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As a small kid, I was told that fish don't feel any pain when they bite into a fishing hook. I'm not sure if this was just something made up so I didn't feel sorry for the fishies.

It appears that the controversy is quite widespread.

Slate even had a piece on it:

There is a new study out that contends fish feel pain. A professor at Purdue and his Norwegian graduate student attached small foil heaters to goldfish. Half of the goldfish were injected with morphine, half with saline, and then the researchers turned on the attached micro-toasters. After the heat was gone, the fish without painkillers "acted with defensive behaviors, indicating wariness, or fear and anxiety." They had also developed a lovely brown crust. These results echo a 2003 study by researchers from the University of Edinburgh who shot bee venom into the lips of trout. The bee-stung fish rubbed their lips in the gravel of their tank and generally seemed pissed off.

[...]

The 2003 Edinburgh study confirmed that trout have polymodal nociceptors around their face and head—i.e., they have the ability to detect painful stimuli with their nervous system. But, according to some definitions of pain, the detection of painful stimuli is not enough. The animal must have the ability to understand it is in pain to really feel pain.

WFN(World Fishing Network) has an article that contradicts the findings on Slate:

Do fish feel pain when we hook them? Well, not according to Dr. James D. Rose.
[...] “Fish have the simplest types of brains of any vertebrates,” he says, “while humans, have the most complex brains of any species. Conscious awareness of sensations, emotions and pain in humans depends on our massively developed neocortex and other specialized brain regions in the cerebral hemispheres. If the cerebral hemispheres of a human are destroyed, a comatose, vegetative state results. Fish, in contrast, have very small cerebral hemispheres that lack neocortex. If the cerebral hemispheres of a fish are destroyed, the fish’s behavior is quite normal, because the simple behaviors of which a fish is capable (including all of its reactions to nociceptive stimuli) depend mainly on the brainstem and spinal cord.”

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A study has found that, even when caught on a hook and wriggling, the fish is impervious to pain because it does not have the necessary brain power.

The research, conducted by a team of seven scientists and published in the journal Fish and Fisheries*, concluded that the fish’s reaction to being hooked is in fact just an unconscious reaction, rather than a response to pain.  

Fish have already been found to have “nociceptors” - sensory receptors that in humans respond to potentially damaging stimuli by sending signals to the brain, allowing them to feel pain.  

However, the latest research concluded that the mere presence of the receptors did not mean the animals felt pain, but only triggered a unconscious reaction to the threat. The latest findings contradict previous research, which suggested that these nociceptors enabled the creatures to feel reflexive and cognitive pain.

*Newby, N.C. and Stevens, E.D. (2008) The effects of the acetic acid “pain” test on feeding, swimming and respiratory responses of rainbow trout (Oncorhynchus mykiss). Applied Animal Behavior Science 114, 260–269

Source : telegraph

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    It might be considered a bit more complicated than that. Wikipedia reports on the controversy. – Oddthinking Mar 14 '13 at 14:11
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    The tl;dr might be: fish, like many "lower" animals, have nociceptors. Some say nociceptors are the source of the experience of pain, while others argue that nociceptors just trigger reflexive avoidance. – Larry OBrien Mar 15 '13 at 1:58

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