Wild fish use visual cues to recognize individual divers

Many animal species have been shown to discriminate between individual humans in captive settings and may use a variety of cues to do so. Empirical evidence remains scarce for animals in the wild, however, particularly in aquatic contexts. For the first time, we investigated discrimination of individual humans by fish in the wild. We first trained two species of fish, saddled sea bream Oblada melanura and black sea bream Spondyliosoma cantharus, to follow a human diver to obtain a food reward. We then investigated whether they could discriminate between two human divers and follow the correct one in an operant-conditioning paradigm. We show that both species were able to quickly learn to discriminate between the two divers when they wore different diving gear. However, they showed no preference when both divers wore identical gear, suggesting that discrimination is based predominantly on visual cues from the dive gear. We discuss the implications of these results for ethical considerations and research practices.