Significance of Sequential Feeding Patterns of Juvenile Rainbow Trout in a Large Lake-Fed River

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Abstract
Rainbow trout fry Salmo gairdneri, feeding during the day in a large river below a lake, far more frequently attacked drifting organisms than substrate‐associated organisms. Recently emerged fry struck at prey more frequently than did older fry. Zooplankton from the lake upstream were the prey most frequently consumed by small fry, whereas larger aquatic and terrestrial insects were the major food of larger fry. Of those zooplankters fed upon by small fry, large calanoid copepods (mainly Epischura nevadensis) and Daphnia galeata mendotae were selectively consumed and smaller Bosmina longirostris rarely consumed, as would be expected of size‐selective predators, but large Cyclops bicuspidatus were also under‐represented in fry stomachs. There was a seasonal increase in size of prey consumed. The relative prey size (size of prey consumed divided by fry size) increased throughout the year, except from fall to winter when fry growth was slow. It is suggested that the sequential change in feeding patterns increased the efficiency of energy intake for the young rainbow trout, conserved their energy expenditure, and thereby improved their likelihood for survival.

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