Absence of evidence is not evidence of absence
Research on pain in fish is not and may never be conclusive. As scientists continue to debate the issue, anglers are often of two minds. Some side with scientists who state that fish "don't have the brains" to feel pain, while others believe that fish do feel pain and angling has an appreciable impact on fish. Many welfare organizations, researchers and anglers believe fish should be given the benefit of the doubt. In the absence of conclusive evidence one way or the other, the precautionary principle should be applied.
In a communication published on January 1, 2015, the Journal of the American Veterinary Medical Association stated the veterinary community is increasingly warming to the possibility that pain for some fish species is a more noxious experience than an unconscious, knee-jerk response. Concerning fish, the American Veterinary Medical Association Guidelines for the Euthanasia of Animals state that considerable evidence is accumulating suggesting it is appropriate to consider the possibility of pain perception in (fish) these species. The Canadian Council on Animal Care (CCAC) states that fish have the capacity to feel pain, and defines pain in fish as a response to a noxious stimulus that results in a change in behaviour or physiology and the same noxious stimulus would be painful to humans.
The majority of respondents (general population) in a small survey conducted in New Zealand and published in 2013 indicated they believed fish were able to feel pain to some degree. Only 1.2% believed that fish had no capacity to feel pain (Muir et al 2013). A similar small survey of fisheries researchers, managers and anglers in North America published in 2011 found that 35 to 58% of respondents believed fish could feel pain (Hasler et al 2011).
More recently (2017, 2018), there has been a flurry of papers published in the scientific literature siding with those who think that fish, as sentient beings, are capable of feeling noxious stimuli that many liken to pain.
In vertebrates (which include fish), pain is mostly associated with tissue damage. There are a few well known and trusted general anesthetics that researchers and veterinarians use when performing minor and major surgeries on fish. These render fish unconscious and immobile while procedures are being done. Much like other vertebrates,fish metabolize the anesthetic and recover fully. Local anesthetics are much less commonly used, although some advocate for their application post surgery and in catch and release fishing.
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Braithwaite, V. (2010). Do fish feel pain?. Oxford university press.
Hasler, C. T., Colotelo, A. H., Rapp, T., Jamieson, E., Bellehumeur, K., Arlinghaus, R., & Cooke, S. J. (2011, May). Opinions of fisheries researchers, managers, and anglers towards recreational fishing issues: an exploratory analysis for North America. In American Fisheries Society Symposium (Vol. 75, No. PNNL-SA-76190). Pacific Northwest National Laboratory (PNNL), Richland, WA (US).
Mettam, J. J., Oulton, L. J., McCrohan, C. R., & Sneddon, L. U. (2011). The efficacy of three types of analgesic drugs in reducing pain in the rainbow trout, Oncorhynchus mykiss. Applied Animal Behaviour Science, 133(3), 265-274.
Muir, R., Keown, A.J., Adams, N.J., and Farnworth, M.J. (2013). Attitudes towards catch-and-release recreational angling, angling practices and perceptions of pain and welfare in fish in New Zealand. Animal Welfare. 22 : 323-329.
Rose, J. D., Arlinghaus, R., Cooke, S. J., Diggles, B. K., Sawynok, W., Stevens, E. D., & Wynne, C. D. L. (2014). Can fish really feel pain?. Fish and Fisheries, 15(1), 97-133.
Weber ES, 3rd. Fish analgesia: Pain, stress, fear aversion, or nociception? Vet Clin North Am Exot Anim Pract 2011;14:21-32.