Direct and indirect effects of gizzard shad on bluegill growth and population size structure

Competition with gizzard shad Dorosoma cepedianum has been shown to influence survival of larval bluegills Lepomis macrochirus as well as growth and size structure of largemouth bass Micropterus salmoides, which prey on these planktivorous species. However, little is known about how the presence of gizzard shad influences bluegills beyond the larval stage. We examined bluegill–gizzard shad interactions across 10 reservoirs with and 10 without gizzard shad to determine direct and indirect effects of gizzard shad on bluegill population size structure. In the presence of gizzard shad, bluegills exhibited smaller adult (>3 years old) size structure. Benthic invertebrate densities were higher in non-gizzard shad reservoirs, which may have contributed to increased bluegill growth in these systems. In contrast, zooplankton densities were similar in reservoirs with and without gizzard shad. Turbidity was higher in reservoirs with gizzard shad, which may have reduced foraging success and growth of bluegill. Finally, higher bluegill densities occurred in reservoirs with gizzard shad, which may have resulted from decreased largemouth bass predation due to gizzard shad availability as alternative prey. These results demonstrate that the presence of gizzard shad is associated with reduced bluegill growth rates and adult size structure and that mechanisms other than direct competition for food resources may be responsible.