A new study by scientists from the Max Planck Institute of Animal Behaviour MPI-AB in Germany has found that wild fish can recognise individual divers based on external visual cues such as different coloured wetsuits or fins.
Beyond the occasional anecdote, there is little scientific evidence that fish can recognise humans. A study found that archerfish bred in captivity were able to recognise computer-generated images of human faces in laboratory experiments, and octopuses have also been found to differentiate between individual humans — but both of these studies were based on experiences in unnatural, laboratory-based conditions. The research team conducted the study eight meters underwater at a research site in the Mediterranean Sea where populations of wild fish have become habituated to the presence of scientists.
To test if the fish could learn to follow an individual diver, Soller fed them while swimming a distance of 50m and wearing a bright red vest over her wetsuit. Once the fish had begun to follow her she repeated the experiment but without the vest, and kept the food hidden until they had swum the full 50m along with her. From the dozens of fish species inhabiting the marine station, two species of seabream seemed particularly willing to engage with the experiment.
Not only that, but the same individual fish began to show up to follow her. After 12 days of training, some 20 fish were reliably following Soller on training swims and she could recognize several of them from their physical traits. Soller and Tomasek dived together wearing dive gear that was slightly different in terms of style and colouring, such as the different sets of fins pictured below.
Both divers started at the same point but swam in different directions, but the fish followed both divers equally. Soller, however, fed the fish that followed her, whereas Tomasek did not. On the second day, the number of fish following Soller increased significantly — with four of six named individuals exhibiting strong positive learning curves during the experiment. When Soller and Tomasek repeated the trials wearing identical diving gear, the fish were unable to discriminate between the two of them, strong evidence that fish had associated the differences in the dive gear — most likely the colours — with the individual divers.
