R. J. Small, Jr. and J. J. Ney, Department of Fisheries
and Wildlife Sciences, Virginia Polytechnic Institute and State University, Blacksburg,
Virginia 24061-0321; Voice 540/231-4458;
FAX 540/231-7580; E-Mail rosmall@vt.edu, jney@vt.edu
Keywords: gizzard shad, Dorosoma cepedianum, trophic
competition, ichthyoplankton, forage fish
In many reservoirs gizzard shad Dorosoma cepedianum may have
negative affects on growth and survival of zooplanktivorous fishes through trophic
competition. The intensity of this impact will depend on interspecific overlap in timing
of larval zooplanktivory, diet composition, and zooplankton availability. Ichthyoplankton
in mesotrophic Claytor Lake, Virginia were sampled in 1997 and 1998 to determine the
impact of an introduced population of gizzard shad on resident forage fish and
young-of-year sport fish. Sampling in both years indicated that peak densities of larval
shad (0.06 fish/m3), alewives Alosa pseudoharengus (0.07 fish/m3),
and sunfish Lepomis spp. (0.50 fish/m3) occur in the limnetic zone from
late-June to late-July. However, larval alewives first appear in mid-May while sunfish
continue to occur in large numbers through mid-August; larval shad are present from
early-June to early-August. All species utilized abundant Sididae and Bosminidae
cladocerans, and copopod nauplii, yet density and composition of zooplankton showed no
relationship to density of larval shad. In Claytor Lake, negative trophic impacts between
larval shad and other zooplanktivores appears to be minimized by moderate temporal
overlap, low shad density, and abundant zooplankton.