Summary: The following "fissue" regards scientific evidence that confirms what animal welfarists have been saying for ages: fish can and do feel pain.

The Great Debate
For years now, the debate over the ability of fish to feel pain has raged between animal welfarists and the pro-fishing crowd. With both sides spewing propaganda, and neither backing their opinion with evidence, it was difficult to determine which group - both of which had good cause to blow smoke - was telling the truth about fish and pain. Certainly, commonalities between fish anatomy and that of other vertebrates has always suggested that fish are quite capable of physical pain, but without conclusive scientific evidence to back it, neither side had a leg to stand on. Thankfully, a 2003 study by UK scientists has laid the great debate to rest: fish are indeed just as capable of feeling pain as you and I.
Proving Pain
Having a nervous system, it seems, is not enough evidence for the pro-fishing crowd. Nor is the presence of pain receptors, including nociceptors and polymodal nociceptors, which are found in "higher" animals like mammals, birds, and reptiles. Indeed, even neural responses to mechanical, thermal, and chemical stimuli in anesthetized fish could not convince hard line "anti-pain" believers that fish can and do feel physical pain. They argued that neural responses to stimuli could be purely reflexive, which shows no evidence of actual pain sensation. So the scientists were forced to bring out the big guns - behavioral confirmation.
In a lab setting, two groups of trout had their lips injected with irritants - bee venom and acetic acid - and a third was administered a control injection of saline. In all instances, scientists observed marked differences in behavior, including "rocking" - a sign of pain in mammals, rubbing the lips on gravel, and reluctance to feed. Not representative of mere reflexes, these profound changes in behavior confirm that pain must actually be felt by fish. In the end, it was confirmed that the research "fulfilled the criteria for animal pain."
Evolutionary Significance
The idea that an animal would not be able to feel pain is contradictory to logic in many ways. Though pain is stressful, it is also vitally important to the survival of an animal. Pain is a tool by which we learn to avoid harmful stimulus. Pain warns us to keep weight off of wounded limbs. Pain lets us know when our body is being attacked. Without the sensation of pain, an animal could be eaten alive without so much as trying to escape; clearly, a species without the ability to feel pain would be a short-lived one indeed.
What's more, suggesting an animal does not feel pain suggests to some extent that the animal feels nothing. Pain is the result of an agitated nerve reaching its threshold and firing. However, before pain, there are a range of other sensations, from gentle, pleasurable feelings, to discomfort and pressure. If fish did not have a developed enough nervous system for such sensations, how could their sensitive mouths pick out the difference between an edible snail and a pebble? How would they itch off parasites on rocks without rubbing their sides raw? How would they engage in mating embrances without harming or killing their partner? Clearly, fish could not function as animals if they could not feel a great range of sensations - including pain.
Common Sense
Simple observation by laypeople could have confirmed that fish feel pain simply based on the actions - both reflexive and behavioral - when exposed to painful stimuli. A fish hooked through the mouth, placed in chlorinated water, wounded in a fight, or suffering from a parasitic skin infection show all of the physical symptoms of pain, and perhaps even some psychological ones. Even reflexive actions could, arguably, prove pain; humans act in reflex to stimuli that are extremely painful and damaging, such as burns. Would we deny that a burn victim feels any pain, simply because his or her body reacted by moving away without thinking and faster than the brain could initially feel?
Furthermore, because fish are vertebrates, with nervous systems so similar to ours that they are increasingly popular subjects in biomedical research, it seems outright foolhardy to suggest that an animal so physiologically similar would be insensitive to pain, simply because it lives in the water and can not cry out when wounded. No one doubts the pain of mammals, birds, and reptiles, because we are able to see the obvious anatomical similarities. While fish are more alien to us, they have many of the same organ systems - including a complex central nervous system.
How have we denied the likelihood of fish feeling pain for so long when pure common sense should have led us to truthful conclusions long ago? It seems likely that, much like the days when dogs were nailed to boards and likened to machines, mankind is once again trying to write off the obvious signs of pain to justify actions that are, on his part, abhorrent.
Ending the Justification of Cruelty
Perhaps sadder than the callous attitudes towards fish revealed by the need for such a study is the fact that a lack of public awareness allows the industrial and sport fishing industries to continue justifying cruelty by denying that fish feel pain. Low opinions of fish are culturally ingrained, and the tradition of sport fishing in many countries helps maintain casual violence towards fish. Even with conclusive scientific evidence proving that fish feel pain just as acutely as any other creature, fish are still hooked through the face, gutted alive, suffocated to death, finned, and fought for sport all over the world. Such appalling cruelty would be illegal if inflicted upon any other animal, which only furthers the case for fighting for equal protection of all vertebrates under the law. However, in a world where it is perfectly acceptable to ruthlessly murder fish for sport, such protections are far and away. Until our culture as a whole is more receptive to fish welfare issues, animal welfarists have a duty and responsibility to spread the word about important scientific findings like this.