Yes, people really do underestimate the cognitive abilities of fish.
Fish are much smarter than humans give them credit for. Consider, for example, the findings reported by a recent study conducted with wild fish in the Mediterranean Sea. This study was inspired by the local fishes who would chase after divers — and steal food that the divers intended to be experimental rewards. That’s not so surprising, but the unusual aspect of this behavior is that the fish would follow only the diver who previously had food, ignoring all other divers. It was almost as though the fish could actually recognize individual human divers and remember who gave them food. But could they?
There is some anecdotal evidence indicating that fish do recognize individual humans, but this evidence comes exclusively from captive aquarium fish (ref). No one knew whether wild fish were capable of recognizing individual humans and remembering their previous interactions with them.
“[N]obody has ever asked whether wild fish have the capacity, or indeed, motivation, to recognize us when we enter their underwater world,” observed the study’s co-lead author, veterinarian Maëlan Tomasek, a doctoral student at the Max Plank Institute of Animal Behavior and the University of Clermont Auvergne. Dr Tomasek is currently studying the evolution of cognitive abilities in wild fishes, particularly those in the wild cichlid fishes of Lake Tanganyika.
To better understand the situation in wild fishes, Dr Tomasek and collaborators designed a simple experiment determine whether wild fish could in fact distinguish between individual divers, and if so, what visual cues did they use to make this identification.
To investigate this, Dr Tomasek and collaborators asked whether wild fish could learn to follow one of two divers who had similar physical shapes, but otherwise differed little in their external appearance (Figure 1). The experiment was conducted in eight meters of open water at a research site in the Mediterranean Sea where the local wild fish are habituated to the presence of divers.
The study fishes were “willing volunteers who could come and go as they pleased,” according to the study’s other co-lead author, Katinka Soller, a bachelor’s student at the Max Planck Institute of Animal Behavior. At the beginning of the study, no fish were observed to spontaneously follow the divers, but after training for 12 days (23 dives), they did.
In the first experiment, Dr Tomasek and Ms Soller asked whether wild fish could discriminate between two divers wearing similar (but not identical) diving gear (Figure 1A).
Throughout the studies, one diver (Ms Soller) was always the correct choice, giving a food reward (small pieces of shrimp) to the following fish, whilst the other diver (Dr Tomasek) was always the incorrect choice. Both divers carried food in a closed zip-lock bag hidden in a pocket of their diving vests.
Dr Tomasek and Ms Soller began their experimental sessions by swimming to the designated starting point and hovering there for 3 min, after which they swam away at a 90-degree angle to each other until they were 50 meters (165 feet) apart. At this point, the fish would received a food treat — or not — depending on which diver they pursued.
On the first day of the experiment, the fish followed both divers equally.
“You could see them struggling to decide who to chase,” Ms Soller remarked. But the fish quickly wised up. On the second day of the experiment, the shoal of fishes following Ms Soller increased tremendously.
Interestingly, of the dozens of fish species that inhabit the marine station, two species of seabream were particularly curious and eager to participate in the training sessions — this despite the fact that humans often kill and eat seabream.
“Once I entered the water, it was a matter of seconds before I would see them swimming towards me, seemingly coming out of nowhere,” Ms Soller noted.
Not only did the seabream learn to follow her, but at the end of the 12-day training session, 20 or so individual fish consistently showed up. In fact, many of the same individuals showed up daily to join the experiment. On average, eight saddled seabream and five black seabream not only recognized Ms Soller, but they followed her. Probably equally amazing is that Ms Soller learned to recognize individual fish — so much so that she even gave them names.
“There was [saddled bream] Bernie with two shiny silver scales on the back and [black bream] Alfie who had a nip out of the tail fin,” Ms Soller said.
When Ms Soller and Dr Tomasek focused their attention on six individual fish amongst the large shoal of followers, they found that four of them showed strong positive learning curves over the course of the experiment (Figure 2).
“This is a cool result because it shows that fish were not simply following Katinka out of habit or because other fish were there,” Dr Tomasek explained. “They were conscious of both divers, testing each one and learning that Katinka produced the reward at the end of the swim.”
What were the fishes using to distinguish between the divers? Were they focusing on the colors of the divers’ wetsuits, or maybe some other physical feature? To answer this question, Ms Soller and Dr Tomasek repeated their experiment but this time, they wore identical wetsuits (Figure 1B). This time, the fishes were confused, unable to distinguish between them (Figure 3).
Were the colors of the divers’ gear what the fishes recognized?
“Almost all fish have color vision, so it is not surprising that the sea bream learned to associate the correct diver based on patches of color on the body,” Dr Tomasek replied.
Human divers do the same when underwater.
“Faces are distorted by diving masks, so we usually rely on differences between wetsuits, fins, or other parts of the gear to recognize each other,” Ms Soller explained.
Interestingly, Ms Soller and Dr Tomasek did notice the fish scrutinizing other parts of their bodies, particularly their hair, their faces and their hands-of-snacks.
“We already observed them approaching our faces and scrutinizing our bodies,” Ms Soller pointed out. “It was like they were studying us, not the other way around.”
As a lifelong aquarium-keeper myself, this study’s findings don’t surprise me, but does serve to reinforce what I’ve long suspected after interacting with and observing my own fish. This study suggests that individual fish can form a bond with individual humans.
This elegant study asked a simple question and used an experimental design that left little room for ambiguities. It suggests that fish can indeed recognize and remember individual humans, and provides a glimpse into what fish rely on as cues.
“It doesn’t come a shock to me that these animals, which navigate a complex world and interact with myriad different species every minute, can recognize humans based on visual cues,” said the study’s senior author, animal ethologist Alex Jordan, an Independent Research Group Leader at the Max Planck Institute of Animal Behavior. Professor Jordan’s expertise lies in understanding the evolution and adaptive value of behavior, and identifying the mechanisms that underlie behavior.
“I suppose the most surprising thing is that we would be surprised they can. It suggests we might underestimate the capacities of our underwater cousins,” Professor Jordan said.
“It might be strange to think about humans sharing a bond with an animal like a fish that sits so far from us on the evolutionary tree, that we don’t intuitively understand,” Dr Tomasek concluded. “But human-animal relationships can overcome millions of years of evolutionary distance if we bother to pay attention. Now we know that they see us, it’s time for us to see them.”
Source:
Maëlan Tomasek*, Katinka Soller* and Alex Jordan (2025) Wild fish use visual cues to recognise individual divers, Biology Letters 21(2) | doi:10.1098/rsbl.2024.0558
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