Russian scientists tested the sensitivity of pain in cod, rainbow trout, carp, and sturgeon.
Painful bursts of electricity were applied to the tail fins. The fish responded by moving their tails.
It was found that the most sensitive areas to pain were the tail, pectoral fins¹, the skin surface around the eyes, and the outer skins of the olfactory sacs².
However, the researchers said that there was sensitivity to pain throughout the whole body, and that the pain threshold for fish was comparable to that in people. They postulated that sensitivity in fish was necessary to detect damage during nest-building and aggression with other fish.
TAIL FIN GIVEN ELECTRIC SHOCKS
"Optico-mechanical system was used to record the response to painful electrical stimulation before and after administration of analgetic agents."
"The fish was semirigidly fixed in a flow chamber (in the region of the mouth and pectoral fins). The gills were continuously moistened with water. The stimulating electrodes were inserted into the caudal fin blade in order to exclude the direct stimulation of muscle fibers."
MOVEMENT OF TAIL RECORDED
"The recording apparatus was a movable wire "fork" embracing the caudal peduncle in the posterior third of the body. In response to painful stimulation (bursts of short pulses 0.5 ms of current 0.5-2.0 mA, with frequency 300/s), the fish moved its caudal peduncle and deviated "the fork" from the zero point. Drugs were administrated by different ways - peritoneally, subcutaneously, intranasally."
MOST SENSITIVE AREAS
"It was found that the caudal, dorsal and pectoral fins, the skin surface around the eyes, and the epithelium of olfactory sacs were the most sensitive nociceptive³ zones."
RECEPTORS PRESENT THROUGHOUT THE BODY
"Studies performed on Cyprinus carpio (carp), Parasalmo mykiss (trout), Gadus morhua (cod), and Acipenser ruthenus (sturgeon) indicated that the fishes possess a developed system of pain sensitivity with receptors (nociceptors) presented on the whole body."
PAIN THRESHOLDS COMPARABLE TO HUMANS
"Nociceptive thresholds of fish under this condition was comparable with human’s one."
HIGH DENSITY OF PAIN RECEPTORS TO
COPE WITH LIFE UNDER WATER
"The high density of nociceptors on fins is likely to be related to the fact, in particular, that fins are damaged in fish during their nest-building activity or agressive interactions."
¹ Two fins located on each side of fish just behind the head.
² Smell organ
³ A nociceptor is a sensory receptor that reacts to potentially damaging stimuli by sending nerve signals to the spinal cord and brain. This process is called nociception.