UV-enhanced fish predation and the differential migration of zooplankton in response to UV radiation and fish

The intensity and spectral composition of visible light are known to influence fish predation on zooplankton. However, in clear-water systems, ultraviolet (UV) radiation (UVR) may also influence fish predation, directly through UV-enhanced foraging, indirectly through alterations in predator-prey overlap, or in a combination of the two. Here we test the hypothesis that UVR facilitates fish predation on zooplankton in an oligotrophic lake. Experiments were conducted in 2.2-m-long UV-transparent or UV-blocking columns suspended in the epilimnion. Zooplankton consumption by young-of-the-year (YOY) largemouth bass (Micropterus salmoides) was compared in the presence and absence of UVR, and the vertical distributions of prey were quantified across light and fish treatments. In addition, a separate experiment was conducted to examine zooplankton vertical responses to UV exposure vs. fish kairomones. Overall, YOY predation on zooplankton was higher in the presence than in the absence of UVR, particularly on diaptomid copepods. This result was only partially explained by zooplankton migratory behaviors, suggesting UV-enhanced searching capabilities in YOY bass. Furthermore, diaptomids displayed a stronger vertical behavioral response to fish kairomones than to UVR whereas Daphnia exhibited a stronger response to UVR.