Do Fish Have Feelings? What Science Knows, for Anglers

Close-up of a lake trout head under water, one eye turned toward the camera — Photo: Eric Engbretson, U.S. Fish and Wildlife Service (public domain)

Fish have something real that looks a lot like feelings: a netted fish pumps out stress hormones, and a fish that watched a friend get hooked avoids the bait. One small reef fish seems to know itself in a mirror. Whether a fish feels any of that the way you feel a fright, no experiment has answered. Scientists on both sides say so.

Carp that only watched a tankmate get hooked were about 55% less catchable afterward. A fish can learn to fear a hook it has never touched.

Do fish have feelings? Yes to states, unknown to feelings

The science on fish minds splits the word in two. “Feelings” has two halves. One half is an emotional state: a change in the body and brain that steers what the animal does next. Fear makes a fish hide. Stress makes it stop eating. Those states can be measured, and fish have them. The other half is the feeling itself, the inside view. Nobody can measure that in a fish, or in a dog. For fish it has to be inferred. Victoria Braithwaite and Culum Brown infer yes. James Rose and Brian Key infer no. Same evidence, opposite readings.

Do fish get scared?

Yes, by the body’s own measure. When a fish is chased, netted or crowded, the stress hormone cortisol floods its blood, along with sugar for a burst of energy. Cory Suski and colleagues sampled tournament largemouth bass within five minutes of weigh-in. Cortisol and glucose sat far above the levels in rested bass and in bass just caught and sampled at once.

A bold fear study came from Sonia Rey’s team in 2015. Six groups of 12 zebrafish lived in a tank with six chambers from cold to hot. Three groups were held in a small net for 15 minutes, then let go; the other three were left alone. For the next four hours the netted fish spent more time in the warm chambers, enough, the authors estimated, to raise their body temperature by about 2 to 4 °C (4 to 7 °F). Stressed mammals show the same rise. Rey called it “emotional fever” and wrote that it “removes a key argument for lack of consciousness in fishes.”

Brian Key and eleven colleagues answered in 2017. By their count of Rey’s own model, only about two more fish sat in the hottest chambers at any moment, not a significant shift. They added that the netted fish may simply have been avoiding the chamber where they were released, which could carry alarm scents from the netting. Rey’s team replied in the same issue. Neither side has settled it.

Do fish recognize themselves?

One species passes the test used on chimpanzees and toddlers. In the mirror mark test, an animal gets a mark it can only see in a mirror. If it tries to remove the mark, it has recognized the reflection as its own body. Masanori Kohda’s team gave the test to the cleaner wrasse, a finger-length reef fish that picks parasites off bigger fish. In 2019 the wrasse tried to scrape the mark off after checking the mirror. In the 2022 follow-up, all 14 new fish scraped a brown mark, and only with a mirror in the tank; across the studies, 17 of 18 fish have passed.

Kohda’s own caution matters as much as the result. His team writes that recognizing itself in a mirror does not mean a fish has other advanced mental abilities, and that self-awareness may differ in degree from one species to the next.

Do fish learn from each other?

They do, and the study was done on carp with a hook. Magnus Lovén Wallerius and colleagues in 2020 let some carp watch a tankmate get caught on a baited hook. The watchers were never hooked themselves. Afterward they avoided the bait, and their catchability fell by about 55% for a short time. Carp that had been caught once were harder still: 57% less catchable within two hours and still 62% less five to seven days later. Fear, in a fish, can be learned by watching. How long such lessons last is the story of fish brains and memory.

Do fish feel pain?

Trout have pain receptors. Lynne Sneddon, Victoria Braithwaite and Michael Gentle found 58 receptors on the face and head of rainbow trout in 2003, 22 of them nociceptors (18 also fired at acid). Acid on the lips made the fish rub them on the gravel and stop feeding for about 170 minutes; morphine sharply reduced the behavior. Rose and Key reply that a receptor is not a feeling. Only about 4% of the fibers in a trout’s face nerve, and 5% in a carp’s tail nerve, are the slow C-fibers that carry burning pain; in a human skin nerve it is about 83%. The pain evidence carries the full argument and the survival numbers by hook location.

What did the key studies find?

Study	What was tested	Result	The catch
Rey et al. 2015	36 zebrafish held in a net for 15 min, 36 left alone, all offered cold to hot chambers	Netted fish spent more time in warm water, an estimated 2–4 °C (4–7 °F) body-heat rise, “emotional fever”	Key et al. 2017: the shift was small, not significant, and may be chamber avoidance
Kohda et al. 2019 and 2022	Cleaner wrasse given the mirror mark test	14 of 14 new fish in 2022; 17 of 18 across the studies	The authors: passing does not imply other advanced abilities
Lovén Wallerius et al. 2020	Carp that watched a tankmate get hooked	About 55% less catchable, short term; hooked carp 57% after 2 hours, still 62% after 5–7 days	Shows learning, not what the carp felt
Sneddon, Braithwaite and Gentle 2003	Rainbow trout lips, acid and bee venom	22 nociceptors among 58 receptors; feeding stopped for about 170 min; morphine sharply reduced the behavior (Sneddon 2003)	Rose 2014, Key 2016: nociception is not pain
Suski et al. 2003	Tournament largemouth within 5 min of weigh-in	Cortisol and glucose far above controls; lactate about five times control levels, back to normal after 24 h (Suski et al. 2004)	Measures stress, not suffering

Why does a hooked fish fight?

Not because it is angry. A fish on a line is running from something it cannot see, at full power, with a mouth it cannot open. Every run is an escape attempt, powered by white muscle that works without oxygen and piles up lactate, the same by-product that builds up in your legs in a sprint. The fish is not fighting you; it does not know you are there. The fight ends when the fish is spent, not when it gives up. That is why the length of the fight decides what happens next.

What does the stress do after you let a fish go?

The bass in Suski’s study tell the story. Cortisol and glucose are up within minutes, and lactate sits at about five times control levels. In a 2004 follow-up, the same team found lactate back at control levels in bass given 24 hours to recover. Until then, that fish is a runner after a sprint, still paying back what the fight cost.

Air makes it worse. In one experiment, rainbow trout were exercised to exhaustion and then held out of the water. With no air, 88% survived. After 30 seconds of air, 62%. After 60 seconds, 28%. The review by Cook, Lennox, Hinch and Cooke is cited for a limit of about 10 seconds. A Mississippi State University study funded by ten state fisheries agencies, reported in AFTCO’s Bass Care 101, found that largemouth bass fought for one minute survived better than bass fought for three.

So whatever a fish feels, the practical answer is the same: short fight, short air time, quick release. The catch and release numbers show what each step is worth by species.

How do you release a fish so it recovers?

Use tackle that ends the fight in about a minute. Match line and rod to the fish and set the drag firm. One minute of fight beat three in the Mississippi State study.
Unhook the fish in the water when you can. Ten seconds of air is the limit; count it out loud. Barbless hooks or crimped barbs come out faster.
Wet your hands and leave the towel in the bag. Slime is the fish’s skin protection, and a dry grip strips it. Hold a bass by the lower jaw, a trout with a hand under the belly, and never touch the eyes or gills.
Take the photo over the water, in one breath. Have the camera ready before the fish comes up. If it is not ready, the fish waits in the water, not on the bank.
Cut the line on a deep-hooked fish. Across 274 published estimates, one review found hook location was the biggest killer. A hook left in the throat beats a hook dug out of it.
Hold the fish upright, nose into the current, until it swims off. Hold it still; do not pump it back and forth. Water flowing over the gills does the work. Warm water slows recovery: 84 °F (29 °C) is the ceiling the Mississippi State study names for a livewell.

A fish that stresses, learns and remembers also rests, which is why fish sleep in their own way. The rest of the questions kids ask are in the fish facts collection.

Sources (18)

Show all 18 sources