Epibenthic Feeding by Rainbow Trout (Salmo gairdneri) in the McCloud River, California

Abstract

SUMMARY (1) The stomach contents of rainbow trout from the McCloud River, California, were compared with the composition of the epibenthic invertebrate fauna and the composition of the invertebrate drift. The epibenthic fauna was dominated by Ephemeroptera and Plecoptera nymphs and immature Trichoptera and Diptera while the drift was dominated by Baetidae nymphs (Ephemeroptera), larval Simuliidae and Chironomidae (Diptera), and flying adult insects of both terrestrial and aquatic origin. Although the different groups of drifting invertebrates had different patterns and rates of drift, the overall peak in numbers of drifting organisms occurred at night. (2) Rainbow trout fry and juveniles relied predominantly upon drifting benthic invertebrates. During the day these fish consumed very little food. The most abundant organisms in the adult trout stomachs were larvae of Trichoptera (especially Dicosmoecus), Plecoptera, Ephemeroptera, Diptera, and terrestrial insects. Nearly half the stomach contents were undigestible algae, debris and exuviae. Feeding appeared to be continuous throughout the day, with no striking peaks in stomach fullness. The stomach contents showed no apparent relationship with the patterns and rates of drifting. (3) The epibenthic feeding behavior of the adult trout is probably due to the turbidity of the river, which makes drift feeding difficult when light levels are low. Epibenthic feeding, coupled with the high level of intraspecific competition probably present in an unexploited trout population, is probably responsible for the slow growth rates of the McCloud River trout.