Diel Feeding Chronology, Rate of Gastric Evacuation, Daily Ration, and Prey Selectivity in Lake Washington Juvenile Sockeye Salmon (Oncorhynchus nerka)

Abstract

Patterns of diel feeding chronology, rate of gastric evacuation, daily ration, and prey selectivity were determined by season and by fish length groups for Lake Washington juvenile sockeye salmon. In summer and fall, sockeye fed intensely during the afternoon through dusk. During the winter a high percentage of the population did not feed. Those fish that fed did so at a much lower rate than observed in summer and fall. No feeding occurred in the hours of darkness at any time of the year. Rates of gastric evacuation decreased with fish body size and increased with water temperature. The daily meal (total intake) generally increased while the daily ration decreased (daily meal/fish body weight) with increasing body size. During the winter the daily meal and daily ration were considerably lower than those observed in the summer and fall. Juvenile sockeye salmon in Lake Washington showed size-selective predation that had marked seasonal trends as well as marked trends among fish size groups. These patterns suggested that sockeye optimally forage by ignoring small zooplankton forms upon encounter.