Despite the common perception that fish can’t feel pain, there is growing evidence that fish and crustaceans are in fact sentient beings capable of experiencing pain, fear and distress. The information below is an overview of the evidence for fish feeling pain & suffering, and the impacts this has on welfare.

Morphology

It has been argued that because fish lack the neocortex responsible for pain perception in the mammalian brain, they are unable to feel pain. However, some experts respond to this by saying:

Studies of the sensory systems, brain structure and functionality provide evidence that different neurological structures provide at least some degree of sentience and pain reception in fish. For example, the forebrain of fish contains the dorsomedial and dorsolateral telencephalon, these brain structures are thought to perform the same function as the amygdala (emotions & fear responses) and hippocampus (spatial learning & memory) in mammals. Simply put, different structures have evolved to fulfil the same role. Therefore, although their perception of pain and suffering may not be identical to ours, fish do have brain structures capable of feeling fear and pain. 
There is also good evidence that some invertebrates such as crustaceans also have the capacity for fear and pain, even though they lack a vertebrate brain system. More information on Crustacean Pain can be found HERE.

Physiology

The following responses provide evidence of a fish’s ability to elicit a physiological pain response:

• Fish react to painful stimuli with primary, secondary and tertiary stress responses. These responses involve increasing levels of cortisol and catecholamines, and an increase in heart rate, blood flow and metabolism. This is the same response observed in mammals.
• Fish have very similar nociceptors (pain receptors) and associated central nervous pathways to other vertebrates. These pathways send information to the same functional areas of the brain in fish as in other animals.
• Endogenous opioids are produced by the brain to reduce pain. They work by interacting with opioid receptors, and all 3 types of opioid receptors known in mammals have been identified in studied fish species. There is also an elevation in the level of precursor chemicals in fish in response to pain, just as there is in people. 
• Analgesics (pain killers) work on fish to reduce the behavioural and stress response to harmful stimuli in the same way that is seen in other vertebrates.
• Scientists have identified that different brain activity occurs in fish in response to a noxious (harmful) stimulus compared to a harmless stimulus, e.g., being touched with a heated prod vs. stroking with a paint brush. This shows activation of the brain structures involved in pain processing and indicates that the reaction to a harmful stimulus is not merely reflexive in fish.

Behaviour

Like mammals, fish have obvious behavioural changes in response to harmful and threatening stimuli. These include innate and learned avoidance behaviour and the suspension of normal behaviour.
The ability to learn is an important aspect of sentience and has been shown in a variety of fish species. In one study, fish learnt to avoid a black compartment when they received an electric shock if they entered it.^* This learned behaviour suggests that they are not just reacting reflexively to harmful stimuli, but rather are motivated to change their future behaviour. Other studies have shown pronounced behavioural changes associated with pain lasting for 3 - 6 hours after the noxious experience, where a merely reflexive reaction would only last a few seconds. These behavioural changes include hovering and inactivity which indicate an increase in fearfulness, reduced appetite, and rubbing of an affected area on the aquarium glass or rocks.

How Can We Reduce Pain & Improve Welfare?

In acknowledging that fish are sentient beings and have the capacity to experience pain, fear and suffering, we then must consider their welfare on a similar level to other animals. Many countries recognise this, and the regulations surrounding welfare in aquaculture and fishing are gaining complexity and significance.
AQUI-S^® is a convenient and effective way to reduce pain relating to husbandry practices. Unlike many common fish sedatives, AQUI-S^® is a true anaesthetic in that evidence suggests it has analgesic (pain reducing) properties as well as reducing activity and the physiological stress response. This 3-fold benefit means greatly improved welfare as AQUI-S^® not only reduces the animals short-term experience of pain and stress, but also the potential handling damages due to avoidance behaviour, struggling and attempting to escape. The long-term detrimental effects of the physiological stress response are also reduced, resulting in healthier and stronger fish. 

*Referenced studies: http://fishcount.org.uk
http://www.efsa.europa.eu/en/efsajournal/pub/954