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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, 2013 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. Mar 15, 2013 at 1:58
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What sounds like a straightforward question is in fact a highly controversial one. There are dozens of research papers on the topic, in addition to a large number of publications more directed towards lay readers. I won't even try to pretend that I've read (much less understood) all the research that is available. But what I think I can say with some confidence is this:

As of 2022, it's unresolved whether fish feel pain or not. While the 2013 newspaper article in the accepted answer cites a single research paper to claim that any reaction to potentially painful stimuli by fish must be unconscious, that answer does not do justice to the complexity of the debate.

The (highly recommendable) article "The Great Fish Pain Debate" (Vettese et al. 2020) summarizes the surprisingly long history of the controversy (apparently, the assumption there are animals such as fish that may be unable to perceive pain was used in South Africa's apartheid ideology to claim that Blacks may be less perceptive than Whites), and it outlines some of the main actors in the debate (including German recreational anglers, the fishing industry, and the scientific community). The article also suggests that due to this history and the configuration of actors, and the resulting politicization of the debate, research may have lost its subjectivity, and has focused on tangential questions instead (emphasis mine):

The number of fish caught by global fisheries each year likely would be counted in trillions. Fish farming kills at least another 80 billion fish each year […]. In contrast to these staggering numbers, recreational angling seems likely to be a relatively minor contributor to whatever suffering fish undergo when they are caught, with estimates suggesting that the annual take from angling numbers in the hundreds of millions.

Thus, a distinctive dimension of our story is how […] science became captured by a quite eccentric set of questions pertaining to one limited dimension of animal welfare. Yet if the perception of pain caused by a single fish hook is a problem so tangled in a web of science and subjectivity as to be unresolvable, what does that say about our ethical obligation to animals?

The review article by Mason & Lavery (2022) comes to a very similar conclusion: the "fish pain" debate is unresolved, with some research suggests that fish do feel pain, while other research claims the opposite (emphasis mine; references omitted).

Some claim that fish have rich experiential lives, able to feel pain, fear, and possibly joy […]. Others claim that fish are essentially unconscious zombies: that being a bass is like nothing, because fish have no phenomenal experience, not even sight […]. The “fish pain” debate is thus rather polarized. […]

As we have reviewed, [phenomenal] consciousness is currently impossible to measure, and impossible to assess in non-humans. Indeed even in humans, assessment is imperfect, relying on the veracity and accuracy of self-report. This makes the claims at both these extremes too strong – and the high levels of agnosticism about fish abilities to feel pain, appropriate.

To summarize, both articles that I've quoted agree that despite a long history of research, we don't know for sure whether fish can feel pain.

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