The International Association for the Study of Pain (IASP) defines pain as an “unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage”.¹

The word “pain” was initially used to describe a human emotional negative experience. It is acknowledged that the experience of pain has a physiological basis. In humans, pain is controlled by four nervous structures: hippocampus, amygdala, cerebral frontal lobes and neocortex.² Rose (2002)¹ introduced the idea of fish insentience based on the absence of neocortex - the neuroanatomical structure which is associated with conscious awareness in humans, thus arguing against the concept of fish feeling pain. Additionally, the simplicity and small size of the fish brain has led many to doubt that fish may have the entire cognitive set required to experience pain and stress in a human sense:³ nociceptors, nociceptive processing brain components, nerve bundles, and cognitive capacity to feel pain.⁴ As Wadiwel (2016) argues in his commentary on Key on Fish Pain,⁵ explaining consciousness is considered the most difficult problem of neuroscience, where demonstrations or proof are not available, nor possible. Even in humans, quantifying pain is considered a challenging task, as it is inherently a subjective experience.^4,6 Opposing to Rose’s inferences, it is argued that pain “is an evolutionary adaptation which helps individuals survive, providing a signal that gives animals the opportunity to remove themselves from damaging situations”. Consequently, pain, which has survival and adaptive value, increases the chances of passing on genetic makeup to future generations.

Pain is seen, in that case, as originating from the most phylogenetically ancient part of the brain, indicating that fish should also have the ability to feel pain. However, the part where most disagreement is shown seems to be when it comes to distinguishing between nociception the physical, unconscious response to noxious stimuli resulting in a behavior change, pain - a psychological (mental) state, and the ability to communicate the feeling verbally, through facial or behavior reactions.¹ It is inferred that not all fish have nociceptors and that fish cannot communicate pain, thus noxious stimuli must not “feel like anything to a fish”.⁶ Some scientists have questioned, however, whether it should be assumed that avoidance responses by fish when they are netted, hooked, live cut, etc., should be communication of pain, which goes back to the survival and adaptive value of pain. Development of the concept of animal welfare, including fish welfare, has resulted in further scientific investigations in fish pain. Professor Gregory Neville, RVC, Univ. of London¹ established the criteria for assessment of pain in fish, which refers to: i. existence of a neurotransmitter, nervous cells, and brain structures similar to those conveying pain in mammals, ii. exposure to painful stimuli and assessment of the response in fish followed by suppression with analgesic drugs and analgesic blockers, and iii. evidence that fish are able to anticipate and thus, avoid the painful stimuli to which they had been exposed to. To demonstrate the first assessment criterion, Sneddon et al.’s study¹ revealed that rainbow trout possess nociceptors capable of detecting physical and chemical stimuli, such as extreme water temperature and pH exposures. There were found nociceptors - located on the head, and nervous fibers similar to those identified in the pain system of other vertebrates. A number of studies had addressed already Gregory Neville’s second assessment criterion showing that fish would respond aversively to electric shock, fin pinching, CO2 saturated water, niddle pricking, and that their pain-like responses decreased at increased levels of analgesics and opioids.^1,7 Morphine increased pain tolerance in fish exposed to what would be considered as painful stimuli to other species. Delivery of naloxone, an opioid receptor blocker, reversed the analgetic effect of morphine.¹ It was concluded that the study results were consistent with the fact that opioid receptors and endogenous (endorphin-like) opioids are located in the spinal cord and brain of fish. As for the third criterion proposed by Neville, it is considered that many studies have fulfilled it, determining that fish are able to learn to avoid aversive stimuli, and thus they can anticipate their effect, which would explain the “keep away from” behavior in given circumstances.¹ Although it is currently accepted that, as in the case of mammals and birds, some fish species can also experience pain, there still remains so much fundamental doubt over whether all fish species - including wild-caught ornamental fish and ornamental fish used in experimental settings (e.g., Danio rerio), and other aquatic animals used for food and non-food purposes, such as crustaceans and mollusks, of which handling, breeding, and keeping may have implications for human practices, may indeed suffer.

References

1.  Yue S. An HSUS report: Fish and pain perception. The Humane Society of the United States. 2008. www.humanesociety.org/assets/pdfs/farm/hsus-fish-and-pain-perception.pdf. Link retrieved May 8, 2017. (VIN editor: Original link was modified on 1–03–2017).

2.  Apkarian AV, Bushnell MC, Treede RD, Zubieta JK. Human brain mechanisms of pain perception and regulation in health and disease. Eur J Pain. 2005;9:463–484.

3.  Huntingford FA, Adams CE, Braithwaite VA, Kadri S, Pottinger TG, Sandoe P, Turnbull JF. Current understanding on fish welfare: a broad overview. J Fish Biol. 2006;68:332–372.

4.  Rose JD. Anthropomorphism and ‘mental welfare’ of fishes. Dis Aquat Organ. 2007;75:139–154.

5.  Wadiwel DJ. Fish and pain: The politics of doubt. Animal Sentience. 2016.038.

6.  Brian K. Falsifying the null hypothesis that “fish do not feel pain”. Animal Sentience. 2016;039.

7.  Gräns A, Niklasson L, Sandblom E, Sundell K, Algers B, Berg C, Lundh T, Axelsson M, Sundh H, Kiessling A. Stunning fish with CO₂ or electricity: Contradictory results on behavioral and physiological stress responses. Cambridge University Press Animal. 2016;10(2):294–301. Published online 2015 May 11. doi: 10.1017/S1751731115000750.