Wild fish and human divers: An unexpected bond

A new study has revealed that wild fish can recognize and follow specific human divers, distinguishing between individuals based on visual cues such as colors on wetsuits and gear. 

The findings suggest that fish, like some other animals, may form differentiated relationships with humans, challenging long-standing assumptions about their cognitive abilities.

The research was conducted by scientists at the Max Planck Institute of Animal Behavior (MPI-AB) in Germany. It builds on years of anecdotal observations from scientific divers who noticed that local fish in the Mediterranean would follow and steal food from the same divers, while ignoring others. 

“Nobody has ever asked whether wild fish have the capacity, or indeed motivation, to recognize us when we enter their underwater world,” said Maëlan Tomasek, a doctoral student at MPI-AB and the University of Clermont Auvergne, France.

Now, the researchers have confirmed that fish do recognize individual humans – and more than that, they remember which ones provide rewards.

Wild fish learn to identify divers

The study took place at a research site eight meters underwater in the Mediterranean Sea, where fish have become accustomed to the presence of scientists. Unlike lab experiments, the tests were conducted in the open sea, meaning fish were free to participate or leave at any time.

Katinka Soller, a bachelor student at MPI-AB and co-first author of the study, designed the training phase to test whether wild fish could learn to follow an individual diver. Wearing a bright red vest, she swam a set 50-meter route while feeding the fish. 

Over time, she gradually removed the red vest, concealed the food, and eventually wore only standard dive gear – yet the fish continued to follow her, anticipating a reward.

Two species of seabream were particularly eager participants. “Once I entered the water, it was a matter of seconds before I would see them swimming towards me, seemingly coming out of nowhere,” Soller said. 

She soon recognized certain individuals who returned day after day and even gave them names, such as “Bernie” and “Alfie.”

After 12 days of training, about 20 fish were consistently following Soller on her swims, proving that they had learned to associate her with food. This set the stage for the next phase of the experiment: testing whether they could distinguish her from another diver.

Presenting fish with a second diver 

To investigate whether the fish were following Soller specifically or simply any diver, the researchers introduced a second diver, Tomasek, who had different-colored gear. 

The two divers started at the same point and swam in opposite directions. On the first day, the fish split evenly between them, unsure of whom to follow. “You could see them struggling to decide who to chase,” Soller said.

However, only Soller provided food. By the second day, the fish overwhelmingly chose to follow her instead of Tomasek. To confirm that they were actively learning to recognize the divers, the researchers focused on six individual fish, four of which showed strong learning patterns over the experiment.

“This is a cool result because it shows that fish were not simply following Katinka out of habit or because other fish were there,” Tomasek explained. “They were conscious of both divers, testing each one, and learning that Katinka produced the reward at the end of the swim.”

How fish recognize human divers

To further investigate what visual cues the fish were using to distinguish between divers, the team repeated the test – this time with both divers wearing identical wetsuits and fins. With no distinguishing features, the fish could no longer tell them apart.

For the researchers, this was strong evidence that the fish were associating specific colors or patterns on the divers’ gear with food. 

“Almost all fish have color vision, so it is not surprising that the seabream learned to associate the correct diver based on patches of color on the body,” Tomasek said.

This ability is similar to how human divers recognize one another 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,” Soller noted. 

With more time, the researchers believe the fish might have learned to recognize subtler details, such as hand shapes or body movements.

“We already observed them approaching our faces and scrutinizing our bodies,” Soller added. “It was like they were studying us, not the other way around.”

Fish and human recognition

This study supports anecdotal accounts of animals recognizing individual humans but goes further by providing controlled experimental evidence in a completely natural environment. 

The ability to distinguish specific people suggests that fish may have more complex cognitive abilities than previously thought.

If wild fish can learn to recognize and follow certain individuals, it raises the possibility that pet fish – or even fish in other natural environments – use similar strategies to identify familiar humans. 

Finding that wild fish can quickly learn to use specific cues to recognize individual divers suggests that many other fish species – our pets included – are able to recognize certain patterns to identify us.

Senior author Alex Jordan, who leads a research group at MPI-AB, finds the results unsurprising. 

“It doesn’t come as 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 Jordan.  

“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.”

Tomasek noted that 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. 

“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.” 

The study was published in the journal Biology Letters.

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