DIEL FEEDING CHRONOLOGY AND DIET SELECTION OF RAINBOW-TROUT (ONCORHYNCHUS-MYKISS) IN THE HENRYS FORK OF THE SNAKE RIVER, IDAHO

Feeding was discontinuous through the day on all dates. Mean stomach content weight was minimal after 0400 MST and sharply increased between 1000 and 1200 MST on all 3 (summer) dates. Feeding apparently did not occur after twilight. Mean stomach content weight was correlated with water temperature on 2 dates and was never correlated with invertebrate drift density for non-age-0 trout. Daily ration (wet weight) was 7% of live weight for non-age-0 trout and 21% for age-0 trout. Trichoptera, Ephemeroptera and Diptera were most important in the diet; terrestrial insects and aquatic vertebrates were rare. Degree of selectivity varied through 24h. Occurrence of low-drift cased Trichoptera larvae in stomachs was correlated with amount of filamentous algae ingested, indicating a degree of epibenthic foraging. Mean length of prey items in stomachs was larger that the drift in 83% of the samples. Findings support experimentally derived decision rules-of-thumb for foraging trout: select larger prey items, select vulnerable prey, and relax selectivity when hungry. -from Authors