Under-ice diel vertical migrations of Oncorhynchus nerka and their zooplankton prey

We used stationary hydroacoustics and measures of environmental conditions (water temperature, dissolved oxygen, light levels, and zooplankton) to det ermine what factors, foraging or predation risk, influenced diel vertical m igrations of Oncorhynchus nerka during winter in three high-mountain lakes. The Sawtooth Valley lakes are deep, oligotrophic, glacial lakes located in central Idaho, U.S.A., and historically contained populations of anadromou s O. nerka. In general, low light intensities limited foraging opportunitie s of O. nerka under ice, especially at night. In Stanley Lake, O. nerka und erwent diel vertical migrations to exploit available light to feed. Oncorhy nchus nerka occupied shallow water at night, where there was still sufficie nt light to feed, but were found in deep water during the day. It is unknow n whether O. nerka occupied deep depths during the day to feed on high dens ities of zooplankton or to avoid predators. In Alturas Lake, O. nerka remai ned in the top 25 m both day and night to feed high densities of zooplankto n found near the surface. In Redfish Lake, O. nerka also showed little migr ation: O. nerka stayed in shallow water both day and night and occupied the same depths with the highest zooplankton densities. The vertical distribut ion of O. nerka during the winter appears to be determined by available foo d and light, but the deep daytime distribution found in Stanley Lake is sti ll unexplained.