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Rethink Fish

Fish are intelligent, sensitive creatures and like many other animals, they explore, travel, socialise, hunt and play. Some species care for their young and use tools as humans do. Fish are sentient animals capable of suffering and feeling pain.

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Most fish have highly developed senses with excellent taste, smell, hearing and colour vision 1. Until fairly recently, many people didn’t realise that fish were sentient or feel pain, and the mental abilities of fish were given limited attention by the scientific community. Now, recent discoveries open up a new world of understanding. Far more complex than we ever realised, fish live rich social lives: communicating; hunting cooperatively; and, in some cases, developing cultural traits 2.

Today, huge numbers of fish are reared in underwater factory farms. Just like on land, these farms are crowded, grim places where the animals suffer immensely. Fish are often killed inhumanely and many endure slow, painful deaths by asphyxiation, crushing or even being gutted alive.

Fish: clever and complex

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Recent experiments have demonstrated the advanced intelligence of fish. On certain mental tests, even tiny cleaner wrasse outperform many primate species including orangutans and chimpanzees using powerful memory and grasp of game theory to solve reef-based food puzzles3.

Very few animals are capable of recognizing themselves in the mirror. Animals that have been observed passing the so called ‘mirror test’ include chimps, dolphins, elephants, and some birds. Now – thanks to new research – we can add the wrasse to the list4. It seems that even the smallest of fish are self-conscious.

The male Japanese pufferfish creates an amazing two metre wide geometric nest in the sand. Working diligently for around a week, he fans the sand with his fins and makes it beautifully symmetrical. He does this to attract and impress females5.

Tusk fish are capable of using tools - crushing clam shells with rocks to get to the meat inside. They carefully select rocks of the right size and shape, and often transport them to other locations to use when foraging6.

Groupers - cooperative hunting between animals of different species is rare and impressive7. Groupers go to caves where they know their moray eel friend lives and use gestures to invite them out to hunt. The eels flush fish out of tight spots on the reef and then the grouper catches them in open water and they share the food. Recently, researchers discovered that groupers also perform the same behaviour with octopuses.

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The emotional lives of fish

Our understanding of the inner lives of aquatic animals is growing rapidly: fish are intelligent, emotional, and far more complex than we ever realised. Exploring the underwater world, we have discovered fascinating behaviours that stem from the same foundations as ours: a brain that can detect pain, process emotions and learn from mistakes, and a drive to survive. Both science and common sense tells us that fish are capable of feeling pain and emotions. Sadly, because of us, they suffer in their trillions each year.

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Fish brains are organized differently from that of mammals. However detailed studies suggest that fish feel pain8. More than this fish can lead complex, emotional lives and have as much right to enjoy their lives as any other animal.

The welfare of fish matters because they are sentient animals9. Evidence for sentience comes from an accumulation of studies testing their brain structure, learning abilities and behaviour. Observing fish in their natural habitats, we are continually surprised by fascinating actions that reveal sophisticated thinking. Problem solving, learning, cooperating, coordinated hunting, tool use, memory, deception. This impressive list just keeps growing.

Feeling pain

Although pain is an unpleasant experience, it helps keep us safe and is vital to our survival. It’s such a valuable phenomenon that it has evolved in multiple animal groups, including fish. Although fish have different bodies and brains to us, there is clear evidence to show their nervous systems are equipped to detect pain, their brains allow them to feel it, and their behaviour changes in response to pain.

Pain relief

Painkillers seem to relieve pain in fish just as they do in humans and other animals. In one experiment, trout that (sadly) had bee venom injected into their lips showed typical signs of pain. They stopped feeding, breathed heavily, rocked back and forth, and rubbed their mouths against the walls of their tank. They didn’t hide from predators and dominant fish became less aggressive - in other words, they appeared to be quite distracted by how they were feeling. When given pain-relieving drugs however, their appetites came back and they returned to their normal behaviour10.

Survival instincts

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Emotions help us survive. They are psychological processes that help animals to avoid danger and work towards desirable resources. For example, fear is really useful because it helps you avoid predators and dangerous situations. Pleasure reinforces things that are good for you and makes you try harder to get more of them.

How do you measure how a fish is feeling? It is a difficult entity to measure in any non-human animal. But we can measure changes in the body and the behaviour that are commonly associated with an emotion in humans, and use our common sense.


Learning to avoid danger is one important reason that animals feel fear. And fish are certainly capable of that. One study showed that if a light was turned on before a net was plunged into a tank, fish learnt to avoid it in future by escaping when the light went on11. In another experiment, fish learnt to avoid electric shocks by remembering where they had been shocked in the past and only swam in ‘safe’ areas of their tank12.


When fish are kept in poor living conditions, they can become ‘pessimistic’, in that they come to expect the worst from a new situation. On the contrary, fish kept in good living conditions become optimists. While we do not condone the following, scientists have used fish to develop new drugs to treat depression in humans. Talking about his work with fish, lead researcher Dr Pittman says “The neurochemistry is so similar that it’s scary”.

In his study, zebrafish are kept drunk on ethanol for two weeks, then their behaviour is observed after they are moved into a new, alcohol-free tank. After a few minutes near the bottom of a new tank, zebrafish usually start to explore their new environment, but those that were withdrawing from alcohol lingered at the bottom. When they were given anti-depression drugs however, they acted normally and moved around the tank13 .

Non-human animals can’t tell us how they are feeling, so it’s impossible to know if they feel depressed in the way we do. But researchers working with ‘depressed’ zebrafish say they lose interest in just about everything: food, toys, exploration.


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Fish have the hormone oxytocin (or its equivalent, isotocin), which is associated with pleasure in humans and other animals.


To find out more about fish sentience, download our briefing


Farming Fish: Underwater Factory Farms

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Although hidden from sight, underwater factory farms harm animals and the environment just like factory farms on land. Fish are crammed into confined spaces where their natural behaviours are restricted. In many cases the water becomes dirty and polluted and the fish are prone to disease, infection and parasites. To treat this, farmers use chemicals and antibiotics, which can be harmful to the marine environment, those who consume the fish, as well as the welfare of the fish themselves. Given the immense scale of fish farming and the trajectories for future growth, we see this as an enormous animal welfare issue that must be tackled.

Lives of abject misery

Currently, billions of farmed fish are living lives of abject misery. In aquaculture, fish are farmed in confined areas and grown to size before being killed and sold. The majority of farming systems are designed with economics in mind, rather than the needs of the fish. Tanks and cages do not adequately meet the needs of fish, as they severely restrict the animal’s natural behaviours. Most farmed fish are reared in what can essentially be considered low-welfare, underwater factory farms. Salmon, while around 75cm long, can be given the space equivalent of just a bathtub of water each.

Overcrowded fish are more susceptible to disease and suffer more stress, aggression, and physical injuries such as fin damage. Along with lack of space, overcrowding can also lead to poor water quality, so the fish have less oxygen to breathe.

The behavioural requirements of most of the fish species used in aquaculture are poorly understood. It is unlikely that the conditions in intensive farming meet even the basic needs of fish. For example, rearing fish in cages prevents their natural swimming behaviour. Salmon are migratory, and would naturally swim great distances at sea14. Instead, they swim in circles around the cage, rubbing against the mesh and each other.

These days, over half the fish that people consume comes from aquaculture – mostly industrial, underwater, factory farms. In 2015, in Europe alone, approximately 1 billion fish were produced in underwater cages15.

Good animal welfare depends on taking care of three important components16: physical well-being, mental well-being and natural living.

In intensive fish farms, all three of these are neglected17 as fish are kept in overcrowded and barren conditions, and are subject to stressful handling procedures and inhumane slaughter methods.

We carried out an undercover investigation and found thousands of fish crammed into a very limited space, with nothing to do but swim listlessly in circles. The fish could not carry out their natural behaviours and showed signs of intense suffering.


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Fish are the most utilised animals on Earth. The numbers are vast. It’s hard to know exactly how many individuals are killed globally, because there are too many to count. But based on how much they weigh, it’s estimated that between one and three trillion (a thousand billion)18 fish are killed for food every year. Only a fraction of these fish are killed humanely.

Globally, most of the fish people consume has been killed inhumanely. This means that the suffering was severe and prolonged. On many fish farms, the routine slaughter method is simply to yank the fish out the water they need to breathe so they slowly “drown” in air (asphyxiation). This method can take over an hour for the distressed animals to die as they fight for their lives, trying to breathe. In the worst cases, fish may be processed (e.g. gutted) while still alive.

Other methods used to kill fish are also inhumane, for example: submersion in a mixture of ice and water; exposure to carbon dioxide and bleeding , among others. These cause considerable pain and fear. Violent struggles demonstrate attempts to escape as they fight for their lives. Suffering can be extremely prolonged. Many fish remain conscious and aware of pain for several minutes after having their gills cut and being left to bleed out.

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Sea bass and sea bream are farmed in large numbers in Europe. It is routine on these fish farms to dump fish into “ice slurry” to die—a practice so cruel the World Organisation for Animal Health has called for it to end. Being dumped in ice slurry slows down the animal’s metabolic rate so it may be kept alive for longer and the suffering is more drawn out. As our undercover footage shows some of these fish are still alive, taking their last breaths and still fighting for their survival as they are packed into boxes to be sent to market – a scenario that would be completely intolerable for any other animal that people eat.

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Humane methods of slaughter exist – British and Norwegian farmed fish are generally stunned before killing - but without legislation in place, most fish operations prefer to keep their costs down and do not buy the relevant equipment to enable a quick death.

For more information about farmed fish slaughter in the EU, read our new report.

Feeding fish to fish

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We face a global crisis of over fishing. Without massive new marine conservation areas and industry regulation, we may see severe and irreparable damage to ocean habitats and biodiversity. Fish farming is responsible for much of the industrial fishing of our decimated oceans. Certain widely farmed species such as trout and salmon are carnivorous. This means that, in the wild, they hunt other fish. In order to farm these animals, they are fed fish feed that is made of wild caught fish. Approximately one quarter of all wild-caught fish are used to make fish feed. This comprises of somewhere between 450 billion and 1 trillion individual fish. In other words, it can take up to 350 wild caught fish to raise a single farmed salmon, so fish farming actually increases the pressure on wild stocks. Since these wild fish die without any form of humane slaughter, the welfare cost of the salmons’ feed is also massive.

report cover of Let The Seas Run Dry. Illustration of ship on sea waves with fish leaping from fish farm cage

For more information about the impacts of using wild-caught fish to feed farmed fish, see our new report, Until The Seas Run Dry, produced in collaboration with Changing Markets Foundation:

Full report 

Scottish salmon

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Salmon farming in Scotland is big business, earning £765,000,000 in 2016. The industry as a whole - its sustainability, and the suffering of the salmon involved - are increasingly coming under the spotlight.

Several operations in Scotland have been recorded holding large numbers of sick fish in cages, in terrible states of torment. Parasite epidemics mean that the fish are being eaten alive by sea lice and have no means to move out of the polluted and congested waters. Salmon are migratory species and travel great distances (for example, some Scottish salmon migrate over 1000 miles out to Iceland19) but on factory farms they can be forced to swim in endless circles.

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Recent scenes of devastation being recorded on Scottish salmon farms and shared with the public are deeply disturbing and distressing, calling into question the viability of the whole industry. Some of the farms were found to be dirty, overcrowded and full of sick and deformed fish. Mortality rates on Scottish Salmon Farms have been increasing to disturbing levels - 10 million animals die during production each year. Recent undercover footage showed lorries dumping thousands of dead animals in enormous open mass graves near the sea.

In light of the terrible state of many salmon farms, Compassion, along with other organisations, is calling for a moratorium on the expansion of Scottish Salmon Farming.


Eat Less. And Better

In the wild, fish numbers are catastrophically plummeting due to over-fishing by humans. By some estimates, 90% of fish populations are fished at or over the maximum sustainable limits. Intensive farming of fish is not the answer. 

There is no doubt about it. The science has been done. The best diet is one with more plants. From the perspective of human health, environmental sustainability and, of course, animal welfare, everyone should be eating less meat and fish, and if they do eat animal products, they should eat ‘better’ – higher welfare - meat and fish.

There has been some confusing messaging from governments and retailers, encouraging consumers to eat more fish. Considering how devastated many marine ecosystems are, if this advice is widely followed there will soon be no fish in the sea. It is possible to get all the health benefits from consuming the right sort of plants – such as nuts, seeds and avocados. Certain fish are actually bad for your health if eaten too frequently. For example, tuna can contain high levels of mercury which can damage the brain20.

The farming of some species of fish, those which are carnivorous, such as salmon, trout, sea bass and sea bream is very problematic. These fish need to eat other fish. To grow one average salmon to maturity can mean up to 35021 smaller fish being caught in the wild and used as feed. All of these ‘feed fish’ deserve to have their welfare protected too, but currently they are being ignored by the fish farming industry. Almost all wild caught fish suffer a slow and excruciating death when pulled out of the water.

If buying farmed fish, it is better to eat species lower on the food chain that do not eat other fish, for example, carp and tilapia. This means that wild fish are not required to feed the farmed fish.

People who do choose to eat fish may want to look at both broad sustainability issues and animal welfare. Currently, it is virtually impossible to find “higher welfare” wild caught fish. Sustainability certification schemes, such as the Marine Stewardship Council (MSC) do not cover animal welfare22. Unless explicitly stated otherwise it is reasonable to assume that fish being sold in supermarkets, restaurants or other outlets could have suffered at some point--during rearing, capture or slaughter.

Unfortunately, as it stands, when it comes to farmed fish inhumane slaughter and welfare issues such as very high stocking densities remain common across the industry so it is hard to find higher welfare fish products. We encourage our supporters to engage with the brands and retailers they shop with and ask about their position on fish welfare. Organic labels, like the Soil Association, require certain minimum standards of welfare for farmed fish, however the issues of the ‘feed fish’ remain for carnivorous species. The main global schemes certifying aquaculture do not have strong enough requirements for fish welfare, for example many do not prohibit cruel slaughter methods. We are asking schemes to update their standards, which could improve the lives of billions of fish.

Find out more

Our references

  1. Brown, C. (2014). Fish intelligence, sentience and ethics. Animal Cognition.
  2. Brown, C. (2014). Fish intelligence, sentience and ethics. Animal Cognition.
  3. Salwiczek, L. H., Prétôt, L., Demarta, L., Proctor, D., Essler, J., Pinto, A. I., … Bshary, R. (2012). Adult Cleaner Wrasse Outperform Capuchin Monkeys, Chimpanzees and Orang-utans in a Complex Foraging Task Derived from Cleaner - Client Reef Fish Cooperation. PLoS ONE, 7
  4. Cleaner wrasse pass the mark test. What are the implications for consciousness and self-awareness testing in animals? Kohda, View ORCID ProfileHatta Takashi, Tmohiro Takeyama, Satoshi Awata, Hirokazu Tanaka, Jun-ya Asai, Alex Jordan doi:
  5. Kawase, H., Okata, Y., & Ito, K. (2013). Role of Huge Geometric Circular Structures in the Reproduction of a Marine Pufferfish. Scientific Reports, 3(1), 2106.
  6. Jones, A. M., Brown, C., & Gardner, S. (2011). Tool use in the tuskfish Choerodon schoenleinii? Coral Reefs, 30(3), 865.
  7. Interspecific Communicative and Coordinated Hunting between Groupers and Giant Moray Eels in the Red Sea Redouan Bshary , Andrea Hohner, Karim Ait-el-Djoudi, Hans Fricke Published: December 5, 2006
  8. Braithwaite, V., 2010. Do fish feel pain? Oxford University Press, New York.
  9. Brown, C. (2014). Fish intelligence, sentience and ethics. Animal Cognition.
  10. Sneddon, L. U. (2003). The evidence for pain in fish: The use of morphine as an analgesic. Applied Animal Behaviour Science, 83(2), 153–162.
  11. Yue, S., Moccia, R. ., & Duncan, I. J. . (2004). Investigating fear in domestic rainbow trout, Oncorhynchus mykiss, using an avoidance learning task. Applied Animal Behaviour Science, 87(3–4), 343–354.
  12. Dunlop, R., Millsopp, S., & Laming, P. (2006). Avoidance learning in goldfish (Carassius auratus) and trout (Oncorhynchus mykiss) and implications for pain perception. Applied Animal Behaviour Science, 97(2–4), 255–271.
  13. Pittman, J. T., & Ichikawa, K. M. (2013). iPhone® applications as versatile video tracking tools to analyse behaviour in zebrafish (Danio rerio). Pharmacology Biochemistry and Behaviour, 106, 137–142.
  14. Hansen, L. P., Jonsson, N., & Jonsson, B. (1993). Oceanic migration in homing Atlantic salmon. Animal Behaviour, 45(5), 927–941.
  15. FAO (Food and Agriculture Organisation of the United Nations). (2018). World Fisheries and Aquaculture. Aquaculture.
  16. Brambell Committee. Report of the Technical Committee to Enquire into the Welfare of Livestock Kept under Intensive Conditions; Command Paper 2836; Her Majesty’s Stationery Office: London, UK, 1965
  17. Ashley, P. J. (2007). Fish welfare: Current issues in aquaculture. Applied Animal Behaviour Science, 104(3–4), 199–235.
  19. Hansen, L. P., Jonsson, N., & Jonsson, B. (1993). Oceanic migration in homing Atlantic salmon. Animal Behaviour, 45(5), 927–941.
  20. Baishaw, S., Edwards, J., Daughtry, B., & Ross, K. (2007). Mercury in seafood: Mechanisms of accumulation and consequences for consumer health. Reviews on Environmental Health, 22(2), 91–114.
  21. The number of fish being fed to one factory farmed salmon varies, depending on the size of the wild fish caught and the composition of the fish feed itself. Our conservative calculations indicate that the number required to feed an average Scottish farmed salmon can vary between 150 to 350.
  22. In order to find out which wild fish can be bought without contributing to the decline of marine ecosystems we encourage our supporters to go the website of the Marine Conservation Society who have a detailed Good Fish Guide. As a rule, it is always better to buy local fish from a small, non-industrial operation.

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If you have any further questions regarding this, or any other matter, please get in touch with us at We aim to respond to all queries within two working days. However, due to the high volume of correspondence that we receive, it may occasionally take a little longer. Please do bear with us if this is the case. Alternatively, if your query is urgent, you can contact our Supporter Engagement Team on +44 (0)1483 521 953 (lines open Monday to Friday 9am to 5pm).