By Alex Kirby
BBC News Online environment correspondent
The first conclusive evidence of pain perception in fish is said to have been found by UK scientists.
Fish have pain receptors like us
This complements earlier findings that both birds and mammals can feel pain, and challenges assertions that fish are impervious to it.
The scientists found sites in the heads of rainbow trout that responded to damaging stimuli.
They also found the fish showed marked reactions when exposed to harmful substances.
The argument over whether fish feel pain has long been a subject of dispute between anglers and animal rights activists.
The research, by a team from the Roslin Institute and the University of Edinburgh, is published in Proceedings B of the Royal Society, the UK's national academy of science.
The researchers, led by Dr Lynne Sneddon, say the "profound behavioural and physiological changes" shown by the trout after exposure to noxious substances are comparable to those seen in higher mammals.
They investigated the fish for the presence of nociceptors, sites that respond to tissue-damaging stimuli.
The researchers applied mechanical, thermal and chemical stimuli to the heads of anaesthetised fish and recorded their neural activity.
Dr Sneddon said: "We found 58 receptors located on the face and head of the trout that responded to at least one of the stimuli.
"Of these, 22 could be classified as nociceptors in that they responded to mechanical pressure and were stimulated when heated above 40 Celsius.
"Eighteen receptors also responded to chemical stimulation and can be defined as polymodal nociceptors."
These polymodal receptors are the first to be found in fish, and resemble those in amphibians, birds and mammals, including humans.
But mechanical thresholds were lower than those found in human skin, for example, perhaps because fish skin is relatively easily damaged.
The mere presence of nociception in an animal is not enough to prove that it feels pain, because its reaction may be a reflex.
Proof requires demonstrating that the animal's behaviour is adversely affected by a potentially painful experience, and that these behavioural changes are not simple reflex responses.
Hurt trout behaved differently
So the researchers injected bee venom or acetic acid into the lips of some of the trout, with control groups receiving saline solution injections or simply being handled.
All the fish had been conditioned to feed at a ring in their tank, where they were collected for handling or injection.
Dr Sneddon said: "Anomalous behaviours were exhibited by trout subjected to bee venom and acetic acid.
"Fish demonstrated a 'rocking' motion, strikingly similar to the kind of motion seen in stressed higher vertebrates like mammals.
"The trout injected with the acid were also observed to rub their lips onto the gravel in their tank and on the tank walls. These do not appear to be reflex responses."
The fish injected with venom and acid also took almost three times longer to resume feeding than the control groups.
Dr Sneddon said the team's work "fulfils the criteria for animal pain". Previous work on fish had looked at the elasmobranchs, fish including sharks, skates and rays with cartilaginous skeletons, and at primitive vertebrates like the lamprey.
Dr Sneddon said: "These studies did not conclusively show the presence of nociceptors.
It's shocking that people will still go fishing for fun
"We believe our study is the first work with fish of the teleost family [those with bony skeletons], and the results may represent an evolutionary divergence between the teleost and elasmobranch lineages."
The Fish Veterinary Society described the research as "an interesting contribution to the debate".
Dawn Carr, director of People for the Ethical Treatment of Animals (Europe), said:
"It's shocking that people will still go fishing for fun.
"For every cruel thing people do, there is a compassionate alternative.
"There are so many ways to enjoy the outdoors - we hope people would go hiking, camping, boating; any sort of sport that doesn't involve animal suffering would be preferable," she said.
The organisation Compassion in World Farming called upon the UK Government to respond to the findings with legislation to improve the living conditions of fish living on fish farms.
The UK's National Angling Alliance described the study's finding's as "surprising".
Dr Bruno Broughton, a fish biologist and NAA adviser, said: "I doubt that it will come as much of a shock to anglers to learn that fish have an elaborate system of sensory cells around their mouths...
"However, it is an entirely different matter to draw conclusions about the ability of fish to feel pain, a psychological experience for which they literally do not have the brains," he said.
He quoted from a study by Professor James Rose of the University of Wyoming, US, in which it was found fish did not possess the necessary and specific regions of the brain, the neocortex.