Utilization of Invertebrate Drift by Brook Trout (Salvelinus fontinalis) and Cutthroat Trout (Salmo clarki) in Small Streams in Idaho

Abstract

In four northern Idaho streams I assessed availability of invertebrate drift in summer 1969. Brook and cutthroat trout inhabited two streams sympatrically and two allopatrically. I compared drift and diet components to assess proportion of drift cropped by trout and extent to which trout segregated into dietary niches. Members of five insect orders (Ephemeroptera, Coleoptera, Diptera, Trichoptera and Plecoptera) comprised 97% of the number of drift organisms and an average of 92% of the number of organisms eaten by both brook and cutthroat trout. The terrestrial component of both drift and diet was insignificant, and predation on fish and/or cannibalism was rare. Most Ephemeroptera, Plecoptera and Coleoptera drifted between dusk and dawn when they may not have been available to trout. Trout selected certain food items. Underyearling brook and cutthroat trout differed little in their food preferences or habits, whether they lived sympatrically or allopatrically, and consumed smaller organisms (mainly Diptera and Ephemeroptera). Older trout of each species living allopatrically differed in utilization of Diptera and Trichoptera, with cutthroat selecting the former and brook trout the latter. These differences were intensified in sympatry.