Fish don't feel pain
Fish have nociceptors, show motivational trade-offs after injury, and learn to avoid harmful stimuli. The scientific picture has shifted substantially.
Detecting damage is not the same as feeling it: fish respond to injury, but in mammals the conscious experience of pain depends on the neocortex, and fish have no neocortex or any clear equivalent. Their avoidance, stress hormones and changed behaviour after harm can all be explained as unconscious protective processing, the kind of thing a simple organism or even a machine could do without anything actually hurting. Without the brain structures we know produce suffering, attributing pain to fish is projection rather than evidence.
Fish possess nociceptors, produce physiological stress responses to injury, and alter their behaviour afterwards in ways that go beyond reflex: they will trade off feeding and predator avoidance against pain relief, and show reduced responses when given analgesia. These are the behavioural markers used to infer pain in other animals.
A minority of researchers maintain that, lacking a neocortex, fish cannot experience pain as mammals do. That remains a live position, but it is no longer where the weight of opinion sits, and it faces the difficulty that different neural architectures can support similar functions. UK law recognises vertebrates, including fish, as sentient, and was extended in 2022 to decapod crustaceans and cephalopod molluscs.
Treating the absence of proof against a claim as proof of it, or the absence of proof for a claim as proof against it.
How it applies here: The inference runs from 'we cannot directly verify their experience' to 'there is none'. The same inferential barrier exists for other humans, and is not treated as decisive there.
Presenting only the evidence that supports a conclusion while leaving out evidence that tells against it.
How it applies here: Rests on the minority position in the literature and presents it as the settled view, while the body of evidence has moved in the other direction.