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

The size of the zooplankton consumed by rainbow trout and yellow perch was studied in two Michigan lakes and compared with the size of the limnetic zooplankton. One of the study lakes was managed for both rainbow trout and warmwater game fish (a “combination” lake), and the other was managed solely for rainbow trout after it was reclaimed with toxaphene. Plankton samples were collected in the summer both before and after the introduction of rainbow trout, and compared with plankton in the stomachs of fish taken at the same time. Daphnids were the only zooplankton consumed by rainbow trout in both lakes and by yellow perch in the combination lake even though there were many other genera of zooplankters. Both species were very size selective and usually consumed only Daphnia over 1.3 mm in size while ignoring the many and often more numerous smaller zooplankton. Despite the broad range in size of these trout (7.9 to 17.2 inches) and yellow perch (2.8 to 9.8 inches), there was no strong evidence of an association between their length and the size of the Daphnia they consumed. Introduction of rainbow trout in the combination lake had no apparent effect on the daphnid population. However, changes in the net plankton did occur in the trout lake after the introduction of rainbow trout, smelt, and fathead minnows. Most obvious changes were: (1) the complete elimination of D. pulex and subsequent replacement by two other smaller species within 4 years, (2) a decrease in the average size of the daphnids from 1.4 mm to 0.8 mm, (3) a decrease in the percentage of daphnids larger than 1.3 mm from an average of 53.8% to 4.7%, and (4) a reduction in the volume and percentage of daphnids comprising the net plankton even though there was no reduction in their numbers. The gap left by the elimination of the large Daphnia was filled by other smaller zooplankters. Effects of predation on Daphnia, as well as the interactions between daphnids and planktivorous fishes are discussed. This study emphasizes the importance of determining the abundance of zooplankton of the proper sizes in the plankton when the survival of planktivorous fishes in lakes is investigated.