Prey community responses to bluegill and gizzard shad foraging: Implications for growth of juvenile largemouth bass

Bluegill Lepomis macrochirus, gizzard shad Dorosoma cepedianum, and largemouth bass Micropterus salmoides are common lentic species that may compete for invertebrate prey resources, and interactions among these species can have important consequences for aquatic community structure. Differential foraging behavior of bluegills and gizzard shad as juveniles, for example, may structure zooplankton and benthos communities and influence the growth of juvenile largemouth bass. We conducted a mesocosm experiment (1.6-m-diameter circular tanks) in which we allowed high and low densities of juvenile bluegills (70-100 mm total length [TL]) and gizzard shad (135-155 mm TL) to forage on established zooplankton and benthos communities for 6 weeks. After this period, we added juvenile largemouth bass (50-80 mm TL) to each tank for 4 weeks to examine growth and diets. Although foraging by bluegill and gizzard shad had limited effects on the total population densities of invertebrates, important taxon-specific effects were observed. At the time that largemouth bass were added to tanks, bluegill treatments exhibited a higher macrozooplankton density, higher turbidity, and lower density of larval hydrophilid coleopterans than did gizzard shad or control treatments. The growth of juvenile largemouth bass was strongly influenced by treatment; largemouth bass grew at similar rates in the gizzard shad and control treatments but lost weight in the bluegill treatments. We suggest that bluegills compete with juvenile largemouth bass for preferred prey items, thereby limiting largemouth bass growth. Further, our results demonstrate that the presence of larger gizzard shad may not have the negative implications observed in previous studies with smaller individuals. Knowledge of the relative, size-specific abundances of bluegills and gizzard shad should provide resource managers with valuable information for developing initiatives aimed at maximizing the growth of juvenile largemouth bass.