Modern scientific research has largely overturned the idea that fish are unintelligent creatures with short memories. For decades, the complexity of fish cognition was underestimated, but new studies reveal capabilities once thought exclusive to mammals and birds. The question of the “most intelligent fish” is complex, as intelligence is a collection of diverse cognitive abilities that vary by species. Answering this requires examining advanced behaviors like social strategy, problem-solving, and self-awareness in the aquatic environment.
Defining Aquatic Intelligence
Measuring intelligence in fish requires metrics focused on behaviors that maximize survival in their specific habitats. A primary measure is behavioral flexibility, the ability to adjust learned responses quickly when environmental conditions change. This adaptation demonstrates a higher level of cognitive processing than simple instinct or conditioning. Research distinguishes between short-term memory, which allows fish to recall recent events, and complex long-term memory, which enables species like goldfish to remember mazes or feeding routines for months.
Social cognition is another important factor, particularly in species that live in complex groups. This includes the ability to recognize individual conspecifics, understand group hierarchies, and cooperate with others. Scientists have also tested for advanced skills like numeracy, spatial learning, and inhibitory control, which is the capacity to suppress an impulsive but incorrect action. While highly intelligent marine life such as the octopus and cuttlefish often come up in this conversation, they are invertebrates and are not classified as true fish.
Leading Contenders for Cognitive Ability
Among the contenders for the highest cognitive ability, the bluestreak cleaner wrasse (Labroides dimidiatus) stands out for its sophisticated social behavior. This small reef fish operates “cleaning stations,” removing parasites from much larger “client” fish, a process that requires managing a complex social network. Cleaner wrasse must remember individual clients, some of whom are predators, and adjust their behavior based on the client’s identity. This social memory and ability to manage complex relationships suggests a high level of individual recognition and strategic thinking.
The Cichlid family, particularly species from the African Great Lakes, also exhibits remarkable intelligence, driven by their complex social structures and demanding parental care strategies. Many cichlids display advanced problem-solving, such as learning complex maze patterns, and exhibit cognitive flexibility when their food sources are altered. Certain species are known for cooperative defense of their young and territory, requiring coordinated effort and communication among group members. Cichlids show a capacity for rapid learning and behavioral adaptation.
Demonstrations of Fish Cognition
The most compelling evidence of high-level cognition in fish came when the cleaner wrasse became the first fish species to pass the mirror self-recognition test, a benchmark previously passed only by a few mammals and birds. When scientists placed a colored mark on the fish’s throat that was only visible in a mirror, the wrasse would repeatedly scrape its body against a hard surface to try and remove the mark. This self-directed behavior suggests the fish recognizes the reflection as its own body with a foreign object, challenging long-held assumptions about self-awareness in non-mammalian species. More recent studies show that the cleaner wrasse can also use the mirror to assess its own body size before deciding whether to attack a rival.
Other species have demonstrated problem-solving and tool use. The archerfish, for example, hunts insects by spitting a powerful jet of water to knock them from overhanging branches, a feat requiring a complex calculation of light refraction and trajectory. Furthermore, studies have shown that some cichlids and stingrays can perform basic arithmetic, distinguishing between quantities and completing simple addition and subtraction problems. These repeatable studies confirm that fish possess advanced cognitive abilities, making the cleaner wrasse and cichlids strong candidates for the title of “most intelligent fish.”