Dietary shifts of a dominant reservoir planktivore during early life stages

We studied dietary shifts in the early life stages of gizzard shad Dorosoma cepedianum, a dominant forage species in North American reservoirs. Larval fish and zooplankton samples were collected weekly during spring in Sardis Reservoir, Mississippi, USA. Diet and prey electivity data suggested the existence of three dietary niches during early life stages: microzooplankton (larvae ≤10 mm total length) in which microzooplankters comprised over 90% by number; crustacean zooplankton (larvae 11–25 mm) in which larval gizzard shad consumed substantial numbers of crustacean zooplankton; and microplankton (larvae >25 mm) in which gizzard shad shifted to filtering protozoans, rotifers, and phytoplankton. There was a high overlap (84%) between the diet of larval gizzard shad and crappies Pomoxis spp. during early May. Larval gizzard shad can potentially reduce microzooplankton density through predation, then shift to crustacean zooplankton and drive their density to decline, then revert to filtration of microzooplankton and exploit phytoplankton. Although, gizzard shad have the ability to influence trophic interactions in reservoir ecosystems, their influence may sometimes be masked by the intensity of bottom-up and top-down effects, as well as population and community interactions.