What they found was that the fish had 58 such receptors around the mouth and actually reacted at lower levels of pain stimulation then humans, perhaps because their skin is more easily damaged. After the fish were injected with the venom they were observed to show a rocking motion akin to that displayed by other mammals when experiencing stress; they also rubbed their lips on the bottom of the tank and against the walls, and took over twice as long to resume feeding then a control group. The study, which was led by Dr Lynne Sneddon of Liverpool University concluded:

U.S. Fish and Wildlife Service Brook Trout

'Administration of noxious substances to the lips of the trout affected both the physiology and the behaviour of the animal and resulted in a significant increase in opercular beat rate and the time taken to resume feeding, as well as anomalous behaviours. The results of the present study demonstrate nociception and suggest that noxious stimulation in the rainbow trout has adverse behavioural and physiological effects. This fulfils the criteria for animal pain.'

In short, they felt pain and reacted to it.

One of the most interesting results of the study is the increase in the length of time it takes for the fish to resume feeding after experiencing pain. This is akin to you falling off your bike as a child and being somewhat reluctant to get back on. Literally the pain is not then just a physical sensation it is psychological. Our poor trout were stressed out. And whilst they are unlikely to win any 'Brain of the Month' awards, it seems churlish to dismiss the feelings they are experiencing as irrelevant simply because they are not making all the mental distinctions we might. Pain is pain is pain when you are the being experiencing it.

And it's not just trout. Another study in the late 80's by Dutch researcher John Verheijen investigated the reaction of carp to being caught with a hook and then released. They found that carp that had been caught and then released abstained from feeding for a considerably longer period of time, and showed stress type behaviour like making rapid darting movements, spitting, diving and shaking their heads. The study concluded that the fish felt pain from being hooked, but that their behaviour was mostly a fear response to possibly being caught again. In effect then, in a similar way that a victim of violent crime may get over the physical injuries quite quickly, but be traumatised for far longer, the fish showed a similar trauma response to being caught.