Wild Fish Can Recognize Humans

On occasion, Undercurrent readers have written in their reader reports that when they return to a spot to dive, the fish seem to recognize them. While one might get a little suspicious of divers who claim a fish recognizes them a year later, if you dive the same spot for a few days, you may be right.

For years, scientists at the Max Planck Harvard Mediterranean Research Center noticed fish would follow specific divers, usually those who had previously fed them, while ignoring others. The fish didn't just beg indiscriminately; they seemed to recognize individual humans. To test this, researchers conducted controlled experiments to determine if the fish actually differentiated between humans based on visual cues. "Do wild fish have the capacity or motivation to recognize us?" asked Maëlan Tomasek, a doctoral student at the Institute.

A Willing School of Subjects

The experiments took place 26 feet underwater, where the free-swimming fish were accustomed to divers. During the initial training phase, assistant Katrinka Soller wore a bright red vest and swam a 164-foot path while feeding any fish that followed. Over 12 days, she gradually removed visible cues like the vest and hid the food, rewarding fish only if they swam the whole distance behind her. Two seabreams, common in the area, emerged as the most engaged participants, returning day after day.

Ultimately, about 20 fish consistently followed Soller during swims, and she learned to recognize some of them individually. With this relationship established, the researchers moved to the next phase: testing whether the fish could tell Soller apart from another diver.

The Two-Diver Test

Soller and Tomasek entered the water together, wearing different variations in wetsuit colors and fins, and swam in opposite directions. On the first day, the fish split between the two divers. But by the second day, many followed Soller exclusively, as Tomasek never fed them.

To confirm learning, the team tracked six specific fish and found four clearly preferred Soller over time. "They weren't just following out of habit," says Tomasek. "They were conscious of both divers, testing each one and learning that Katrinka produced the reward at the end of the swim."

However, when the same test was repeated with both divers wearing identical gear, the fish could no longer tell them apart. This indicated that the fish relied on visual details--likely color patches or gear design -- to recognize individuals. "Fish have color vision, so it makes sense they'd use it to tell us apart," says Tomasek.

Fish Studying Humans

Interestingly, this visual recognition mirrors how divers identify each other underwater. Because masks distort facial features, divers often rely on wetsuits, fins, or gear to know who's who. The fish, it turns out, were doing exactly the same. "They would approach and scrutinize our faces and bodies, as if studying us," says Soller.

The findings support longstanding anecdotal claims that fish -- and potentially other wild animals -- can build specific relationships with people. It also hints at cognitive abilities we often overlook in fish. "These animals navigate complex environments and interact with many species. Of course they can learn who we are," says senior author Alex Jordan.

"It might seem strange to think about forming bonds with a fish," says Tomasek. "But this study shows that human-animal relationships can cross millions of years of evolutionary separation -- if we pay attention."

A synopsis of a study published in Behavioral Biology.