The Diel Vertical Movements of Kokanee Salmon, Oncorhynchus nerka, in Granby Reservoir, Colorado

Abstract

Sonar equipment was used at Granby Reservoir, Colorado, 1963 to 1966, to correlate environmental factors with diel vertical movements of kokanee salmon introduced into the reservoir in 1952. Twenty-four hour operation of this equipment revealed that salmon schools were located throughout the upper 9 meters of water during the day, followed by migration and concentration at depths from 9 to 18 meters at night, where temperatures averaged 10–13 C. Light intensity at the lower water strata appeared to be correlated with upward movements of the schools at dawn. Zooplankton, utilized by kokanee salmon as food, was present in major concentrations above the 9 meter depth. No evidence of vertical migration by zooplankters over the diel period was established. Analysis of 359 kokanee stomachs obtained by vertically set gill nets revealed Daphnia pulex as the organism most frequently ingested. Tentative conclusions based on observations made during summer months indicate that kokanee feed diurnally on zooplankton at shallow depth and begin descent to areas within and below the metalimnion in early evening, presumably to rest and assimilate food. At dawn, possibly triggered by penetration of light to the lower strata, the fish begin upward migrations and resumption of feeding activities.