What Is the Smartest Fish in the World? What Lab Studies Show

No study ranks fish species on one task, so no fish holds a measured title as the smartest fish in the world. The best-supported candidates are the cleaner wrasse, the archerfish, the grouper and the cichlid, each tested on a different skill. The samples are small: 4 marked wrasse, 14 groupers, 8 goldfish. The goldfish results also sit badly with the 3-second memory myth.

The new finding is how little each famous fish result rests on, and what the goldfish study actually measured: a feeding time that came once in 24 hours. Below: six fish studies side by side, the freshwater and pet-fish evidence, and the myth set against the measurements.

What is the smartest fish in the world?

No one can say, because the studies use different tasks and different species. Here is what each famous fish result tested, with the catch behind it.

• Cleaner wrasse, mirror mark test: Kohda and colleagues (PLOS Biology, 2019) marked 4 fish under the throat. In front of a mirror, 3 of the 4 scraped the mark, and none did in the control conditions. Catch: 4 fish, and the editor's note says the paper received both positive and negative reviews
• Archerfish, learning by watching: Schuster and colleagues (Current Biology, 2006) report that archerfish learn to hit prey moving fast at height. Every fish in a group learned the skill by watching a group member shoot. Catch: a hunting skill, and the abstract gives no sample size
• Cichlid (Astatotilapia burtoni), reasoning about rank: Grosenick and colleagues (Nature, 2007) show that males watching fights between rivals later inferred the hierarchy, a form of transitive inference. Catch: one species in a lab tank
• Grouper and giant moray eel, joint hunting: Bshary and colleagues (PLoS Biology, 2006) followed 14 recognised groupers in the Red Sea. All 14 shook their heads at a moray at least once to signal, and in 58% of 120 observations the moray left its crevice. Catch: field observation, not an experiment
• Goldfish, timing: Gee and colleagues (1994) trained 8 goldfish to press a lever for food, then shrank the feeding window to 1 hour in every 24 for 4 weeks. The fish pressed in anticipation of feeding time. Catch: 8 fish in one study
• Goldfish, driving: Givon and colleagues (Behavioural Brain Research, 2022) trained goldfish to move a wheeled water tank towards a target on land. Catch: we could read only the abstract, which gives no animal count

Read down the list. Every result is strong evidence for one species on one skill. None of them puts two species on the same scale. So the smartest fish species has no measured answer.

How much can 4 fish tell you?

Not much on their own. A pass rate of 3 of 4 sounds strong. With so few animals, an exact 95% confidence interval runs from 19% to 99%. The data fit anything from about 1 wrasse in 5 passing to almost all of them (our arithmetic). The grouper study counts 120 observations, but they come from only 14 fish, so its real sample is closer to 14 than to 120.

A fair ranking needs one task given the same way to many species. Our smartest animals study has a self-control test for 39 species: 29 mammals and 10 birds, and no fish. Until someone runs a task like that on fish, any list of the smartest fish is a list of separate stories.

What is the smartest freshwater fish?

The smartest freshwater fish is unknown, because no study compares freshwater fish with each other. The freshwater evidence here is for cichlids and goldfish. The cichlid (a freshwater species) inferred rank from watching fights. Goldfish learned a daily timing task and steered a vehicle. The cleaner wrasse and the grouper live in the sea.

The sea has its own contenders. Our article on the smartest animal in the ocean sets fish beside dolphins and octopuses. Our research page on the smartest animals shows why one list never fits every measure.

What is the smartest pet fish?

The goldfish has the most evidence of any pet fish we found. It is the smartest pet fish only in the sense that it has been tested more than other tank species. The smartest aquarium fish, tested the same way, is unknown.

In the Gee study the goldfish worked a lever, and the feeding window shrank to 1 hour in every 24. In a second experiment the timing skill could develop under constant light, so a day and night signal was not the only cue. The Givon study adds the driving task. Both are lab tasks with trained animals, so they say what a goldfish can learn, not what a pet one will do. Test your own memory in our memory game.

Do goldfish really have a 3-second memory?

We found no study that measured a limit of 3 seconds. In the Gee study, the goldfish learned to respond in anticipation of a feeding window that opened once a day. A fish that expects food at a set time of day is tracking something that happens once in 24 hours.

That is our reading of the abstract, not a memory test. The study was about telling the time of day. It ran 4 weeks at the 1-hour setting, then 6 days with the food switched off, and the pattern of responding persisted for a limited number of days. The claim the data support is a daily rhythm learned over weeks, not a memory of months.

Can a fish recognise itself in a mirror?

Possibly, in one species, but experts disagree. In the Kohda study, 3 of 4 throat-marked cleaner wrasse scraped the mark in front of a mirror and not in the controls. The authors ask whether this shows self-recognition or another process, and the journal printed a companion Primer on how to read the result.

For other animal groups, see our articles on the smartest reptile, the smartest insect and the smartest bird, and for the primates, the smartest primate. The wider question is in the smartest thing in the world.

To see where your own reasoning sits on a scale built for people, take the IQ Metrics IQ test.

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