Fish possess the capacity to feel painful events. This is a basic survival necessity. They have nerves, as we do, to detect cause pain - heat, chemicals, and pressure.
Hooking, time out of the water, and handling, cause stress which can last a day. There is increased heart rate, unnatural behaviour, permanent tissue damage, and the fish is more likely to die.
After fish have been violently taken from water and then returned, they may die. Sometimes this may be after several hours. Studies have found that a third, half, or more, fish can die at a site.
Fish are not the mindless automatons that they are often portrayed as. They can have complex social lives, take great care of their young, and sometime pair for life.
Professor Braithwaite’s, book, “Do Fish Feel Pain?”, concludes that fish feel pain, and can suffer. She said that the evidence for this is as good as any that we have for birds and mammals.