CHANGES IN HABITAT USE AND FEEDING CHRONOLOGY OF JUVENILE RAINBOW-TROUT (ONCORHYNCHUS-MYKISS) IN FALL AND THE ONSET OF WINTER IN SILVER-CREEK, IDAHO

We assessed changes in density, distribution, and microhabitat of age-0 rainbow trout (Oncorhynchus mykiss) in Silver Creek, a partially spring-fed stream, by periodic snorkeling in August 1987 through January 1988. We examined trout stomach contents and invertebrate drift samples in diel collections in August, September, October, and January to test if the period of feeding shifted from daytime to nighttime, concurrent with a transition to day concealment. In late September, fish aggregated briefly during the day and then began to conceal themselves in macrophyte beds, undercut banks, and submerged sedges and grasses along streambanks as temperature dropped below 8 degrees C in early October. Fish emerged from concealment at night, and numbers of trout visible were greatest 30-60 min after sunset and about 30 min before sunrise. Periods of peak feeding changed from afternoon and evening in August and September, when fish were day active, to mainly at night in October after the initiation of day concealment. Trout did not feed upon abundant chironomids in the daytime drift in October. In January, fish fed at 1-4 degrees C on mayflies, and stomachs were fullest in the early morning. Observations suggest that Silver Creek trout experienced a metabolic deficit that began in September.