Polarization vision seldom increases the sighting distance of silvery fish.

Although the function of polarization vision, the ability to discern the polarization characteristics of light, is well established in many terrestrial and benthic species, its purpose in pelagic species (squid and certain fish and crustaceans) is poorly understood [1]. A long-held hypothesis is that polarization vision in open water is used to break the mirror camouflage of silvery fish, as biological mirrors can change the polarization of reflected light [2,3]. Although, the addition of polarization information may increase the conspicuousness of silvery fish at close range, direct evidence that silvery fish - or indeed any pelagic animal - are visible at longer distances using polarization vision rather than using radiance (i.e. brightness) vision is lacking. Here we show, using in situ polarization imagery and a new visual detection model, that polarization vision does not in fact appear to allow viewers to see silvery fish at greater distances.