Seasonal and Diel Food Habits of Rainbow Trout Stocked as Juveniles in Lake Washington

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Abstract
I examined food habits of rainbow trout Oncorhynchus mykiss in Lake Washington, Washington, for relationships to the spatial and temporal distribution of their prey. Rainbow trout smaller than 250 mm (fork length) ate primarily Daphnia pulicaria during summer and autumn; larger fish were piscivorous throughout the year. Longfin smelt Spirinchus thaleichthys made up the largest fraction of fish prey in the diet of rainbow trout from the nearshore zone during autumn and winter, whereas prickly sculpin Cottus asper (in 1984) and yellow perch Perca flavescens (in 1985) were most important in spring and summer. Rainbow trout in the offshore zone (>15 m deep) ate mostly longfin smelt in spring and summer. Longfin smelt exhibited a 2-year cycle of abundance that appeared to influence the feeding habits of rainbow trout. During 1984, the adult longfin smelt population was large and provided the major prey of rainbow trout. When adult longfin smelt were less abundant during the 1985 winter, the fish fract...