Most people wouldn’t guess ‘fish’ when asked about intelligence in animals, but new research in the journal Biology Letters shows that fish might be smarter than many of us think. 

Previous lab studies have shown that captive archerfish, can recognise human faces in controlled settings, however there is little evidence that wild fish can do the same. 

To learn more about wild fish, researchers from the Max Planck Institute of Animal Behaviour ran experiments to test whether they could distinguish between different humans based solely on visual recognition. 

They started by getting a diver to attract the attention of local fish 8 metres underwater in the Mediterranean sea. The diver wore a bright red vest and fed the fish while swimming in a straight line for 50 metres.  

Every day the diver repeated this process, but started to remove some of the visual cues on their dive gear like the red vest. They also stopped feeding the fish continually during the 50m swim, instead waiting until they had swam the whole 50m with them until feeding them. 

Two species of wild sea bream willingly engaged in the swimming task and after 12 days of training, around 20 fish would reliably follow the diver for 50m in order to get some food at the end. 

In the next part of the experiment, two divers entered the water, with the new diver wearing a different coloured wetsuit and fins.  The divers swam 50m in opposite directions and on the first day the fish were confused as to which diver to follow. At the end of the 50m swim, only the original diver fed the fish and by day two the fish ignored the new diver and followed the original diver who fed them at the end.  

To see how the fish were recognising the original diver as the deliverer of food, the next part of the experiment involved both divers wearing exactly the same colour and style of dive gear. This confused the fish and they didn’t know which diver to follow which suggests that wild fish can quickly learn to use specific cues like colour to recognise individual human divers and is a finding that challenges long-held assumptions about fish cognition. 

This study not only sheds light on the sophisticated cognitive abilities of fish but also prompts a re-evaluation of how we perceive and interact with marine life. 

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