Food Consumption by Larval Gizzard Shad: Zooplankton Effects and Implications for Reservoir Communities

—Because peak abundance of larval gizzard shad Dorosoma cepedianum occurs simultaneously with the midsummer decline of macrozooplankton in Ohio reservoirs, we hypothesized that zooplanktivory by larval gizzard shad caused this decline. To test this hypothesis, we compared larval food consumption with zooplankton productivity in two reservoirs. Larval gizzard shad began to influence zooplankton production in a reservoir with high zooplankton productivity (exceeding 125 mg-m -d ') only after peak zooplankton biomass occurred, even at high larval densities (38 shad-nr). However, consumption by early juvenile (25-30-mm) gizzard shad severely reduced zooplankton in this reservoir. Conversely, relatively low densities of larval gizzard shad (3-7 shad-m ) had variable effects on zooplankton in a reservoir with low zooplankton productivity (at most 4 mg-m-'-d"). Depending on larval gizzard shad density and zooplankton production, larval gizzard shad alone or in conjunction with early juveniles may control zooplankton assemblages, increasing competition for limited resources at a time when zooplankton are critical to sport-fish recruitment. Predation often influences community structure (OBrien 1987). Aquatic systems tend to be less structurally complex than terrestrial ecosystems, affording prey fewer refugia than their terrestrial counterparts (Murdoch and Bence 1987). As a result, aquatic communities may be strongly mediated by predation. Competition in aquatic communities also is common and may arise from size-structured interactions where similar-sized individuals of different species compete for a limited resource (Stein et al. 1988). Predation by a particularly numerous or efficient species may directly influence the competitive interactions among its prey or with competitors at its trophic level. Two species that interact as predator and prey as adults may be competitors as larvae or juveniles. These mixed predation-competition interactions may play an important role in aquatic communities. Gizzard shad Dorosoma cepedianum are abundant in many reservoirs throughout the southeastern USA, often dominating the fish community (Cramer and Marzolf 1970; Noble 1981; Johnson et al. 1988). As preferred prey for many piscivores (Carline et al. 1986; Johnson et al. 1988; Matthews et al. 1988; Wahl and Stein 1988), they have been stocked in attempts to improve predator growth rates (Noble 1981). Historically, however, little effort has been directed toward quantifying the role of gizzard shad in freshwater systems and the possible direct and indirect effects of this species on aquatic communities (but see Guest et al. 1990; DeVries et al. 1991). A review by DeVries and Stein (1990) suggested that shad Dorosoma spp. may not be ideal forage fishes. Gizzard shad can consistently produce large numbers of offspring from few adults (Miller 1960; Pierce 1977), and their larvae may compete with other fishes for zooplankton (DeVries and Stein 1992). Furthermore, because gizzard shad grow quickly (Bodola 1966), they often reach a size refuge from most predators by the end of their first year (Adams and DeAngelis 1987; Johnson et al. 1988). Impressive larval production coupled with fast growth limits predator consumption to a maximum of 30% of gizzard shad production, at least in Ohio reservoirs (Carline et al. 1984; Johnson et al. 1988). Most importantly, however, gizzard shad are opportunistic omnivores, feeding on zooplankton as larvae (Barger and KJlambi 1980; DeVries et al. 1991), but capable of switching to phytoplankton or detritus as juveniles and adults (Kutkuhn 1958; Miller 1960; Bodola 1966; Pierce et al. 1981). Consequently, gizzard shad can drive zooplankton to extinction, yet still survive and