R. J. Small, Jr. and J. J. Ney, Department of Fisheries and Wildlife Sciences, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061-0321

Abstract. Gizzard shad Dorosoma cepedianum may negatively affect zooplantivorous forage-fish and young-of-year sport fish through trophic competition. By influencing zooplankton density and composition, larval shad may alter prey availability to other young-of-year fishes. The intensity of this impact will depend on interspecific overlap in timing of zooplanktivory, diet composition, and zooplankton availability. We are currently examining the temporal distribution, abundance, and diet of young-of-year fishes in addition to the abundance and composition of zooplankton in mesotrophic Claytor Lake, Virginia. Extensive ichthyoplankton sampling in 1997 indicated that larval shad are present from late May to mid-July, overlapping in time with larval alewives Alosa pseudoharengus and black basses Micropterus spp.; abundance of larval sunfish peaked in late July. Larval shad and larval alewives primarily consumed cyclopoid copepods and cladocerans, yet alewives selected larger individuals than shad. Young-of-year black bass consumed littoral cladocerans before switching to benthos. Density and composition of limnetic zooplankton showed no relationship to density of larval shad. In Claytor Lake, trophic competition between larval shad and other zooplanktivores appears to be minimized by moderate temporal overlap, abundant zooplankton, and low similarities in diet.