Size-selective Predation on Daphnia by Rainbow Trout and Yellow Perch

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Abstract
The size of the Daphnia consumed by rainbow trout and yellow perch was compared with the size of the daphnids present in two Michigan lakes. Despite the presence of numerous smaller individuals, 96% of those eaten by trout and 82% by perch were larger than 1.3 mm. No other members of the limnetic zooplankton population were found in the stomachs. The following dramatic changes in the net plankton occurred in one lake after the introduction of trout, smelt, and fathead minnows: within 4 years D. pulex was eliminated and replaced by 2 smaller species of daphnids; the average size of the daphnids dropped from 1.4 mm to 0.8 mm; the percentage of daphnids larger than 1.3 mm in the population declined from an average of 53.8% to 4.7%; and there was a reduction in the volume and percentage volume of daphnids comprising the net plankton but no reduction in number. When the larger daphnids disappeared the trout did not utilize the zooplankton. This study demonstrates the importance of including length-frequency data for zooplankton in studies on relationships between plankton and fish.