How to train your fish

European researchers have made the startling claim that fish can recognise individual divers.

In a peer reviewed paper published today in Biology Letters, the research team from the Max Planck Institute of Animal Behavior (MPI-AB) in Germany conducted a series of experiments while wearing a range of diving gear, finding that “fish in the wild can discriminate among humans based on external visual cues.”

They said for years, scientific divers at a research station in the Mediterranean Sea had a problem: at some point in every field season, local fish would follow them and steal food intended as experimental rewards.

“Intriguingly these wild fish appeared to recognise the specific diver who had previously carried food, choosing to follow only them while ignoring other divers.

“And, more than that, they follow specific divers they know will reward them. This finding, published in Biology Letters, lends credence to the possibility that fish can have differentiated relationships with specific humans.”

The paper describes the research and how it concluded that fish have these never before considered abilities.

Three videos from the study showing: fish following a diver during Experiment 2; both divers departing from the starting point during Experiment 1; and fish spontaneously following a diver in 2019, which inspired the study. Credit: Tomasek, Soller, Jordan (2025) Biology Letters.

“The research team conducted the study 8m underwater at a research site in the Mediterranean Sea where populations of wild fish have become habituated to the presence of scientists. Their experiments took place in open water and fish participated in trials as ‘willing volunteers who could come and go as they pleased,’” explains Katinka Soller, a bachelor student from MPI-AB who was co-first author on the study with Maëlan Tomasek.

The first experimental phase—the training—tested if fish could learn to follow an individual diver. The training diver, Soller, started by trying to attract the attention of local fish; she wore a bright red vest and fed fish while swimming a length of 50 meters. Over time, Soller removed the conspicuous cues until she wore plain dive gear, kept the food hidden, and fed fish only after they had followed her the full 50 meters.

“Of dozens of fish species inhabiting the marine station, 2 species of seabream in particular willingly engaged in the training sessions. Sea bream are best known to us as fish that we buy to eat, yet they surprised the scientists by their curiosity and willingness to learn.

“Once I entered the water, it was a matter of seconds before I would see them swimming towards me, seemingly coming out of nowhere,” says Soller. Not only were bream learning to follow her, but the same individuals were showing up day after day to join the lessons. Soller even took to giving them names: “There was Bernie with 2 shiny silver scales on the back and Alfie who had a nip out of the tail fin,” she says.

Soller feeding fish during training. Credit: Tomasek, Soller, Jordan (2025) Biology Letters.

After 12 days of training, roughly 20 fish were reliably following Soller on training swims and she could recognise several of them from physical traits. By identifying individual fish participating in the experiment, the stage was set for the next experimental phase: testing if these same fish could tell Soller apart from another diver – Tomasek.

Both divers started at the same point and then swam in different directions. On the first day, the fish followed both divers equally. “You could see them struggling to decide who to chase,” says Soller.

But Tomasek never fed the fish who followed him. So from the second day, the number of fish following Soller increased significantly. To confirm that fish were learning to recognise the correct diver, the researchers focused on 6 fish out of the large group to study individually, finding that 4 of these 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,” says Tomasek. “They were conscious of both divers, testing each one and learning that Katinka produced the reward at the end of the swim.”

But when Soller and Tomasek repeated the trials, this time wearing identical diving gear, the fish were unable to discriminate them. For the scientists, this was strong evidence that fish had associated the differences in the dive gear, most likely the colours, with each diver.

“Almost all fish have colour vision, so it is not surprising that the sea bream learned to associate the correct diver based on patches of colour on the body,” says Tomasek.

Senior author Alex Jordan, who leads a group at MPI-AB, says: “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 recognise humans based on visual cues. 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.”

Adds Tomasek: “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 survey surprised Nadine Huth, the operations manager at Townsville’s Adrenalin Dive, who has more than 20 years diving experience all over the world.

“I’ve never had a fish recognise me,” Huth told Cosmos. “Fish are curious and I think they can recognise behaviour.”

She says fish learn to associate divers with food, interaction, or threats – in which case they may hide behind rocks.

“For example in marine protected areas where feeding is allowed, certain species become accustomed to specific divers who regularly provide food.

“Conversely, fish that have been speared or chased by divers may learn to avoid people wearing certain gear or behaving in a specific way.”

Huth says fishing and feeding is not allowed in the areas on the Great Barrier Reef where she trains divers.