PHILLIP GAINES AND JEFF BOXRUCKER, Oklahoma Fishery Research Laboratory, 500 East Constellation, Norman, Oklahoma 73072, USA

Larval shad food consumption and changes in zooplankton community structure were evaluated on Hugo Reservoir, in southeastern Oklahoma. The objectives of this study were to determine if larval gizzard shad (Dorosoma cepedianum) are selective as to prey type and size and if this may influence zooplankton community structure. Larval shad and zooplankton were collected weekly from May 1 through June 27, 1996. Crustacean zooplankton density (No./L) fluctuated, but generally increased during the study period. Chesson’s alpha, a measure of prey preference, was calculated for each zooplankton taxa within larval shad length groups across collection dates. Larval shad in all length groups exhibited high preferences for cyclopoid copepods and the large cladoceran Diaphanosoma sp., and an avoidance of other cladocerans and rotifers as well. Differences between length groups were noted, with larval shad £ 10.0 mm also having a high preference for nauplii. Larval shad also selected for prey items of greater mean length than were found in the environment. However, these preferences for large individuals and specific zooplankton taxonomic groups do not fully explain the changes documented in zooplankton community structure and zooplankton mean length during this study.