I have taken a lot of this out of my replies from a thread almost a year old now, but it is a summary of some of the scientific literature out there that supports the idea that fish can indeed feel pain:
Oidtmann B, and Hoffman RW. "Pain and suffering in fish" BERLINER UND MUNCHENER TIERARZTLICHE WOCHENSCHRIFT, vol 114, issue 7-8, jul-aug 2001. pages 277-282.
Here is the abstract of that article "The question on the capability of fish to feel pain and of suffering are still subject of discussion nowadays. In the article presented, the information available in the literature to date is summarised. Based on this knowledge, the conclusion is drawn that fish are capable of feeling pain and that they are able to suffer in the sense of the word as used in the German animal welfare law."
Yue, Moccia and Duncan, Applied Animal Behavior Science 2004, talks about training trout, and their response to fear.
From Yue et al. 2004:
Although the term ‘fear’ is used in everyday vernacular to describe the negative affect that
most animals are assumed to feel during, or in anticipation of, some frightening stimulus,
this term is more cautiously used today when referring to fish. This is partly due to the disbelief,
by some, that fish have the capability to experience conscious feelings. Rose (2002)
believes that conscious experiences like fear and pain are neurological impossibilities, due
to the lack of a neocortex in fish—the presumed place where consciousness dwells in higher
vertebrates. He therefore proposed that behavioural responses to noxious stimuli are separate
from psychological experiences (of fear for example)—behavioural responses to frightening
or aversive stimuli are merely reflexive responses and are not accompanied by a negative
feeling. Nonetheless, the term ‘fear’ has been widely used to describe fish behaviour for
some time (Pinckney, 1967; Gallon, 1972; Huntingford, 1990; Ledoux, 1990; Noakes and
Baylis, 1990). Others have put forth the idea that fish derive conscious experiences through
some mechanism other than the neocortically based consciousness of humans and other
highly evolved mammals (Verheijen and Flight, 1997). Recent anatomical, physiological,
neuropharmacological and behavioural data suggest that fish are likely to feel subjective
experiences, like fear, in much the same manner as tetrapods. A full review of
this evidence is beyond the scope of this paper, but briefly, the major argument lies in
the fact that the neuroanatomical structure and function between fish and higher vertebrates
are more similar than previously thought (Rakic and Kornack, 2001; Chandroo et al.,
2004).
Here is the reference to the Chandroo et al. article:
Title: Can fish suffer?: perspectives on sentience, pain, fear and stress
Author(s): Chandroo KP, Duncan IJH, Moccia RD
Source: APPLIED ANIMAL BEHAVIOUR SCIENCE 86 (3-4): 225-250 JUN 2004
There are detractors, A Prof. Rose is one of the most prolific. (quoted in the article above and several of the previous threads has works by Rose)
One of the more interesting points is that Rose wrote his articles for the fish farming industry (and is often cited by sport fishermen websites, too) and the conclousions he draws supports their pratices. Is there a chance that the funding is driving the results of the research? Or, is it a question that the research agreed, and then got the funding? The former is far too often the case on issues like gun control or the affects of global warming.
Either way, it is fair to say that this is a very open debate, and even the most recent articles (2004) say that there is much more experimentation that will need to be done to more definitively answer this question.