Diel vertical migration (DVM) is a widespread phenomenon in aquatic organisms, yet the adaptive significance of this behavior is still unclear. In particular, diel vertical migration by juvenile sockeye salmon (Oncorhynchus nerka) has received considerable attention. We studied how changes in the light environment affect juvenile sockeye DVM in Alaskan lakes through changes in foraging rates and predation risk. Using hydroacoustics to track temporal changes in fish distribution, we found clear patterns of DVM and a strong, significant correlation between the mean depth of the sockeye and the amount of light at the lake surface. However, we could not detect diel changes in the depth distribution of fish large enough to be sockeye predators. Given a lack of diel vertical migration in the zooplankton community, it appears that juvenile sockeye were not simply tracking their food supply. Calculations of the in situ light experienced by individual juvenile sockeye suggested that they migrate to maintain a constant light environment. This light environment allowed the sockeye to exploit an antipredation window whereby they could forage on zooplankton while reducing the odds of visual detection by their predators. Furthermore, this antipredation window was continuous in early summer, but was split into two discrete time periods during the crepuscular hours in late summer. These data support the hypothesis that changes in habitat use by juvenile sockeye salmon reflect a dynamic strategy to minimize the ratio of predation risk to foraging gain that changes dramatically over the course of diel cycles in pelagic ecosystems.
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