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 d etritus as they grow longer than 30 mm. However, in laboratory studies without detritus, gizzard shad longer than 30 mm consume crustacean z ooplankton. To explore the composition of diets of omnivorous 30-100-m m gizzard shad, we completed 1-h laboratory feeding trials with differ ent amounts of zooplankton and detritus and quantified the diets of gi zzard shad in reservoirs. In both laboratory and field, gizzard shad a te primarily detritus but also ate zooplankton, consuming more as more became available, which demonstrates that this species is a facultati ve detritivore. In the field, zooplankton consumption declined as gizz ard 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 sha d may compromise the potential for food web manipulations based on the trophic cascade hypothesis in Ohio reservoirs.