It is difficult to determine the nature of the response to pain in fish or whether their experience is similar to that observed in mammals. Although there have been few species-specific studies, there is evidence that fish exhibit a pronounced initial response to injuries or to contact with nociceptive stimuli or chemical algesics but their response to chronic stimuli has not been characterized.

Generally, fish react to noxious stimuli (such as puncture with a hypodermic needle) with strong muscular movements, and when exposed to a noxious environment (such as an acidic solution) show abnormal swimming behavior, attempts to jump from the water, and more rapid opercular movements. Such effects indicate some, perhaps considerable, distress, but it is not possible to describe the distress unequivocally as pain-induced.