Although many have a view on this there has actually been little research on the subject. People seem to form a view which is convenient for themselves, anglers will argue that fish do not feel pain as we do while other who are concerned with fish welfare such as fish keepers will argue that they are capable of sensing pain.
Pain is a valuable warning, its presence can prevent further damage being done to an injury or as a message to immediately stop an activity and withdraw. So it would seem odd that fish wouldn't have such a warning system simply from a common sense point of view.
In mammals and birds there are bundles of fibres inside the nerves. Two types of fibres transmit nociceptive information. The first type is called the A-delta fibres and is associated with the first sensation of pain (in simple terms - the initial 'ouch' part of pain). The second fibres within the nerve are called the C fibres and these behave differently because they are much thinner and better insulated meaning they transmit the information more slowly. The C fibres are responsible for transmitting the dull, throbbing and ache type of sensations.
Both of these fibres are present in the nerves of fishes, but there are far fewer C fibres in fish than in other vertebrates. Usually 50 - 60% of the fibres are C fibres but in fish they make up just 4%.
When tested with pressure, heat, weak acid ect these nerves did respond by transmitting information to the fishes brain. Interestingly not all the nerves behaved in the same way, some only responded to one particular stimulus while other reacted to all stimuli. Of course simply demonstrating that pain information is transmitted from the skin to the brain doesn't in itself prove the fish feels pain because it has still to be proved that the brain is able to process that information.
To test how the fish reacted to pain a group of fish were observed behaving normally, their breathing rate, activity level, feeding patterns and so on. The the fish were caught anaesthetised and placed in to one of four possible groups.
The fish were then replaced in an aquarium on their own so that they didn't react with other fish around them and then they were observed through small slits in a screen - again so that they didn't react to being observed.
Initially all the fish showed signs of stress with increased breathing rates which were 40% higher in groups three and four but were 80% higher in the groups one and two and all of the fish had lost interest in food.
The fish given the noxious injections were also observed rubbing their snouts on objects in the aquarium including the aquarium sides. Further observation were taken every fifteen minutes.
Little changed over the first hour but then groups three and four began to return to a normal breathing pattern and they began to show some interest in food while the fish in groups one and two remained obviously agitated with increased breathing, flashing and still showed no interest in food. Only after three and a half hours had passed did they show signs of returning to normality. This experiment does show that fish do respond to bee venom in such a way that indicates pain but it doesn't really prove it.
An experiment using trout was devised. This involved adding a foreign object to the their aquarium and observing their reaction to it. Trout are quite wary creatures by nature and tend to avoid anything perceived as a possible threat. Sure enough all the trout gave the object a wide berth and deliberately avoided getting close to it. Having demonstrated this behaviour the original experiment using bee venom and unharmed fish was repeated to see if the different groups of fish behaved differently to a foreign object in their aquarium.
The group which had simply been handled still showed their normal behaviour and avoided the object but the group treated with the bee venom were less cautious and tended not to deliberately avoid the object. Which again demonstrated that the bee venom was a distraction to them.
This experiment was again repeated but this time the fish given the bee venom were also given an analgesic to counter its effects and when a foreign object was placed in their aquarium they showed their natural avoidance behaviour. Unknown to the scientist who carried out these experiments a team of Russian scientist had been working on the same thing and had reached the same conclusions and had further demonstrated that not only trout behaved in this way but carp and cod behaved in the same way and that the nociceptors were widely spread over the fishes entire body.
Other studies using pin pricks or mild electric shocks have shown that the information goes up the spine to different parts of the brain. Under anesthetic part of a salmons skull was removed and electrodes placed in different regions of the brain in order to see if there was any activity as a result of these different stimuli. The area of the brain called the telencephalon or fore brain which is the same region of the brain where this information is processed in mammals was found to be the main area of activity and that the activity was different for different types of stimuli. Even different levels of electric shocks were shown to produce different levels of activity which demonstrates that the fish didn't only register the stimuli but could tell the difference between a very mild electric shock and a more severe one.
These experiments do indicate that fish do indeed feel pain but they don't really answer the question if the fish actually is caused to suffer from pain.
In mammals and other vertebrates it is the neocortex part of the brain where feelings and emotions are processed. Fish do not have a neocortex and it is argued that with out a neocortex there is no emotion or feelings and as such a fish is incapable of suffering from pain.
Further studies have shown that even without a neocortex experiments have shown that fish are indeed sentient.
Edmund Rolls a professor of psychology at Oxford university describes emotions as states that occur because an animal experiences something positive and rewarding or negative and punishing. Experiments have shown that fish are able to learn to keep away from something that causes them to receive a mild electric shock and that they can also learn simple tasks to gain a reward.
Researchers from Seville have demonstrated that fish use different parts of their brain to process information and that their brains are far more sophisticated than had previously been thought.
An abstract (below) from Fish Cognition and Behavior, Second Edition. Edited by Culum Brown, Kevin Laland and Jens Krause. C 2011 Blackwell Publishing Ltd. Published 2011 by Blackwell Publishing Ltd.
Amongst the vertebrates, fishes have suffered the most from the common misconception of the evolutionary ladder. However, over the last few decades this fallacy has begun to be redressed. Researchers now realised that, like the rest of the vertebrate kingdom, fishes exhibit a rich array of sophisticated behaviour and that learning plays a pivotal role in behavioral development of fishes.
Gone, or at least redundant, are the days where fishes were looked down upon as pea-brained machines whose only behavioral flexibility was severely curtailed by their infamous 3-second memory (`a la Dory in Disney’s Finding Nemo). As
2003; Odling-Smee & Braithwaite 2003).
Fishes have evolved complex cultural traditions and pursue Machiavallian strategies of manipulation, deception and reconciliation (Bshary et al. 2002; Brown & Laland 2003). They not only recognise one another, but can monitor the social prestige of and dominance relations amongst others (McGregor 1993; Griffiths 2003; Grosenick et al. 2007) and cooperate in a variety of ways during foraging, navigation, reproduction and predator avoidance (Huntingford et al. 1994; Johnstone & Bshary 2004;
Fitzpatrick et al. 2006).
It is clear that the recent developments in our understanding of fish behaviour require a substantial reappraisal of their behavioral flexibility that warrants further investigation.
Although we will probably never really know exactly how fish perceive pain. It does seem extremely likely that they do feel pain and that it does cause suffering to them.
Nociceptive - Of, relating to, or denoting pain arising from the stimulation of nerve cells.
Braithwaite, Victoria (2010-03-25). Do Fish Feel Pain? (p. 53). Oxford University Press. Kindle Edition.
Issues in Behavioral Psychology: 2011 Edition - Goldfish brain research.
European Journal of Neuroscience, Vol. 21, pp. 2800–2806, 2005 - Emotional and spatial learning in goldfish is dependent on different telencephalic pallial systems
Fish Cognition and Behavior, Second Edition. Edited by Culum Brown, Kevin Laland and Jens Krause. C 2011 Blackwell Publishing Ltd. Published 2011 by Blackwell Publishing Ltd.