Do Fish Feel Pain? The Science of Fish Nociception

The question of whether fish feel pain has moved from philosophical debate to a central concern in fisheries science, aquaculture, and veterinary medicine. The scientific evidence now demonstrates that fish possess nociceptors, display pain-related behavioral changes, and respond to analgesic drugs in ways that closely parallel mammalian pain responses. This article reviews the molecular, neurobiological, and behavioral evidence on fish nociception and pain perception, and explains what this means for practical fish husbandry, welfare assessment, and research protocols.

Defining Nociception and Pain in Fish

Nociception is the simple detection of potentially damaging stimuli, usually accompanied by a reflex withdrawal response. Pain, by contrast, involves a negative affective component that alters future behavior. In order to survive, animals must avoid injury and detect potentially damaging stimuli through nociceptive mechanisms. If the injury is accompanied by a negative affective component, future behavior should be altered, and one can conclude the animal experienced the discomfort associated with pain. This distinction matters for fish welfare because reflex responses alone would not require the same welfare considerations as genuine pain experience.

The empirical evidence for nociception in fishes spans molecular biology, neurobiology, anatomy of nociceptors, and whole animal behavioral responses. Studies demonstrate the evolutionary conservation of nociception and pain from invertebrates to vertebrates. Research in fish has shown that the biology of the nociceptive system is strikingly similar to that found in mammals. Potentially painful events result in behavioral and physiological changes such as reduced activity, guarding behavior, suspension of normal behavior, increased ventilation rate, and abnormal behaviors, all of which are prevented by the use of pain-relieving drugs. Fish also perform competing tasks less well when treated with a putative painful stimulus. There is ample evidence to demonstrate that it is highly likely that fish experience pain and that pain-related behavioral changes are conserved across vertebrates.

The Neuroanatomy of Fish Nociception

Fish possess specialized sensory receptors called nociceptors that detect potentially damaging stimuli. These nociceptors are similar to those found in mammals. The neurophysiological basis of nociception or pain in fish is demonstrably similar to that in mammals. Bony fish possess nociceptors that respond to chemical, thermal, and mechanical stimuli, and the neural pathways that carry this information to the brain show striking anatomical parallels with mammalian pain pathways.

Veterinarians need to adapt methodologies for examining, performing diagnostics, and treating fish patients to decrease stress, decrease fear, and avoid or decrease nociception. Because fish are aquatic and poikilothermic, there are several unique anatomic and physiologic considerations that must be understood when working with these animals. The increasing use of fish resources and a greater understanding of aquatic animal medicine demands providing evidence-based veterinary care for these animals.

The debate over whether fish feel pain has persisted for several years, yet veterinarians have been mostly absent from the discussion. While this controversy has its place, it is unlikely to be resolved in the near future. Fish welfare could instead be improved by pursuing more clinically applicable research to increase knowledge of fish behavior and physiology. Such research would assist in learning the optimal environment for their specific needs, as well as compiling verified indicators of pain in fish.

Behavioral Evidence for Pain in Fish

Behavioral indicators are valid and have been shown to profoundly differ between nonpainful and painful treatments in fish. However, these are not universal, and species-specific differences exist in behavioral responses to pain. A range of general, behavioral, and physiologic indicators can be used when assessing pain in fish. Many of these can be used at the tank side and are termed operational welfare indicators, whereas some require further computer or laboratory analysis.

Early studies reported the occurrence of prolonged rocking movements in trout and rubbing of their lips if they were injected with acetic acid. Subsequent studies examined the role of morphine in reducing these activities and examined shifts in attention when responding to noxious stimuli. The responses of fish go beyond those expected of mere nociceptive reflex. The responses clearly involve central processing, and pain experience, although not proven, is a distinct possibility.

Contemporary studies over the last decade have demonstrated that bony fish possess nociceptors similar to those in mammals, that they demonstrate pain-related changes in physiology and behavior that are reduced by painkillers, that they exhibit higher brain activity when painfully stimulated, and that pain is more important than showing fear or anti-predator behavior in bony fish.

At a Glance: Key Studies on Fish Pain

The Zebrafish Model in Pain Research

The zebrafish is already a well-established animal model in many other research areas like toxicity testing, as a model for diseases, or regeneration, and has great potential in pain research. Methods of electrophysiology, molecular biology, analysis of reflexive or non-reflexive behavior, and fluorescent imaging are routinely applied, but it is the combination of these tools that makes the zebrafish model so powerful. Simultaneously, observing complex behavior in free-swimming larvae, as well as their neuronal activity at the cellular level, opens new avenues for pain research.

Current methods to study nociception and pain in zebrafish include chemical, thermal, and electric stimuli. Treatments with the best algogenic potential have been identified, and options of analgesia to counter effects of nociception and pain include opioids, non-steroidal anti-inflammatory drugs (NSAIDs), and local anesthetics. These practices require critical evaluation, and gaps in knowledge remain.

Mammalian models have long dominated research on nociception and pain, but there is increasing evidence for comparable processes in fish. The need to improve existing pain models for drug research and the obligation for 3R refinement of fish procedures facilitated the development of numerous new assays of nociception and pain in fish.

Practical Assessment of Pain in Fish

Operational Welfare Indicators

Operational welfare indicators are practical measures that can be used at the tank side to assess pain in fish. These include behavioral and physiologic indicators that have been validated in research settings. Behavioral indicators have been shown to profoundly differ between nonpainful and painful treatments in fish, though species-specific differences exist.

General behavioral indicators of pain in fish include reduced activity, guarding behavior, suspension of normal behavior, increased ventilation rate, and abnormal behaviors. These changes are prevented by the use of pain-relieving drugs, which provides evidence that they are pain-related instead of simple reflex responses.

Physiologic Indicators

Physiologic indicators of pain in fish require further computer or laboratory analysis. These may include measures of stress physiology, immune function, and other biomarkers. The combination of behavioral and physiologic indicators provides a more complete picture of pain status in fish.

Species-Specific Considerations

Behavioral responses to pain are not universal across fish species. Species-specific differences exist in behavioral responses to pain, which means that welfare assessment protocols must be tailored to the species being managed. What works for rainbow trout may not be appropriate for zebrafish or Atlantic salmon.

Thermal Stress and Nociception in Aquaculture

Thermal treatment has become the most used delousing method in salmonid aquaculture. Concerns have been raised about it being painful for the fish. A study of Atlantic salmon acclimated to 8°C examined behavioral responses when transferred to temperatures in the range of 0 to 38°C. Exposure time was 5 minutes or until fish reached the endpoint of losing equilibrium and laying on their side, a sign of imminent death.

At temperatures below 28°C, none of the fish reached endpoint within the 5-minute maximum. At 28°C, four of five fish reached endpoint, and fish reached endpoint more rapidly as temperature increased further. Fish transferred to temperatures above 28°C had higher swimming speed immediately after transfer and maintained a high swimming speed until just before loss of equilibrium. Their behavior was from the start characterized by collisions into tank walls and head shaking. Just before loss of equilibrium, they started breaking the surface of the water, swimming in a circle pattern, and in some instances displayed a side-wise bending of their body. Salmon transferred to temperatures above 28°C showed instant behavioral responses indicative of nociception or pain.

This research has direct implications for aquaculture operations that use thermal treatments for sea lice control. The temperature thresholds identified in this study provide a basis for establishing welfare limits in commercial operations.

The Role of Analgesia in Fish Management

The use of analgesic drugs in fish is an emerging area of veterinary practice. Current use of analgesics for fish patients is summarized in the veterinary literature, and the ongoing controversy regarding pain in fish affects clinical decision-making. Research has demonstrated that pain-related behavioral changes in fish are prevented by the use of pain-relieving drugs, which provides both evidence for pain perception and a practical tool for welfare management.

Options for analgesia in fish include opioids, non-steroidal anti-inflammatory drugs (NSAIDs), and local anesthetics. The choice of analgesic depends on the species, the procedure being performed, and the regulatory framework in the relevant jurisdiction. Veterinarians need to adapt methodologies for examining, performing diagnostics, and treating fish patients to decrease stress, decrease fear, and avoid or decrease nociception.

The Debate on Fish Pain and Sentience

Arguments Against Fish Pain

Some argue that because the fundamental structures involved in pain perception and experience in mammals are not found in fishes, there is no potential for these animals to experience pain. This position holds that fish lack the neuroanatomical complexity required for conscious pain experience.

Others believe that because fish demonstrate strong avoidance behaviors to noxious stimuli, other neurosensory processing systems to process pain information, yet to be identified, may have evolved, and the possibility for pain perception exists. Further work regarding the neurophysiological mechanisms and behavioral responses involved in nociception and potential pain perception in fishes is required.

Arguments Supporting Fish Pain

The empirical evidence for nociception in fishes demonstrates the evolutionary conservation of nociception and pain from invertebrates to vertebrates. Studies in fish have shown that the biology of the nociceptive system is strikingly similar to that found in mammals. Potentially painful events result in behavioral and physiological changes that are prevented by the use of pain-relieving drugs.

The responses of fish to noxious stimuli clearly involve central processing, and pain experience, although not proven, is a distinct possibility. The idea of pain cannot be dismissed by the argument that fish respond only by reflex.

The Role of Phylogenetic Distance in Perceptions of Fish Pain

Humans form deep attachments to some nonhuman animals, yet these attachments are unequally distributed across the tree of life. Empathy and affective preference are typically stronger for phylogenetically closer species, especially mammals, than for distant taxa such as reptiles, fish, or arthropods. Signal recognizability, including faces, gaze, vocal formants, and biological motion, and predictive social cognition facilitate mind attribution to mammals. Conserved neuroendocrine systems, such as oxytocin, further amplify affiliative exchange, particularly in domesticated dyads.

This phylogenetic bias affects both scientific debate and public perception of fish pain. Boundary cases such as cephalopods, cetaceans, and parrots show that perceived agency, sociality, and communicative transparency can overcome phylogenetic distance. Calibrated anthropomorphism, hands-on education, and messaging that highlights agency, parental care, or ecological function reliably broaden concern for under-represented taxa.

Fish Sentience in the Scientific Literature

A review of the scientific literature for evidence of fish sentience searched the journal database Science Direct using 42 keywords that describe traits or elements of sentience. The review returned 470 results for fish sentience in 142 different species and subspecies of fish, featuring 19 different sentience keywords. The top four keywords were stress, anxiety, fear, and pain. The findings highlight an abundance of evidence for fish sentience in the published scientific literature.

Fish are traded, caught, farmed, and killed in their trillions every year around the world, yet their welfare is often neglected and their sentience regularly disregarded. Legislation governing the treatment of fish and attitudes toward their welfare require scrutiny so that their welfare can be safeguarded across the globe.

Public Perception of Fish Sentience and Welfare

Public perception of fish sentience varies considerably. A survey of highly educated citizens from Bogotá, Colombia, and Curitiba, Brazil, found that the percentage of participants who perceived fish as sentient animals was 79.7% and 71.8%, respectively. The classification of sentience perception among taxonomic groups seems in accordance with the phylogenetic proximity to humans, suggesting participants were more likely to perceive sentience in mammals than in other animals.

The descending order related to the highest perception of fish suffering in different scenarios was fishing with hook and line, municipal live fish fair, fish-and-pay ponds, fish kept as laboratory animals, fish farming, fish in pet stores, production of ornamental fish, fish in aquarium exhibits, and fish kept as pets. Lack of knowledge about the conditions of capture, handling, transport, and sale of ornamental fish may justify the perception of low level of suffering in the last scenarios.

Regarding humane slaughter, 57.0% and 55.0% of respondents were unaware of the issue. After reflection induced by the questionnaire, 76.0% and 72% of participants believed that fish should be included in humane slaughter regulations.

Welfare Implications for Fish Farming and Research

Practical Welfare Assessment Steps

1. Establish baseline behavior for the species and population under normal conditions. This requires observation over time to understand normal activity patterns, feeding behavior, and social interactions.
2. Identify operational welfare indicators appropriate for the species. Behavioral indicators such as reduced activity, guarding behavior, suspension of normal behavior, increased ventilation rate, and abnormal behaviors can be assessed at tank side.
3. Monitor fish during and after procedures that may cause pain. This includes handling, transport, thermal treatments, and surgical procedures.
4. Record observations systematically. Documentation of behavioral changes and physiologic indicators allows tracking of welfare status over time and identification of patterns.
5. Escalate to veterinary assessment when pain is suspected. Veterinarians can provide guidance on analgesic use and pain management protocols.

Records and Measurements

Accurate records are essential for welfare assessment in fish facilities. Records should include water quality parameters, stocking density, feeding rates, behavioral observations, and any procedures performed. Behavioral observations should note the frequency and duration of specific behaviors, as well as the context in which they occur.

For research facilities, the welfare of fish in an experimental setting is key to the quality of the research. Ensuring optimal environmental conditions in a minimal-stress environment is the best means for accomplishing that goal. Although there remain many unanswered questions, the well-being of these animals is a significant factor in quality science.

Common Failure Patterns in Fish Welfare Assessment

One common failure pattern is relying on a single indicator of pain instead of a combination of behavioral and physiologic measures. Behavioral responses to pain are not universal across species, so a behavior that indicates pain in one species may not be reliable in another.

Another failure pattern is failing to establish baseline behavior before assessing pain. Without knowledge of normal behavior for the species and population, it is impossible to identify pain-related changes.

A third failure pattern is ignoring the effects of stress on fish welfare. Stress and pain are interrelated, and stress can both cause and exacerbate pain responses. Veterinarians need to decrease stress, decrease fear, and avoid or decrease nociception when working with fish patients.

Escalation Criteria

Professional escalation is warranted when fish show persistent behavioral changes indicative of pain, when pain-related behaviors do not resolve with analgesic treatment, when fish show signs of severe distress such as loss of equilibrium, or when there is uncertainty about the appropriate analgesic protocol for a particular species or procedure.

Veterinarians should be consulted for guidance on analgesic use, pain management protocols, and welfare assessment in fish. The ongoing controversy regarding pain in fish should not prevent the provision of evidence-based veterinary care for these animals.

The Cognitive Spectrum Framework

Recent theoretical work proposes a unifying framework in which cognition is understood as an organizational property of living systems, grounded in information embodied in their physical structures and in their ongoing interactions with the environment. Within this info-computational perspective, living systems engage in behavior, learning, and anticipation by dynamically transforming embodied information through distributed, physically realized processes that support viability and self-maintenance.

These processes are present from the onset of life and become progressively more integrated and temporally extended with increasing biological organization. The framework provides explanatory continuity across biological scales and clarifies how complex forms of cognition, awareness, and mind arise as elaborations of basic life-regulatory dynamics.

This framework has implications for the fish pain debate because it suggests that cognition and sentience exist on a spectrum instead of being all-or-nothing phenomena. Fish, as vertebrates with complex nervous systems, would fall on this spectrum in a position that supports pain perception capacity.

Red Herrings in the Fish Pain Debate

Some measures held to indicate sentience have the same problem as nociceptors: they are necessary but not sufficient for true pain. These red herring measures are also present in non-sentient organisms, notably those lacking nervous systems, like plants and protozoa, spines disconnected from brains, decerebrate mammals and birds, and humans in unaware states.

These subjects can show approach and withdrawal, react with apparent emotion, change their reactivity with food deprivation or analgesia, discriminate between stimuli, display Pavlovian learning, including some forms of trace conditioning, and even learn simple instrumental responses. Consequently, none of these responses are good indicators of sentience.

Potentially more valid are aspects of working memory, operant conditioning, the self-report of state, and forms of higher order cognition. The question of whether fish feel pain, or indeed anything at all, therefore stimulates sometimes polarized debate. Debates around fish ability to feel pain concern sentience: do reactions to tissue damage indicate evaluative consciousness, or mere nociception?

Ethical Frameworks for Fish Farming

Systems that farm non-typical wild species for human consumption are on the rise globally, in contrast to more typical livestock production. In some instances, wildlife farming may arguably help alleviate poverty, provide sustainable animal protein, and be a useful strategy for conservation through reducing wildlife poaching or breeding some animals on farms for reintroduction. However, it is unclear whether farming non-typical species within variable and often unregulated systems truly offers these benefits or outweighs the costs, including animal welfare implications, public health concerns, and normalizing or intensifying the consumption of wild animals.

A sentiocentric ethical decision-making framework for the farming of wild species has been proposed and piloted with academic key informants. The framework outcome for 11 of 13 appraisals was that the chosen species may be suitable for farming. However, erroneous responses were likely in places, and there was some uncertainty over definitions of framework terminology. The framework can be applied proactively or reactively by different stakeholders, including governments, businesses, and NGOs.

Regulatory and Policy Context

Fish welfare is still a relatively new field. Regulations and protocols to ensure fish welfare are currently limited and vary considerably in different jurisdictions. This is in part because of the ongoing controversy as to whether or not fish feel pain.

The abundance of evidence for fish sentience in the published scientific literature suggests that legislation governing the treatment of fish and attitudes toward their welfare require scrutiny so that their welfare can be safeguarded across the globe. The conclusion from comparative studies is that both fish and decapods should be awarded consideration for their welfare.

Limitations of Current Research

Several limitations affect the current state of knowledge on fish pain. First, behavioral responses to pain are not universal across fish species, which complicates the development of standardized welfare assessment protocols. Second, the neurophysiological mechanisms involved in nociception and potential pain perception in fishes require further investigation. Third, the ongoing controversy regarding pain in fish affects both research funding priorities and regulatory development.

The debate over fish pain is unlikely to be resolved in the near future. Fish welfare could instead be improved by pursuing more clinically applicable research to increase knowledge of fish behavior and physiology. Such research would assist in learning the optimal environment for their specific needs, as well as compiling verified indicators of pain in fish. This would then lead to improved studies that could help to determine if and when analgesic drugs can be beneficial in fish, as they are in many other species.

Frequently Asked Questions

What is the difference between nociception and pain in fish?

Nociception is the simple detection of potentially painful stimuli, usually accompanied by a reflex withdrawal response. Pain involves a negative affective component that alters future behavior. If an injury is accompanied by a negative affective component, future behavior should be altered, and one can conclude the animal experienced the discomfort associated with pain. Fish possess nociceptors similar to those in mammals, and they demonstrate pain-related changes in physiology and behavior that are reduced by painkillers.

Do fish have nociceptors like mammals?

Yes. Bony fish possess nociceptors that are similar to those in mammals. The neurophysiological basis of nociception or pain in fish is demonstrably similar to that in mammals. Studies in fish have shown that the biology of the nociceptive system is strikingly similar to that found in mammals.

What behavioral changes indicate pain in fish?

Behavioral indicators of pain in fish include reduced activity, guarding behavior, suspension of normal behavior, increased ventilation rate, and abnormal behaviors. These changes are prevented by the use of pain-relieving drugs. Fish also perform competing tasks less well when treated with a putative painful stimulus. However, behavioral responses are not universal, and species-specific differences exist.

Can analgesic drugs be used in fish?

Yes. Research has demonstrated that pain-related behavioral changes in fish are prevented by the use of pain-relieving drugs. Options for analgesia in fish include opioids, non-steroidal anti-inflammatory drugs (NSAIDs), and local anesthetics. Veterinarians need to adapt methodologies for examining, performing diagnostics, and treating fish patients to decrease stress, decrease fear, and avoid or decrease nociception.

What happens to salmon during thermal treatments above 28°C?

Atlantic salmon acclimated to 8°C and transferred to temperatures above 28°C show instant behavioral responses indicative of nociception or pain. These include higher swimming speed immediately after transfer, collisions into tank walls, head shaking, breaking the surface of the water, swimming in a circle pattern, and in some instances side-wise bending of the body. At 28°C, four of five fish reached the endpoint of losing equilibrium within the 5-minute exposure period.

Why does the fish pain debate continue despite the evidence?

The debate continues because some argue that the fundamental structures involved in pain perception and experience in mammals are not found in fishes, so there is no potential for these animals to experience pain. Others believe that because fish demonstrate strong avoidance behaviors to noxious stimuli, other neurosensory processing systems to process pain information may have evolved. The controversy has persisted for several years and is unlikely to be resolved in the near future.

How does public perception of fish sentience compare to scientific evidence?

Public perception of fish sentience varies. In surveys of highly educated citizens in Bogotá, Colombia, and Curitiba, Brazil, 79.7% and 71.8% of participants perceived fish as sentient animals. Perception of sentience among taxonomic groups seems in accordance with phylogenetic proximity to humans. The scientific literature contains abundant evidence for fish sentience, with 470 results for fish sentience in 142 different species and subspecies of fish.

What are operational welfare indicators for fish pain?

Operational welfare indicators are measures that can be used at the tank side to assess pain in fish. A range of general, behavioral, and physiologic indicators can be used when assessing pain in fish. Many of these can be used at the tank side, whereas some require further computer or laboratory analysis. Behavioral indicators are valid and have been shown to profoundly differ between nonpainful and painful treatments in fish, though species-specific differences exist.

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References and Further Reading

• NCBI Literature Resources. National Center for Biotechnology Information.
• PubMed. National Library of Medicine.
• Evolution of nociception and pain: evidence from fish models.. Philosophical transactions of the Royal Society of London. Series B, Biological sciences, 2019.
• Fish analgesia: pain, stress, fear aversion, or nociception?. The veterinary clinics of North America. Exotic animal practice, 2011.
• Pain Recognition in Fish.. The veterinary clinics of North America. Exotic animal practice, 2023.
• Current Methods to Investigate Nociception and Pain in Zebrafish.. Frontiers in neuroscience, 2021.
• Potential Pain in Fish and Decapods: Similar Experimental Approaches and Similar Results.. Frontiers in veterinary science, 2021.
• Laboratory fish: impacts of pain and stress on well-being.. Contemporary topics in laboratory animal science, 2003.
• Pain in aquatic animals.. The Journal of experimental biology, 2015.
• The Controversy on Fish Pain: A Veterinarian's Perspective.. Journal of applied animal welfare science : JAAWS, 2019.
• A Kettle of Fish: A Review of the Scientific Literature for Evidence of Fish Sentience.. 2022.
• Perception of Fish Sentience, Welfare and Humane Slaughter by Highly Educated Citizens of Bogotá, Colombia and Curitiba, Brazil.. 2017.
• De-anthropomorphizing the mind: life as a cognitive spectrum in a unified framework for biological minds.. 2026.
• Why Humans Prefer Phylogenetically Closer Species: An Evolutionary, Neurocognitive, and Cultural Synthesis.. 2025.
• To Farm or Not to Farm? Pilot Testing a Sentiocentric Ethical Framework for Farming Non-Typical Species.. 2026.
• What Is It Like to Be a Bass? Red Herrings, Fish Pain and the Study of Animal Sentience. Frontiers in Veterinary Science, 2022.
• Sudden exposure to warm water causes instant behavioural responses indicative of nociception or pain in Atlantic salmon. Veterinary and Animal Science, 2019.
• Differential effects of NMDA and group I mGluR antagonists on both nociception and spinal cord protein kinase C translocation in the formalin test and a model of neuropathic pain in rats. Pain, 2001.
• Fish. Nociception and pain. A biological perspective. 2012.
• Nociception and pain in teleost fish. 2012.
• SENSORY SYSTEMS, PERCEPTION, AND LEARNING | Nociception or Pain in Fish. Encyclopedia of Fish Physiology from Genome to Environment Volume 1 3, 2011.
• Fish oil concentrate delays sensitivity to thermal nociception in mice. Pharmacological Research, 2011.

This article is educational and does not replace institutional policy, professional advice, or applicable safety and regulatory requirements.