Wild fish identify human divers based on gear colors

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

The same individual fish were also showing up day after day to join in with the lessons. Soller also gave a few of them names.

“There was Bernie with two shiny silver scales on the back and Alfie who had a nip out of the tail fin,” she says.

After a training period of 12 days, about 20 fish were reliably following Soller on training swims. She could also recognize several of them from physical traits–like the missing piece of tail fin on Alfie. Identifying the individual fish participating in the experiment set the stage for the next experimental phase.

[ Related: New evidence suggests dogs may ‘picture’ objects in their minds, similarly to people. ]

Upping the ante

Next, it was time to see if these same fish could tell Soller apart from another diver, Maëlan Tomasek, a study co-author and a doctoral student at the Max Planck Institute of Animal Behavior and the University of Clermont Auvergne in France.

Tomasek joined Soller for these dives, wearing a wetsuit and fins with more color. Both divers started at the same point in the water and then swam off in different directions.

On the first day, the fish followed both Tomasek and Soller equally.

“You could see them struggling to decide who to chase,” says Soller.

Tomasek never fed the fish who followed him. From the second day, the number of fish following Soller began to significantly increase.

To confirm that the fish were learning to recognize the correct diver, the team focused on six fish to study individually. Four out of the six showed strong positive learning curves 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 said in a statement. “They were conscious of both divers, testing each one and learning that Katinka produced the reward at the end of the swim.”

However, when Soller and Tomasek repeated the trials wearing identical diving gear, the fish were unable to tell them apart. According to the team, this indicates that the fish are associating the differences in the dive gear, most likely the colors, with each individual diver.

“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,” says Tomasek.

‘It’s time for us to see them’

Humans also do this when swimming underwater, since faces are distorted by diving masks. It’s the differences between fins, wetsuits, masks, and other diving gear that are often the best way to recognize someone.

With more time, the fish may learn to pay attention to more subtle human features, such as hands or hair. According to the team, observing that wild fish can quickly learn to use specific cues to recognize an individual human diver provides more evidence that other fish species can recognize certain patterns to identify us.