Do Fish Feel Pain? And How Do We Know?

fish swimming by an orange coral

For many of us, the answer to the question "do fish feel pain?" might seem obvious. But this question sparked heated debate in the scientific community for years, until relatively recently. However, about 15 years ago, a series of highly sophisticated experiments were published, and according to most biologists the results settle the debate once and for all: fish are capable of sensing and feeling pain just like birds and mammals are. Let's take a look at how scientists were finally able to prove that fish can feel pain.

Step 1: Evidence of pain in the body and the brain

For starters, scientists discovered that fish (like birds, reptiles, amphibians, and mammals) have many sensory receptors – and just like in the human body some are sensitive to touch, some to heat, and some to burning. These sensory receptors are particularly concentrated, like ours are, in sensitive areas like the face. The transmission of signals upon damage to the nervous system was also found to be very similar to our own.

Examining the activity of areas in the brain also revealed results very similar to those found in the brains of humans, dogs, birds, and frogs: scientists showed that eating activates mechanisms in a fish's brain associated with pleasure – and that, in contrast, electrifying fish activates areas of the brain known to be indicative of fear.

But fish don’t feel pain consciously like we do??

It seems that the existence of all the known components of the human pain system, from the sensory receptors to the processing areas in the brain, wasn’t enough to settle the debate. Why did nature (or God) build up this whole complex system of pain transmitters in the body and processors in the brain just like we have, if not to feel pain like we do? What is the likelihood that such overwhelmingly similar systems function completely differently? There were some scientists who were yet to be convinced. They suggested that these could be automatic receptors, perhaps representing a type of "unconscious pain". These claims were made mostly by researchers associated with the fishing industry, but their approach still represented standard scientific skepticism. Scientists are trained to be skeptical, after all.

blue fish hiding around green coral

Step 2: Behavioral experiments

In order to test whether fish were really suffering, scientists set out to conduct behavioral experiments on fish. In one experiment, they simply injected a caustic, pain-inducing substance (bee venom or acid) into the lips of fish, while a control group was injected with neutral substances – and then tested their reactions.

It became clear that:

  1. The fish that were exposed to the caustic substance lost interest in food (similar to us when we are in pain).
  2. Their breathing rate increased by a greater amount and for a longer time than fish in the control group.
  3. The level of cortisol increased for a much longer amount of time compared to the control group. This is the known distress hormone in their bodies and in ours.
  4. They exhibited irregular behavior (like we do when we’re in pain). This included side-to-side movements at the bottom of the tank and an attempt to rub the affected organ.

All behavioral evidence clearly indicates that fish feel pain.

fish swimming in a bright sea with more fish in the background

But are fish really aware of the pain??

But this time too, there were scientists who countered that perhaps this was merely evidence of basic physical reactions or behavioral reflexes of creatures without any conscious awareness. It must be admitted that this skepticism does seem more forced, but still there were scientists who refused to see this as scientific proof that fish are suffering.

Step 3: Experiments with painkillers

To disprove claims of basic unconscious reflexes, further behavioral experiments were conducted using painkillers. One experiment was based on two facts:

  • When we are consciously in pain, we concentrate on the pain and pay less attention to other things.
  • A fish in a normal state moves away from a foreign object placed into their tank and keeps distance until certain that this object does not present a threat.

Researchers discovered that when a foreign object was placed in front of a fish that had just been injected with acid, the fish did not move away from the object. The researchers concluded that the pain diverts the fish's attention away from the possible danger. To prove that their hypothesis was correct, the researchers repeated the experiment but this time injected the fish with morphine, a strong painkiller, in addition to the acid. The pain relief caused the fish to behave this time, as expected, in a more alert, cautious, and typical manner.

In another experiment, fish were first given the choice between an empty tank, or a tank with vegetation and rocks. It's not hard to guess which the most popular choice was, right? All the fish preferred the tank with vegetation and rocks, where they could hide and play. In the experiment, researchers injected the fish with a painful acid, and added painkillers to the empty tank. If fish were able to feel pain, the researchers expected them to choose the empty tank with the pain relief. What happened? As you may have guessed, the injected fish did all move straight to the tank with no plants, but with a painkiller.

So the experiments using painkillers all showed that fish feel pain, confirming the findings of the physical and behavioral experiments done before.

In fact, since these findings and the flood of evidence that fish are conscious and feeling creatures like us, depression and self-starvation in fish have also become legitimate areas of research. We’ve come a long way.

Conflict of interest

These and similar experiments have convinced most scientists that fish feel pain. It seems that today the only scientists who insist on doubting whether fish can feel and suffer are those receiving the funding for their research from the fishing industry. Fearing the application of animal welfare laws to fish, the fishing industry is happy to fund research that could cast doubt on the fact that fish can feel. Not unlike the oil and fossil fuel industries funding suspect studies questioning climate change. But for those of us with a real interest in understanding the world around us, it seems that the evidence is beyond clear.

close up on a big blue fish

Why is it important if fish feel pain?

Because when a creature can feel pain this obliges us to consider their suffering. In the famous words of the philosopher Jeremy Bentham:

“The question is not, can they reason? Nor, can they talk? But, can they suffer?”

It is for precisely this reason that it is also impossible to read these findings and ignore the cruelty involved in them. True, animal studies can be fascinating, but our curiosity cannot justify torturing and killing innocent creatures. Some would say that such studies are important to convince people that fish suffer, and thus reduce suffering overall. Unfortunately, the mounting evidence of the ability of fish to feel pain, fear, and even depression have not yet caused humanity to reduce the suffering inflicted on fish – or even to reduce the number of fish tortured and killed, a number that stands at trillions of fish every year.

So let's draw the only clear conclusion from all these experiments: fish are capable of feeling pain (and fear and suffering, and many other feelings, also positive of course). And it's unethical to cause suffering to someone else when it can easily be avoided. Fish are capable of suffering, so it is bad to hurt and mistreat them.

Let's let the fish swim and thrive in the ocean, and stop packing them into fish ponds and trawling them out of the ocean like they're objects. They are sentient beings just like us. When they are stabbed it hurts, just like it hurts us – and when they are taken from the water they are literally dying to breathe.

many fish trapped and crushed in a net

The bottom line

Do fish feel pain? Yes. Just like other animals do (vertebrates at least). Their bodies are similarly equipped with pain receptors, a nervous system for transmitting pain signals, pain processing areas in the brain, and hormonal mechanisms designed to regulate pain and deal with it. This, in addition to the abundant behavioral evidence for pain felt in fish outlined above.

What next?