How sensitive are fish to pain?

Russian scientists recorded the responses of various fish to painful electrical shocks. The fish jerked their tails. They were given the fish pain killers and then repeated the shocks. Analgesics reduced pain sensitivity by up to 89%. The most sensitive areas to pain were: tail and pectoral fins, skin around the eye, and olfactory sacs. Pain sensitivity was comparable to humans.
pain in fish comparable to humans
In research carried out at Manchester University in England, the face of the trout was stimulated while responses in the trigeminal nerve in the brain were recorded. The scientists found that skin receptors of trout are more sensitive to mechanical stimulus than mammals and birds. They conjecture that fish are continuously exposed to water pressure, bacteria and fungus. Fish were also pain sensitive to lower thresholds of heat than mammals.
trout more sensitive to pressure and heat

Pain Sensitivity of Fishes and Analgesia Induced by Opioid and Nonopioid Agents (pdf), Lilia S. Chervova, Dmitii N. Lapshin, Proceedings of The Fourth International Iran & Russia Conference, Moscow
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."
Nociception in fish: stimulus–response properties of receptors on the head of trout Oncorhynchus mykiss, Ashleya, P, Sneddon, McCrohan, C, Brain Research, 2007, University of Manchester, University of Liverpool, UK
Trout skin sensitive to pressure
"Trout cutaneous receptors recorded here are more sensitive overall to mechanical stimuli than those of mammals and birds, with some responding to stimuli as low 0.001g."
Sensitivity to pain
"It is likely that fish, continuously exposed to external water pressure and to bacterial and fungal agents, require greater cutaneous sensitivity, including for nociception, than terrestrial animals."
Trout more sensitive to lower temperatures than mammals
"The heat thresholds recorded here from mechanothermal and polymodal receptors ranged from 20°C, though mean values were around 29 and 33°C, respectively. This range is below that considered noxious in mammalian nociception, but can be explained by the proposition that nociceptors in fish have evolved to match habitat temperatures."

Fish Pain