Environmental Biology of Fishes 63: 223–228, 2002. © 2002 Kluwer Academic Publishers. Printed in the Netherlands. How much does ultraviolet radiation contribute to the feeding performance of rainbow trout, Oncorhynchus mykiss, juveniles under natural illumination? Vanina Rocco a,b , Juan Pablo Barriga b , Horacio Zagarese a & Mariana Lozada a Centro Regional Universitario Bariloche, Universidad del Comahue, U.P. Universidad, 8400 Bariloche, Argentina (e-mail: vrocco@crub.uncoma.edu.ar) a Consejo Nacional de Investigaciones Cient´ ıficas y Tecnol´ ogicas (CONICET), Argentina b Agencia Nacional de Promoci´ on Cient´ ıfica y Tecnol´ ogica (ANPCyT), Argentina Received 22 September 2000 Accepted 6 June 2001 Key words: UV vision, planktivory, predation, prey detection Synopsis Most research on environmental effects of ultraviolet radiation (UVR) has focused on its potential negative conse- quences. However, natural UVR can also be beneficial to living organisms (e.g., vitamin D synthesis, UV vision, germicide activity). UV vision has been demonstrated in a variety of animals including several invertebrates and vertebrates. Juvenile rainbow trout, Oncorhynchus mykiss, has a retinal photoreceptor, which is sensitive to UVR between 360 and 370 nm. Among other functions, UV vision has been proposed to contribute to prey detection by enhancing the contrast between the prey and its background. We performed a series of feeding experiments with juvenile rainbow trout and several zooplankters as prey. The fish were allowed to feed either under full solar radiation, or under solar radiation from which the UV component had been removed using a long-pass cut off filter. We found that the removal of UV wavelengths had no effect on the number of prey eaten or on the preference for particular food items. This is contrary to published studies reporting prey detection enhancement mediated by UV vision in rainbow trout. This disparity in the results may be due to our use of natural radiation instead of artificial UV sources, in which the visible component is poorly represented. Although our results do not disproof the presence of UV vision in juvenile rainbow trout, they do cast doubts about its significance in enhancing feeding performance in a natural light environment. Introduction Most recent research on the environmental effects of ultraviolet radiation (UVR) has been stimulated by the realization of stratospheric ozone layer thinning (Farman et al. 1985), and consequent increases in UVB fluxes (Madronich 1994). Retrospectively, it seems rea- sonable that research in the early years had primar- ily focused on the negative effects of UVR (Worrest 1986, H¨ ader et al. 1995). However, natural UVR can also be beneficial to several living organisms. Perhaps, the most paradigmatic example of a beneficial role of UVR is the synthesis of vitamin D (Carpenter & Zhao 1999). Another positive function of UVR is related to vision: it has long been known that the longest wavelengths within the UV spectrum (UVA) contribute to orientation and pattern recognition in insects (von Frisch 1967, Wehner 1992). UV vision has also been demonstrated for various aquatic invertebrate (Smith & Macagno 1990, Martin et al. 2000) and vertebrate (Jacobs 1992) animals. In fish, UV vision has been pro- posed to help in orientation (Hawryshyn 1992) and prey detection by enhancing contrast between the prey and its background (Browman & Hawryshyn 1992, Loew et al. 1993, Browman et al. 1994, Losey et al. 1999). Juvenile rainbow trout, Oncorhynchus mykiss, has a retinal photoreceptor, which is sensitive to UVR between 360 and 370 nm. UV vision in juvenile trout