Feeling pain is as vital to fish as it is to other animals - it teaches them to avoid things that cause harm. Pike, for example, remember to avoid anglers’ hooks, as do carp.
Fish have nerve receptors, similar to those found in amphibians, birds, and mammals, which detect painful events. In one experiment, twenty-three pain receptors were found on the face of rainbow trout.
Russian scientists discovered, in various types of fish, that the most sensitive areas to pain were the tail and pectoral fins, skin around the eye, and olfactory sacs. Their pain sensitivity is similar to ours.
When suffering harm, signals are sent to the spinal cord and brain.
The fish brain is constructed differently to humans, having different areas to process and feel pain.
Fish behave as if they are in pain. In one experiment, goldfish, after having a weak acid injected into their lips, experienced a doubling of respiration, stopped feeding for three hours, and reduced swimming. They rocked back and forth, balancing on either pectoral fin, while resting on the gravel, and they also rubbed their lips into the gravel and tank walls.
Pain killing drugs act on fish.
Zebrafish, when given a choice between an empty chamber in a tank, and a chamber with gravel and plants, preferred the second. They were then injected with a chemical known to cause pain. Then, the choice offered to the fish was the chamber with gravel and plants, or a chamber with painkiller in it. They chose the pain relief.