It may be time to offer Dory an apology… and perhaps quit our badmouthing of the goldfish, because when it comes to matters of memory, new findings would suggest that an elephantfish – and indeed many other types of wild fish – may never forget.

In a first-of-its-kind study into the cognitive behaviour of fish living in the sea, researchers from the Max Planck Institute of Animal Behaviour – a research facility with headquarters in Germany – have found that wild fish can recognise individual humans.

Not only that, but fish can even be savvy enough to favour the individual humans they know are going to feed them; lending credence to the very possibility that fish can have differentiated relationships with specific humans.

But let’s take a step back. How on earth did we get here?

This study has its roots in a very singular problem experienced – for many years – by scientific divers at a research station in the Mediterranean Sea. That problem was, that at some point in every field season, local fish would follow them and steal food intended as experimental rewards. Nothing out of the ordinary, so far. Fish like food.

But these scientists noticed something intriguing: these wild fish actually appeared to recognise the specific diver who had previously carried food, choosing to follow only them while ignoring the other divers.

Looking to discern fact from fiction in this fishy tale, a team from the Max Planck Institute of Animal Behaviour in Germany set out to conduct a series of experiments while wearing a range of diving gear, to decide once and for all whether fish in the wild had the cognitive ability to discriminate among humans all based solely on external visual cues.

Put simply, are fish capable of telling people apart?

Overall, there has been little scientific evidence to suggest that fish can recognise humans at all. One captive-bred species, the archerfish, was famously able to recognise computer-generated images of human faces in laboratory experiments.

“But nobody has ever asked whether wild fish have the capacity, or indeed motivation, to recognise us when we enter their underwater world,” said Maëlan Tomasek, a doctoral student at the Max Planck Institute of Animal Behaviour and the University of Clermont Auvergne, France.

To make their discovery, the research team conducted their study eight metres 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,” said Katinka Soller, a bachelor student from the Max Planck Institute and co-first author on the study.

The first experimental phase – the training – tested if fish could learn to follow an individual 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 metres. Over time, she 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 metres.

Of the dozens of fish species inhabiting the marine station, two species of seabream in particular willingly engaged in the training sessions. A fish best known to us as the fish we buy to eat, the curiosity and “willingness to learn” scientists witnessed the seabream express took them by quite the surprise.

“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 researchers concluded that not only were the bream learning to follow her, but it was the same individuals that were showing up “day after day to join the lessons”.

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

Thus commenced phase two of the experiment: the two-diver test.

This time Soller and Tomasek – whose dive gear differed slightly from hers, notably in some colourful parts of the wetsuit and fins – dived together; both starting at the same point before swimming in different directions. On the first day, the fish followed the divers equally.

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

But Tomasek never fed the fish who followed him, so by 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 six fish, finding that four 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,” said Tomasek. “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, this time wearing identical diving gear, the fish were unable to determine one from the other. For the pair, this was strong evidence the 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,” said Tomasek.

With more time, the authors say, fish might have learned to pay attention to subtler human features like their hair or hands, to distinguish the divers. 

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

The researchers suggest that with these findings – that wild fish can quickly learn to use specific cues to recognise individual human divers –  it stands to reason that many other fish species could recognise certain patterns to identify us. It’s this mechanism, after all, that is the foundation for special interactions between individuals, even across species.

Senior author, Alex Jordan, who leads a group at the Max Planck Institute, said: “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 recognise humans based on visual cues.

“I suppose the most surprising thing is that we would be surprised that they can. It suggests we might underestimate the capacities of our underwater cousins.”

Tomasek acknowledges just how odd it may seem to those from an outside perspective, sharing a bond with an animal like a fish; one that “sits so far from us on the evolutionary tree” that it could be hard to understand.

“But human-animal relationships can overcome millions of years of evolutionary distance if we bother to pay attention,” he said. “Now we know that they see us, it’s time for us to see them.”

The study, Wild fish use visual cues to recognise individual divers was published in the scientific journal, Biology Letters.

Click here for more from the Oceanographic Newsroom.