http://news.bbc.co.uk/2/hi/sci/tech/2983045.stm
Old news. Fish do feel pain and do elicit measurable pain responses. Countless studies confirm it, look for yourself.
Suffering, that's another story.
So you link to an article that quotes Dr Lynne Sneddon. Perhaps it interests you to learn that Sneddon also writes "scientific reports" for several European animal rights organizations. Sneddon and I had it once out with each other on a BBC live show where she insisted that animal training is cruel because according to her animal training by its very nature includes physical force and pain. When I, as a professional animal behaviourist, proved her point for point wrong with scientific facts she left the studio in a hissing fit. Of course if an animal is trained by a layperson then it is possible that pain is involved and that is wrong, but we were talking about professional circus animal trainers.
I could go on and explain in great detail about pain receptors and how many there are per square inch in a fish v. mammal or humans and how different brains register pain. Suffice to say that a fish does not perceive pain the way humans or mammals do. There are countless scientific research papers written by scientists that do not sit on a bandwagon that show fish been hocked several times within a couple of hours in a labor setting without registering any pain or prolonged discomfort. There is prove that a fish can suffer considerable negative stress when it is reeled in. Fish that have been reeled slowly "played" suffer almost no stress whereas fish that have been "horsed" in can suffer fatal stress.
As to the quote in the article where Sneddon mentions the acid test on fish. If a fish has its lips dowsed in acid it will be in discomfort since the acid affects a much larger body surface (more pain receptors registering) than a tiny puncture wound form a hook.