We know that all vertebrates have the capacity to suffer because they are sentient beings, but individuals of fish species not just suffer, but suffer like we do.
Animals are sentient beings, which means that they can have positive, negative, or neutral experiences based on the information they obtain from their senses and its processing in their nervous systems. When negative experiences linger because whatever the animal tries to do to solve the problem did not work fast enough, we call this suffering (which can be manifested as pain, but also as negative moods, depression, discomfort, fear, anxiety, etc.).
The question of whether individuals of fish species can suffer is no longer seriously disputed within reputable scientific circles. The old-fashioned idea that these animals are little more than reflex machines drifting through life without awareness has been dismantled by decades of empirical research. Today, most scientists who study animal behaviour, neurobiology, or welfare accept that these non-mammalian aquatic vertebrates can experience pain and distress. The real question is not whether they suffer, but how their suffering compares to ours. Is it fundamentally different, diminished, or perhaps more similar than many are willing to admit? To approach this question, we can examine three key areas: their sensory systems, their brains, and their behaviour.
We both have the same biological basis of pain
Any meaningful discussion of suffering must begin with nociception, the biological capacity to detect harmful stimuli. In humans and other mammals, nociceptors are specialised sensory receptors that respond to potentially damaging events, such as extreme heat, pressure, or chemical irritants. For a long time, it was assumed that individuals of fish species lacked such structures, but this claim was directly challenged by research in the early 2000s.
In 2002, Dr Lynne Sneddon, working with Dr Victoria Braithwaite and Dr Michael Gentle, published a landmark study demonstrating the presence of nociceptors in rainbow trout (Oncorhynchus mykiss). These receptors responded to mechanical, thermal, and chemical stimuli, similarly to those found in mammals. This discovery alone dismantled the argument that these animals are incapable of detecting injury as part of their natural survival mechanisms.
However, detecting harmful stimuli is only part of the story. Critics often argue that nociception is merely a reflex, not evidence of subjective experience, yet further research by Sneddon in 2003 showed that when these animals were exposed to noxious substances, they displayed prolonged behavioural changes, such as reduced feeding, rocking motions, and rubbing their mouths against tank walls. Crucially, when given analgesics like morphine, these responses were significantly reduced. This suggests not just detection, but modulation of pain — something that aligns more closely with an experience rather than a simple reflex.
Ashley’s 2007 review in Applied Animal Behaviour Science synthesised these findings and concluded that the evidence strongly supports the capacity for pain perception in these animals. The presence of nociceptors, combined with behavioural and pharmacological responses, forms a compelling case that their experience of harmful stimuli is not dissimilar to ours.
We both have big brains that process more than reflexes
Beyond sensory receptors, the structure and function of the brain play a crucial role in determining whether pain can be consciously experienced as suffering. Detractors have often pointed to the absence of a neocortex (a structure prominent in mammalian brains) as evidence that individuals of fish species cannot suffer. However, this argument relies on a narrow, mammal-centric view of neurobiology.
Comparative neuroanatomy reveals that these animals possess brain regions functionally analogous to those involved in emotional processing in mammals. The pallium, for instance, is considered homologous to parts of the mammalian cortex. Research by Northcutt (2002) and later by Salas and colleagues (2003, 2005) demonstrated that this region plays a role in spatial learning, memory, and behavioural flexibility, which are functions associated with higher-order processing. There is plenty of evidence that fish species can solve problems like us, so they have a brain well equipped for it (some can even recognise themselves in mirrors as they have passed the “mirror test”).
In contrast to insects, whose nervous systems are far more decentralised and limited in processing capacity, non-mammalian aquatic vertebrates exhibit a level of neural integration that supports complex behaviours similar to those of other vertebrates. Studies by Broglio et al. (2005) showed that goldfish (Carassius auratus) could perform tasks requiring spatial cognition, relying on brain regions analogous to the hippocampus in mammals. This suggests not only memory, but also the ability to form internal representations of their environment.
More importantly, research into affective states (the emotional components of experience) has begun to reveal that these animals may possess more than just cognitive abilities. Sneddon’s work, along with subsequent studies, has shown that they can learn to avoid places associated with painful experiences, a phenomenon known as conditioned place avoidance. When an animal remembers a location as unpleasant and actively avoids it, it suggests that the experience carried a negative value. This aligns with the broader scientific consensus that pain is not merely a sensory event but an emotional one.
To argue that these animals do not suffer because their brains are not exactly like ours is to fall into what could be called neurochauvinism, the assumption that only brains like ours can generate meaningful experiences. Evolution has produced multiple solutions to the same problem, and consciousness may arise from different architectures, not just one. But even if we allow for neurochauvinism, the brain of fish species is not that different to the brain of many mammals, so if we can see suffering in invertebrates like octopuses or crustaceans, we should expect to see it too in animals closer to us from a phylogenetic point of view.
We both behave similarly in distress
If biology provides the mechanisms, behaviour provides the final evidence. One of the most compelling arguments for comparable suffering lies in how individuals of fish species act when they are harmed or distressed.
When exposed to painful stimuli, these animals do not simply withdraw and carry on as if nothing happened. Instead, they exhibit prolonged changes in behaviour that resemble those seen in mammals. As mentioned earlier, trout injected with acetic acid engage in rocking motions and rub their mouths against surfaces, and they also reduce feeding because they are prioritising coping with the injury over other essential behaviours. The fact that these animals alter their priorities suggests a central processing system weighing competing needs, much like we do when we are in pain.
Another striking example comes from their response to oxygen deprivation. When removed from water, individuals of fish species exhibit frantic, desperate movements (gasping, thrashing, and attempting to return to a breathable environment), often dismissed as mere reflexes, yet the parallel with human experience is difficult to ignore. When a human is submerged and unable to breathe, the response is immediate and intense (panic, involuntary movements, and a desperate drive to reach air), like in fish species. The behavioural similarities are unmistakable. In both cases, the organism is reacting to a life-threatening lack of oxygen, and the urgency of the response suggests an aversive experience of considerable intensity. We both behave similarly to the same problem because we both suffer from it equally.
Regarding other types of suffering less connected with direct pain, when placed in confinement for life, individuals of fish species also suffer as we do, showing the same types of stereotypic behaviours we encounter in captive mammals and birds.
We cannot ask these animals to describe what they feel, just as we cannot directly access the subjective experiences of other humans while they are having them (recollecting the experience and verbalising it may not be totally accurate), yet science does not require identical experiences to recognise similar suffering.
Taken together, the evidence shows that individuals of fish species possess nociceptors that detect harmful stimuli, brains capable of processing and integrating those signals as suffering, and behaviours that indicate distress, learning, and prioritisation. While their subjective experiences may not be identical to ours, the weight of evidence suggests that their suffering is not categorically different to ours.
Therefore, there is no justification to continue exploiting these animals, be that in food production or “sport”, because doing this causes them suffering not unlike the one we would experience if we were the victims of fishing vessels or angling rods.
