. Rainbow trout injected with acetic acid in the lips rocked from side to side, rubbed their lips on the tank and breathed faster, and morphine reduced both the odd behaviours and the faster breathing. Sneddon and colleagues found that such changes last up to 6 hours, so they are not simple reflexes.

But critics push back. James Rose argued that fish lack a neocortex and have few C-type nerve fibres, and that pain depends on awareness; he and, more recently, Brian Key argue that the fish brain is so different from ours that fish are probably not conscious as we are. Sneddon replies that a different evolutionary path could produce different neural systems doing the same job, and other researchers believe consciousness does not require a neocortex. The American Veterinary Medical Association accepts that fish feel pain, while the question of what a fish experiences, if anything, stays open.