The surprisingly profound debate over whether fish feel pain

What must it feel like to be a fish — to glide weightlessly through the sea, to draw breath from water, to be (if one is lucky) oblivious to the parched terrestrial world above?

Maybe you suspect there isn’t much to fish — and you could hardly be blamed for it. For centuries, Western natural philosophy maligned sea creatures as primitive, dim-witted, perhaps not even conscious. It’s a prejudice that goes back at least as far as Aristotle, whose scala naturae ranked fishes near the bottom of the hierarchy of existence. According to Plato, fish were characterized by “the lowest depths of ignorance.”

And so it remains today: Humans use fishes in far greater numbers than we do land animals — for food, for amusement as pets, and more — but our species shows strikingly little interest in what these experiences might be like for them. We even use fish as bywords for stupidity and poor brain function, like the proverbial goldfish mind that resets every three seconds — a myth fabricated out of thin air.

But I should speak for myself. Although I’m professionally obsessed with the ethics of our relationships with nonhuman animals, I’m somewhat embarrassed to admit I’ve given little thought to the massive class of animals that we call fish. I’ve hardly written a word about the hundreds of billions killed — quite brutally — by the commercial fishing and fish farming industries every year, nor much considered why it is that aquatic animals are treated as an afterthought to those who live on land.

Fish are hard to empathize with. They lack facial expressions we can readily understand, their bodies are scaly and cold to the touch, and although they make plenty of sounds to communicate with one another, we generally can’t hear them. Their entire world — built on senses and signals that we land-bound creatures cannot access — is as alien to us as ours no doubt is to them.

“We know very, very little about their day-to-day lives,” Becca Franks, an assistant professor of environmental studies at New York University, told me. “Rather than seeing that big blank murky fogginess as a mystery that is waiting to be unveiled…there’s a cultural expectation that their lives are simple and not interesting.”

In recent decades, however, our aperture on marine life has widened tremendously. Scientific advances have increasingly shown that we’ve misjudged fish. They have complex social relationships and cognitive abilities, maintain long-term memories, and use tools. Octopuses — invertebrates with even more genetic distance from humans than fish — have become international celebrities for their intelligence. The more we attempt to look into the minds of animals farther and farther from us on the evolutionary tree, the more we discover how much we’ve been underestimating them.

Yet even as it has shed new light onto the piscine creatures that populate most of our planet, the science of fish minds has been mired in debate over a startlingly, deceptively simple question: Can fish feel pain?

It might sound weird that a capacity as basic as pain is still contested in animals who can distinguish between individual human faces and migrate thousands of miles. But the question remains unsettled because pain is a subjective feeling that science cannot prove definitively. And so, even as experimental evidence that fish experience pain has accumulated over the last quarter-century, some prominent skeptics continue to doubt it.

The great fish pain debate shows us how scientific knowledge is shaped not just by linear progress, but also by historical contingency, cultural biases, philosophical roadblocks, and internal ethical paradoxes. And it reflects back a story about us humans — our endeavor to find sentience in our fellow creatures, and our countervailing impulse to deny morally important qualities in them.

Perhaps nowhere are these competing tendencies more evident than in our attempts to understand fish, alien as they are to our warm-blooded, mammalian selves.

What we talk about when we talk about fish pain

Most accounts of what we know about pain in fish go something like this: Until very recently, it was widely believed that fish don’t feel pain, or much of anything. There’d been little empirical work on the question. Then, in the early 2000s, a group of University of Edinburgh researchers — Lynne Sneddon, the late Victoria Braithwaite, and Michael Gentle — transformed how we see fish.

They discovered that fish have nociceptors, or neurons that send signals to the central nervous system when an animal is injured. Nociceptors are considered necessary for experiencing pain, just as photoreceptors are for vision, but on their own, they aren’t sufficient — the animal needs to be able to perceive pain in the brain. So Sneddon and the team, along with other researchers after them, ran behavioral experiments designed to figure out whether fish really feel pain — whether their nociceptors aren’t just reacting reflexively to noxious stimuli, the way a human involuntarily pulls away after touching a hot stove microseconds before feeling any pain.

Over and over again, the findings have pointed to yes. Goldfish and trout prodded with needles showed not just reflexive responses, but also activity in parts of their brains associated with higher processing. When rainbow trout were injected with painful substances, like acetic acid or bee venom, their respiration rates spiked, their appetites dropped, they rocked back and forth, and they rubbed the affected areas against the gravel and walls of their tanks, like pressing your tongue against a sore tooth. In other experiments, trout injected with acid altered their social behavior and diminished their fear responses to predator cues and novel objects, findings that, the scientists believed, suggested the fish were directing attention to the pain and away from normal behaviors. Goldfish avoided swimming to parts of their tanks where they received electric shocks, sometimes even (depending on their hunger levels) forgoing food.

In the decades following World War II, scientists had increasingly begun studying animal well-being in the novel field of animal welfare science, as food production industrialized and the public grew concerned about the treatment of animals in the meat industry. But before Sneddon started her work on fish pain in the late ’90s, fish had mostly been ignored by the field; the focus had been on land animals like chickens, pigs, and cows. “At that time, even vegetarians would say, ‘Oh, I eat fish because they don’t experience pain,’” Sneddon, now a professor at the University of Gothenburg in Sweden, told me. “You even hear it in the Nirvana song,” she said, where Kurt Cobain drones, “It’s okay to eat fish, ’cause they don’t have any feelings.”

Sneddon and other scientists like her do this work because they care deeply about aquatic animals and hope their findings can inform animal welfare policy, from which fish are so often excluded. Still, when I first learned about this research, something about it unsettled me. I found the idea that we needed to inflict painful, extremely invasive experiments onto fish — including not just pricking them and injecting them with acid, but also surgically opening their heads to place electrodes before killing them at the end — in order to prove they feel pain to be ethically self-defeating. Sneddon told me she’s had “guilt dreams” where she was the one being injected with acid. Were there not less harmful ways to ask questions about the capabilities of fish?

And, surely, I thought, this was not the first time that scientists had ever thought about fish pain, nor the only way one could think about it. Although the history of Western thought is littered with disparaging ideas about aquatic life, we have a rich corpus to draw from for alternative ways of understanding the natural world and the experiences of fish. Charles Darwin, for example, wrote in The Descent of Man that “the lower animals…manifestly feel pleasure and pain, happiness and misery.” He did not wonder about whether fish feel pain, Becca Franks, the NYU professor, told me, “because he assumed that they did. He didn’t find it to be surprising. He spoke easily and fluently about animal emotions across the animal kingdom.”

And even in modern science, Franks points out, land animals, unlike fish, have not been put through an intensive battery of experiments to test whether they can feel pain; researchers start from the assumption that they do. “Nobody’s done these tests with chimpanzees,” she said, nor has anyone felt obliged to set up a comparable research program in chickens to confirm whether their pain is real.

But with fish, “their inferior status is evident even in arguments designed to grant them greater moral standing,” Franks and a group of co-authors wrote in a chapter of the book Animal Dignity. “Fishes, but not other vertebrates, are repeatedly asked, in increasingly elaborate experiential designs, to prove that they can feel pain.”

Darwin’s ideas belong to an intellectual lineage that takes animal feeling for granted, as a constitutive part of what it means to be an animal. “He said animals must feel pain, because it is an alarm system that alerts them to danger and stops you from injuring yourself,” Sneddon said. But to understand why the fish pain debate remains a debate, we have to understand another, competing tendency that has shaped our view of animals since the Scientific Revolution: the idea, associated with the 17th-century French philosopher René Descartes and others, that animals are akin to machines without thought or feeling.

Science still doesn’t understand consciousness

“Debates around fishes’ ability to feel pain,” scientists Georgia Mason and J. Michelle Lavery wrote in 2022, “are essentially debates about consciousness.” The sensation of pain, after all, requires an animal to be conscious — to have something that it’s like to be them. Sentience, a more specific form of consciousness, refers to an animal’s ability to have positively or negatively valenced feelings, like pain and pleasure.

Science is still far from understanding how and why consciousness arises, how widespread it is across our planet, and what the content of conscious experiences is — what it’s like inside the mind of, say, a fish that evolved to navigate a world so different from our own.

“On the one hand is the brain, a tangible lump of tissue that can be handled, weighed, measured, sliced and publicly examined. On the other are our conscious experiences — vivid and all-consuming but private and known only to the one person experiencing them,” biologist Marian Dawkins wrote in 2017. “How the two are connected is still so unknown to us that to say that there is something mysterious and almost magical about it seems entirely appropriate.” That means the scientific methods we have cannot prove that an animal — or even a human — is conscious.

Descartes, one of the fathers of modern science, helped codify the divide between the physical and the mental. He “split mind from matter, arguing that they are totally distinct: Only humans have mind,” as Vox’s Sigal Samuel has put it. And he crystallized a premise now embedded deep in the foundation of modern science: that science is responsible only for the objective examination of things that can be independently verified, which almost by definition excludes subjective experiences.

Today, scientific approaches to animal minds land somewhere between the Cartesian and the Darwinian views. Most scientists, like most modern people generally, do not actually believe, as Descartes likely did, that animals are unfeeling automata. For the animals who are closest to us — like the baby pigs who cry out when they’re castrated on factory farms, or even the chickens bred to grow so big they struggle to walk — the capacity for pain is not seriously in doubt. But scientific consensus has not extended that courtesy to fish. And because consciousness is, at bottom, a philosophical question, whether or not fish possess it can be debated endlessly.

Some scientists argue that fish lack parts of the brain that they believe are necessary to feel pain, particularly the neocortex, a brain region that only mammals have. This is a minority view, dismissed by most experts who have researched pain perception in fish, who point out that there is no good evidence to believe that fish brains must process pain in the same way mammalian ones do. Birds also lack a neocortex, and even in humans, pain processing is distributed across different parts of the brain. We don’t understand either consciousness or the brain enough to claim that lacking a particular brain architecture forecloses the possibility of feeling pain. And it’s worth noting here that this very critique of fish pain, according to Franks and co-authors Jennifer Jacquet and Troy Vettese, traces back to fishing interests, who, reacting to the rise of animal welfare laws, helped create a research agenda disputing fishes’ capacity for pain.

Most experts in the field now do believe fish feel pain. But a more thoughtful, more serious challenge to the evidence comes from Georgia Mason, a behavioral biologist at the University of Guelph in Canada, who, in 2022, co-authored a review of the fish pain literature that became a touchstone in the field. She and co-author J. Michelle Lavery called for tougher evidentiary standards in fish pain research, arguing that many of the types of pain tests that have been used in fish can also be passed by non-conscious subjects, like humans in unaware states and animals whose spines have been disconnected from their brains.

Decerebrate rats and chicks, for example, or animals who have had their forebrain function removed and are used as non-conscious comparators (controls) in consciousness debates, still show strikingly affective behavior, including licking or biting at injuries and vocalizing. If unconscious animals can do that, the authors argue, then a fish rubbing at their injuries is not very strong evidence of consciously felt pain.

In Mason’s estimation, existing fish pain research consists of “more opinion than hard empirical work that was directly relevant to how fish feel,” she told me. “Faced with the question, ‘How do you know this shows sentience?’ I think a lot of researchers just start hand-waving at that point.” She and her co-author propose ideas for experiments that they argue could more convincingly show that consciousness, rather than just reflexes, are involved in a pain response.

But Mason doesn’t believe that fish don’t feel pain — just that we haven’t proven beyond a reasonable doubt that they do. She thinks we should take a precautionary approach, treating fish as though they’re capable of pain and suffering until higher-quality evidence comes in. “I really think that’s the right thing to do,” she said. That contrasts with the preferred approach of other prominent fish pain skeptics, who argue that giving fish the benefit of the doubt could lead to animal welfare regulations that imperil the fishing industry and undermine food security.

Despite the inherent limitations of consciousness research, the Cartesian split that quarantined private experiences from science, Mason and plenty of other researchers like her believe that rigorous experimental designs can convincingly approximate consciousness. “We don’t have a perfect marker of being sentient,” she acknowledged. “We’re never going to know for sure. And the trick is to try and close that inferential gap so it’s as small as it can possibly be, so that really you have to do elaborate backflips to argue that an animal isn’t sentient.”

On a personal, human level, Mason said, she has felt the pull of recognition and connection to nonhuman animals, including fish. “Watching free-living fish in the wild on a reef, I think you can immediately be convinced that of course they’re conscious,” she said. “But the question is: Is that feeling tapping into something real, or are we just tricking ourselves?”

Here is another way of thinking about fish. Although there’s a common misconception that fish are primitive, in evolutionary terms — because humans, along with every living animal species, are descended from aquatic ancestors — the fishes alive today are nothing like the fish we’re descended from. Fish have not stood still but have continued to evolve and speciate, just as the lineage that would become humans did. “There has been ample time for fishes to evolve complex and diverse behavioural patterns as well as the cognitive hardware that goes with it to match the diversity of ecological niches they occupy,” wrote Australia-based ecologist Culum Brown.

Part of the confusion stems from the very language we use. To even talk about “fish” as one undifferentiated mass impoverishes our understanding of them; it would be as sweeping as lumping all land animals into one category. There are just as many (and possibly more) fish species as there are land vertebrates, and at least as much diversity among fishes as there is between toads, ravens, lions, and gorillas.

Fishes are, to state the obvious, very different from us. But there’s no reason to assume that evolving to live in the watery realm that covers most of the Earth has necessarily made them less conscious or cognitively complex. Cleaner wrasse fish can recognize themselves in a mirror, a finding that Mason calls “amazing and mind-blowing — it gives you goosebumps.” As Vox contributor Garrison Lovely wrote, “guppies have friends, salmon probably jump for fun, monogamous convict cichlid fish exhibit more pessimistic behavior after a breakup, and Japanese puffer fish make flirtatious art.”

Franks suggests there are more productive routes than trying to get to the bottom of the consciousness question. “We don’t even understand how this works in humans,” she said. Rather than ask “is there a light on or not?” inside fishes’ minds, we might ask what different species of fishes want, what motivates them, and what interests them by observing their behavior and simulating the actual contexts in which they live.

“The more that we can fill that out in its rich complexity rather than on one single narrow track, the better it is for fish and for our understanding,” Franks said. It might also make the general public more likely to empathize with fish than do depressing, undignified experiments where they’re repeatedly wounded or made to flee electric shocks.

Mason, too, suspects that if we’re trying to figure out our ethical duties to fish, or to any animal, pain may be too narrow a question. “You could have no ability to feel pain but still a completely different type of sentience,” she said, like an animal that feels terrible in the presence of a strong magnetic field. It’s possible to imagine a species that “can’t feel pain, but can feel terror or other negative affective states that we humans can’t imagine.”

We’ll maybe (probably) never know what it’s like to be a fish. Which is kind of sad, but there’s something thrilling about it, too. The countless ways of experiencing the universe, far beyond our terrestrial comprehension, is our planet’s grandest mystery. If consciousness is unknowable, then we must decide, rather than determine, who ought to be treated as if they can feel. That leaves us to do what humans already do so well — to see a life unlike our own and recognize a fellow subject.