Can Fish Feel Pain?

The science of fish pain raises greater questions about the philosophy of consciousness and animal welfare

By Amy McDermott

The sun hung high and hot, beating down on my sunburned shoulders. Hornets hummed around old bait and drowned in soda-bottle bug traps. Gulls cried their shuddering craww craw and I peered off the pier. Seafoam green paint and old fish scales peeled from the salt-rotted wood and stuck to my palms. They smelled like vinegar, like rotting flotsam and tanned skin. The juniper-green sea sighed gently below, swirling around dark, mussel-covered pilings.

Rod in hand, I willed a fish to bite. Their dark silhouettes circled my gossamer line; soft nibbles rippled up its length like notes on a guitar string. Calico sea bass, Spanish mackerel, and opaleye cautiously inspected my bait, slipped it off the hook unscathed. They were dastardly; they were smart.

By noon, I was out for blood. A big opaleye—a dark shadow with two white spots across its back—circled my line, but wouldn’t commit. My breathing grew calm and quiet, the rod was steady in my hands. And then, a violent shake: the opaleye was mine!

I reeled it up, giddy at the thrill, until I saw the animal hanging from my line.

The opaleye was big indeed, large enough to make a meal, but it also looked scared. The spines along its dorsal fin stood defensively straight, its red gills flared, the hook was buried deep and bloody in its throat.

And then I was scared too, scared that this fish would die, scared that I would kill it.

Fear was unexpected. I had grown up fishing, been holding a rod since I was five years old, learned to gut and filet before I was ten. It wasn't my first time on the green pier; it wasn't even my first opaleye. But it was my first taste of piscine empathy, the first time I saw somebody on the end of my line.

I wrapped a yellow dishtowel around the opaleye's head, and smoothed its dorsal spines down along its back, then took a pair of needle nose pliers and wrested the hook from its soft, pink throat.

Running down the dock, cradling the opaleye in my yellow towel, I willed it not to die. And when I reached the beach, I plunged the towel into the ocean, and waited for the fish to move.

It didn’t. It was dead. I had killed it.

***

Over the years, I’ve told that story more than a few times. Guilt brings it to mind. I worry that the opaleye was in pain as it suffocated, that it was scared and suffering at the end. Most people reassure me that fish can’t feel pain, that I’m anthropomorphizing it more than I ought. But I wonder if that’s true. Can fish feel pain? Can they suffer?

A House Divided

Press your fingernail into your arm. Feel that shadow of pain? When you touch your skin, nerves at the point of contact send an electrical signal up your spine and into your brain, telling you that you’ve been injured. You don’t know that you’ve been hurt as the signal races up your back; you only feel pain once the impulse reaches your brain.

If you’ve ever touched a hot stove, for instance, you may have noticed that your hand reflexively recoiled from the heat moments before you realized that you had been burned. Why? Because signals racing up your spine triggered a reflexive reaction, before they reached your brain. Your spine responded to damage even before your brain informed you that you had been hurt.

Most animals share this unconscious, reflexive reaction. Injury is bad news for living cells, so even the simplest of creatures have evolved reflexes to avoid it. Even nematodes (tiny, brainless worms), will hurl themselves away from danger, because localized nerves in the threatened area of their body trigger a reflexive response.

A nematode does not know that it is hurt, however. It does not have a brain to tell it that it has been injured. As with the reflexive movement of your hand from that hot stovetop, the worm is responding unconsciously. This is the mechanical aspect of pain, the automated response of discomfort to injury. Without a brain, there is no way to feel it.

The feeling of discomfort, on the other hand, is completely psychological.

Awareness that you personally are in pain is a conscious experience. Soldiers, horribly injured on the battlefield, often won’t feel pain until hours after, because their brain is so distracted by the mission at hand. Conversely, people who are heartbroken or who suffer from depression often report physical pain as well as emotional sadness, despite actual injury.

For this reason, pain is officially defined both by discomfort, and by the awareness of that discomfort. The International Association for the Study of Pain defines pain as “an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage…It is unquestionably a sensation in a part or parts of the body, but it is also always unpleasant and therefore also an emotional experience." Pain is therefore both sensory and psychological.

The definition of pain is based on human experience. We can evaluate our subjective state through conversation. If you’re in pain you can tell your doctor, and rate your discomfort on a sliding scale. Of course, pain is relative, so the worst thing you’ve experienced might not feel that bad for somebody else. But overall we are able to use language to effectively convey our physical and emotional state.

The same cannot be said for fish. They can’t tell us how they feel, so we have to study their subjective experience indirectly. Since human behavior is often an important indicator of our emotions (and pain-response), researchers look to both physiological and behavioral cues in order to assess whether fish are experiencing pain in the same ways we do.

In order to feel pain as humans do, fish need the physical equipment: nerves that sense touch, temperature, and chemical damage, which stimulate electrical activity in the brain. But they also need a brain capable of self-awareness; a brain that knows that it, personally, is in pain. The kind of self-awareness that is unique to conscious, sentient life.

Researchers who study pain in fish are divided on the question of consciousness. They have broken into two major camps: those who think fish might be conscious, and those who think they’re definitely not.

In the last decade Dr. Lynne Sneddon, Director of Bioveterinary Sciences at the University of Liverpool and a leading voice in the “maybe” camp, has shown that rainbow trout have the physical equipment for an unconscious reaction to pain. Just like us, they have nerve cells in their skin that signal the brain in response to injury.

What’s more, Sneddon has also observed that injured trout changed their behavior. Fish whose lips had been injected with acid or venom rubbed their mouths against the walls of their aquaria for up to an hour and a half, avoiding feeding and other normal behaviors. In subsequent experiments trout chose morphine-dosed food over undosed food after injection. The logic is that an animal in pain will choose food dosed with morphine because the anesthetic feels better.

So fish respond to physical damage both mechanically and behaviorally, but the question of awareness is still on the table. Opponents of fish cognition are unconvinced that the behaviors demonstrated by Sneddon’s trout are evidence of sentience. Dr. James Rose, Professor Emeritus of Zoology at the University of Wyoming, is one prominent voice. He argues that unconscious, reflexive reactions to physical injury can be more elaborate than you might expect. Reflexes aren’t necessarily brief or simple, and behaviors that look like consciousness may not be. For instance, when the major brain regions associated with sentience were removed from laboratory rats, the rats still resisted receiving an injection. They bit at the syringe and licked the puncture site, although they were not mentally capable of consciousness.

So then, what is consciousness, and how do we recognize it?

Even in humans, it’s hard to say. Many neuroscientists believe that our sentience depends on a region of the brain called the frontoparietal neocortex. The whole neocortex forms the majority of that pink, wrinkled matter that gives our human brain its characteristic look. It's a well-developed outer sheath, wrapped around our more primitive, interior brain structures.

The neocortex is divided into four lobes (frontal, parietal, occipital, and temporal), each of which is associated with a different set of functions. Emotional memories, language, and socially-mediated choices all light up in the frontal lobe. The parietal lobe sits just behind the frontal neocortex, and integrates sensory information (especially touch) with recognition of an object or idea. For instance, if I traced my finger across your back in the shape of the letter E, your parietal neocortex would integrate the memory of my touch with the shape of the symbol. Although you would not see the movement of my hand, you would feel the shape I had drawn, and read the “E” by touch alone.

Although other, deeper areas of the brain, like the amygdalae (a pair of almond-shaped structures) work in concert with the neocortex, some neuroscientists believe that the cortex alone is crucial for conscious perception. The neocortex is unique to mammals, and is most well-developed in humans.

Teleost fish like trout are very far from us on the evolutionary tree: our last shared ancestor lived 450 million years ago. We are very distant, and so are our brains.

Fish don’t have a neocortex, which many researchers take as evidence that they are not sentient or self-aware. Dr. Rose and others have argued that even if fish do have the mechanics for unconscious perception of injury, they still can’t feel pain—they don’t have the brain for it.

Not everyone agrees that the neocortex is the only brain structure necessary for consciousness though. The amygdalae, for example, are also associated with emotions like fear and disgust. “It's your amygdala, which is a very primitive part of the brain, that is necessary for conscious perception of disgust and fear. So it's not the neocortex,” says Dr. Sneddon in her light Scottish burr over Skype earlier this month. We had sat down to discuss the prospect of pain in fish. “A lot of different areas of the brain are involved in the perception of different types of stimuli. I don't think there’s a specific seat of consciousness in the brain.”

What’s more, Sneddon may have found an analogous structure to our neocortex in the brains of trout. Using brain-imaging techniques, she has found that a region of the trout brain called the telencephalon appears to be a very rudimentary substrate of the human cortex. Human embryos also have a telencephalon, part of which develops into the cortex as we grow. If fish share this rudimentary structure, could they also share a form of our consciousness? “I don’t think we know enough about how the fish brain works to know whether it copes with everything that the neocortex copes with, but it could be,” explains Dr. Sneddon. “Certainly fish can operate at a very complicated level with their very small brain. They can navigate spatial mazes, they can remember where landmarks are, they can remember and recognize up to 47 different fish, and they can recognize their own odor distinct from other fishes' odor. So we think the possibility is there.”

The Right Question?

In short, consciousness is very difficult to define. It’s hard to know which animals are sentient, but the question matters, because animal welfare regulations hinge on the ability to feel pain. Some animals are considered conscious, while others aren't, and so are exempt from humane practices. In an effort to reduce individual suffering, regulations have been based on the assumption that some animals are sentient, and therefore can experience the subjective states of pain and fear, while others cannot.

In the US today, The Animal Welfare Act defines the legal treatment of animals. It doesn’t even mention fish. There are no federal laws regarding their humane treatment. 

You heard right: welfare definitions are based on the conscious perception of pain, although sentience is impossible to prove. The Animal Welfare Act aims “to assure the humane treatment of animals…to ensure that animal pain and distress are minimized." But “humans are the only organisms that we know with certainty are conscious” explains Dr. Rose. “I’m not excluding others from the consciousness club, just basing this statement on the fact that we have no unequivocal way of establishing consciousness in other species.” So if consciousness is impossible to prove, and welfare laws are based on consciousness, then why do some animals have legal protection, while others don’t?

Basing animal welfare on a concept that is elusive, and ultimately impossible to prove, is not the best foundation for legislation. It’s as subjective as consciousness itself. Luckily, there is no single definition of welfare; alternative approaches exist.

In recent years, function-based and behavior-based definitions have gained popularity in academic circles. Rather than interpreting subjective mental states, function-based definitions are based exclusively on animal physiology; an animal with well-functioning body systems that is in good health, and that has low stress levels is considered well cared for.

But physiology isn’t tremendously useful to interpret individual experience. Even in humans, we can’t know another person’s thoughts or feelings just by measuring their heart rate. A fast-beating heart could imply nervousness, but it could also imply excitement; it’s hard to observe any nuance beyond that.

Behavior, on the other hand, can provide insight on animal perspectives.

Mink, for example, will press themselves against a weighted door to access a bath on the other side. They won’t press the door if the bath is unavailable, suggesting that mink really like to swim. Behavior could offer new insight into animals' wants and aversions, and therefore into their humane treatment.

Although Drs. Sneddon and Rose disagree about the potential for consciousness in fish, they are unified in the belief that an animal’s behavior is indicative of its internal experience and should therefore shape welfare regulations. “I think the human definition of pain is just not useful when you apply it to animals,” says Dr. Sneddon, “if you learn to avoid something, it means that internally it was negative. It wasn't a positive experience or else you would keep doing it.” Dr. Rose echoes the same feelings, “There are clear, objectively identifiable behavioral indicators of what animals consider to be ‘good’ and ‘bad.’ Welfare standards can be based on those indicators without resorting to speculations… on presumed consciousness.” In the absence of words, an animal's actions should guide us.

The opaleye I reeled up onto the pier flopped and struggled in the yellow towel. Defensive spines stood sharp and upright along its back. It fought hard on the line, but was hooked too deep in the end. Based on its behavior, it is safe to say that the fish did not want to be caught. Perhaps it didn't experience pain, but it nonetheless resisted its fate. It gasped as it suffocated, oxygen-deprived in an ocean of air.

I’m not saying that fishing is inhumane, or that it should stop. I’m not saying we shouldn’t eat meat. But I do think that fish should be killed for food, not sport, because a hook in the throat is clearly not an experience they enjoy.

And when we do kill a fish, both Dr. Sneddon and Dr. Rose also agree that the most humane death is a quick one, a concussion followed by pithing. “A brisk blow to the head is lethal,” explained Dr. Rose; Dr. Sneddon concurred, adding that “fish have a layer of cartilage on the top of their head, and if you hit it really hard you basically either stun them or cause brain death. You should follow that with a pithing, which involves the insertion of an implement to make sure the brain is actually destroyed, or exsanguination, which means blood loss.” You catch the fish, and knock it unconscious immediately. You kill it with the speed and mercy every living thing deserves.

Pain and welfare go hand-in-hand for people. Our wellbeing is compromised when we hurt. The same is not true for animals; human definitions are too hard to apply to non-human brains. But behavior can give us clues into the subjective experience of animals, and researchers from both camps agree that it should define humane treatment. Ultimately, we can’t know what (if anything) a fish consciously feels. We can’t even know what another person feels. Maybe it shouldn’t matter.

Amy McDermott is a science writer and graduate student at Columbia University, where she studies marine conservation biology. She can be reached at agm2159@columbia.edu