Feeding Preferences of Omnivorous Gizzard Shad as Influenced by Fish Size and Zooplankton Density

In Ohio reservoirs, larval gizzard shad Dorosoma cepedianum less than 30 mm total length consume only zooplankton but frequently switch to detritus as they grow longer than 30 mm. However, in laboratory studies without detritus, gizzard shad longer than 30 mm consume crustacean zooplankton. To explore the composition of diets of omnivorous 30-100-mm gizzard shad, we completed 1-h laboratory feeding trials with different amounts of zooplankton and detritus and quantified the diets of gizzard shad in reservoirs. In both laboratory and field, gizzard shad ate primarily detritus but also ate zooplankton, consuming more as more became available, which demonstrates that this species is a facultative detritivore. In the field, zooplankton consumption declined as gizzard shad body size increased. We believe gizzard shad maximize growth by supplementing their low-protein detritus diet with more zooplankton as more becomes available. With this strategy, omnivorous gizzard shad may compromise the potential for food web manipulations based on the trophic cascade hypothesis in Ohio reservoirs.This work was supported by Federal Aid in Sport Fish Restoration project F-57-R, administered jointly by the U.S. Fish and Wildlife Service and the Ohio Division of Wildlife, to R.A.S