DIETARY SHIFTS OF A DOMINANT RESERVOIR PLANKTIVORE DURING EARLY-LIFE STAGES

We studied dietary shifts in the early life stages of gizzard shad Dor osoma cepedianum, a dominant forage species in North American reservoi rs. Larval fish and zooplankton samples were collected weekly during s pring in Sardis Reservoir, Mississippi, USA, Diet and prey electivity data suggested the existence of three dietary niches during early Life stages: microzooplankton (larvae less than or equal to 10 mm total le ngth) in which microzooplankters comprised over 90% by number; crustac ean zooplankton (larvae 11-25 mm) in which larval gizzard shad consume d substantial numbers of crustacean zooplankton; and microplankton (la rvae > 25 mm) in which gizzard shad shifted to filtering protozoans, r otifers, and phytoplankton. There was a high overlap (84%) between the diet of larval gizzard shad and crappies Pomoxis spp. during early Ma y. Larval gizzard shad can potentially reduce microzooplankton density through predation, then shift to crustacean zooplankton and drive the ir density to decline, then revert to filtration of microzooplankton a nd exploit phytoplankton. Although, gizzard shad have the ability to i nfluence trophic interactions in reservoir ecosystems, their influence may sometimes be masked by the intensity of bottom-up and top-down ef fects, as well as population and community interactions.