Fish are little understood by humans. We have false perceptions about their lack of intelligence or the ability to feel pain. Recent studies on fish have shown that not only do fish feel pain, but they also have intelligence, the capacity for long-term memories, an ability to use tools and form sophisticated social structures.
Intelligence in fish
Studies undertaken by behavioural scientists have shown higher than expected intelligence levels in fish. Fish have been known to anticipate the arrival of food based on cues such as a change in water temperature or the sound of a whistle. Fish can also make use of visual landmarks (including differentiating between similar looking landmarks) to locate hidden foods. Fish have been trained to push a lever to obtain food. Studies have also shown an ability to differentiate between friendly and unfamiliar fish.
Fish communicate through low frequency sounds, inaudible to human ears. Aggression, courtship, warnings and social / reproductive status are communicated through grunts, croaks, hums, moans, thumbs, clicks etc.
Ability to feel pain
It is well-established that fish experience chemical and physiological stress responses in a manner similar to mammals. Fish produce the same stress hormones and release them within a similar physiological pathway. While stress in fish is well documented, confusion related to their ability to feel pain is related to the lack of a neocortex (the part of the brain used in humans to feel pain) in fish.
However, multiple studies since then have clearly demonstrated not only the ability of fish to feel pain, but also their avoidance and reactions to the same. Just because a particular function does not occur in the same place as man (pain felt in the neo-cortex), does not mean that it does not exist.
As quoted by John Webster (University of Bristol, emeritus professor of Animal Husbandry) -- “A powerful portfolio of physiological and behavioural evidence now exists to support the case that fish feel pain and that this feeling matters. In the face of such evidence, any argument to the contrary based on the claim that fish ‘do not have the right sort of brain’ can no longer be called scientific. It is just obstinate.”
A number of fish species exist within hierarchy based social orders, where dominant fish are typically larger than it’s subordinates. Fish demonstrate lower stress when they are in tanks with ‘familiar’ fish or with fish with an already established social structure. Stress levels are accordingly higher, when a fish is introduced to a tank with 'unfamiliar' fish where social structures have not yet been clearly defined. This indicates clear comprehension amongst fish, with relation to the members that belong to their group / school.
Fish have an ability to create cognitive maps of their watery holes, using cues such as polarized light, sounds, odors and visual landmarks to guide them on migratory routes and feeding grounds. Some fish, like birds, are known to migrate thousands of kilometers in search of feeding or breeding grounds.