Diel vertical migration by juvenile sockeye salmon and zooplankton in a stained and a glacial lake

Abstract

Vertical distributions of both juvenile sockeye salmon (Oncorhynchus nerka) and zooplankton in a stained (Secchi depth 4.3 m) and a glacial (Secchi depth 0.2 m) lake in Alaska were measured at four times within a 24-h period. Stained-lake juveniles avoided surface during daylight, crossed large temperature gradients, and preyed upon cladoceran zooplankton >0.30 mm long. Glacial-lake juveniles preferred surface (<1.5 m deep) during the daylight, experienced little temperature change, and preyed upon copepod zooplankton >0.30 mm and terrestrial insects. Zooplankter depth distributions overlapped with day and night juvenile sockeye depth distributions in the glacial lake, but they overlapped only at night in the stained lake. The amplitude of juvenile sockeye vertical migration is positively correlated with euphotic zone depth. Sockeye vertical distribution is not related to absolute light intensity, zooplankton abundance or distribution, or thermal stratification. Most applicable are multiple factor explanations that incorporate predation risk and forage abundance for the juvenile sockeye.