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. Gizzard shad Dorosoma cepedianum is a common freshwater fish species that can predominate fish biomass in many central and southern U.S. reservoirs (Johnson et al. 1988; DeVries and Stein 1992; Stein et al. 1995). Occupying an intermediate trophic level, gizzard shad can have tremendous influence on reservoir systems through middle-out community regulation (Stein et al. 1995). Foraging by gizzard shad can influence zooplankton and phytoplankton communities (Cramer and Marzolf 1970; DeVries and Stein 1992; Schaus and Vanni 2000) and the abundance and size structure of piscivores (Swingle 1946; DeVries et al. 1991; Michaletz 1998a, 1998b). Gizzard shad can affect growth and size structure of piscivores through competition with other zooplanktivorous prey-fish species (e.g., Dettmers and Stein 1992; Garvey and Stein 1998a). The links between these competitive interactions at intermediate trophic * Corresponding author: [email protected] 1 Present address: Department of Evolution, Ecology, and Organismal Biology, The Ohio State University, 1680 University Drive, Mansfield, Ohio 44906, USA. 2 Present address: Minnesota Department of Natural Resources, 1801 South Oak Street, Lake City, Minnesota 55041, USA. Received September 7, 2001; accepted June 19, 2002 levels and timing of piscivory, growth rates, and size structure of piscivores have received considerable attention (Kirk and Davies 1987; Guest et al. 1990; DeVries and Stein 1992; Pope and DeVries 1994; Dettmers and Stein 1996; Olson 1996; Roseman et al. 1996; Garvey and Stein 1998a; Michaletz 1998a, 1998b). However, information about eventual effects on the adult populations of the interacting zooplanktivore species is generally lacking. Bluegills Lepomis macrochirus commonly compete with gizzard shad for food resources in reservoir systems. Larvae of both species prey extensively on zooplankton, and that competition influences growth and survival of larval bluegills (DeVries and Stein 1992; Welker et al. 1994; Garvey and Stein 1998b; Garvey et al. 1998) and reproductive success of adults (Kirk et al. 1986). The mechanism responsible is probably exploitative competition for zooplankton, which gizzard shad have the potential to control and significantly reduce (Cramer and Marzolf 1970; Dettmers and Stein 1992, 1996; DeVries and Stein 1992; Welker et al. 1994; Stein et al. 1995). These interactions may be mitigated by the timing of bluegill spawning. Bluegills are protracted spawners, and the timing of their spawning bouts and the subsequent emergence of larvae relative to that of gizzard shad