Occurrence and Utilization of Zooplankton by Juvenile Chinook Salmon in the Lower Columbia River

Abstract

The stomachs of juvenile chinook salmon, Oncorhynchus tshawytscha, taken in the Prescott-Kalama area of the Columbia River during July-November 1968 and March-December 1969 were examined to obtain information on their contents. Zooplankton, especially Daphnia, were the major item in the diet of the young salmon from July through October, whereas insects were the most important constitutents of the diet during spring and fall. The zooplankton in the area was sampled from January 1968 to November 1969 and examined in relation to the relative abundance of juvenile chinook salmon and their stomach contents. Daphnia, Bosmina, and cyclopoid copepods were major zooplankters in the area, and their periods of highest abundance were associated with high water temperatures of summer. Samples taken during daylight hours indicated Bosmina were most numerous at the surface, Daphnia were most numerous below 5 m, and cyclopoid copepods were relatively uniform in their vertical distribution. The zooplankton populations were generally increasing during the season of juvenile chinook abundance (April-July). Juvenile chinook were selective in their feeding habits and consistently consumed Daphnia in a much higher percentage than it was found in the zooplankton samples.