Selective Predation on Zooplankton by Gizzard Shad

Food selection by a planktivorous fish, the gizzard shad, Dorosoma cepedianum, was studied in Tuttle Creek Reservoir, Kansas. To determine if selection was occurring a fish stomach sample and a plankton sample were taken concurrently at 22 separate sampling times during the study. Percent composition of the various food items in paired stomach and plankton samples was compared by an electivity (E) index. Four least squares analyses of variance were performed on the E index values to determine which factors (i.e., species of plankton, date of collection, time-day or night, size-class of fish or size-class of plankton) explained most of the variation in E and thus was responsible for selection by gizzard shad. Differences between the species of plankton in the analysis was found to be the most consistent factor contributing to selection. Differences in size, mobility, behavior, and so forth might be considered as causative agents responsible for positive or negative selection by gizzard shad. Of th...