Do fish feel pain? Scientists in the UK studied rainbow trout. They found nerves on the fish's head that react to heat, pressure and acid. These nerves are like "pain sensors."

When scientists put a little acid into the fish's lips, the fish acted strangely for a long time. They also waited longer before eating. But some scientists say fish brains cannot feel pain like ours. The debate continues.

Whether fish feel pain has been debated by biologists for more than 20 years. In 2003, a British team found nerves called nociceptors in the heads of rainbow trout. These nerves reacted to strong pressure, heat above 40°C and acid, much like those of mammals and birds. When acid was injected into the trout's lips, the fish moved their gills faster, took longer to start eating, and showed unusual behavior for a long time.

Later studies reported that painkillers reduced such changes. However, some scientists argue that fish lack the brain structures humans need to feel pain. Fish clearly have the sensors. The dispute is whether they consciously feel something unpleasant.

For more than two decades, biologists have argued over whether fish feel pain, and both sides cite serious evidence. In 2003, Sneddon and colleagues recorded nerves in the heads of rainbow trout that responded to strong pressure, heat above 40°C and 1% acetic acid. These nociceptors resembled those of mammals and birds. When acid was injected into the lips, the fish showed faster gill movements, delayed feeding and unusual behaviors that persisted for a long time, which the team took as a sign of possible discomfort.

A 2015 review added that such changes can be reduced by painkillers and that responses to injury may take priority over fear responses. Critics, however, draw a sharp line between nociception, the detection of harmful stimuli, and pain, a subjective experience. In 2016, Key argued that feeling pain requires specific cortical structures that fish lack, while a 2012 review by Rose and colleagues had suggested that the behavioral changes could occur without consciousness. Because there is no way to measure subjective experience directly, the dispute is unlikely to end soon, raising the practical question of whether fish should be treated with care in the meantime.