Ontogenetic Changes in Feeding Habits of Juvenile Cutthroat Trout

Abstract

Abstract Laboratory experiments demonstrated an ontogenetic shift in feeding habits of juvenile cutthroat trout Salmo clarki. Fish smaller than 6.0 cm in standard length preferred planktonic prey Daphnia pulex to each of three benthic prey (species of Hyalella, Callibaetis, and Enallagma). Larger fish consumed benthic prey significantly more often. Diet of cutthroat trout reared in a field enclosure also shifted from primarily planktonic prey to more frequent use of benthic prey once fish grew to about 7.0 cm. Changes in feeding habits were not due to differences in ability to capture evasive prey or to handle large prey. Rather, the motivation of cutthroat trout to feed on daphnia seems to be different between small and large fish. Concurrent changes in morphology of the mouth suggest that onset of a new phase of physical development may be responsible for the shift in feeding habits of juvenile cutthroat trout.