In the limnetic zones of small, highly productive reservoirs, young-of-year (YOY) gizzard shad (Dorosoma cepedianum) or threadfin shad (D. petenense) (henceforth, shad) often attain high densities during spring. Environmental factors facilitating early growth and survival of shad plus potential interspecific competition for zooplankton may reduce growth and survival of YOY bluegill (Lepomis macrochirus), another common species in reservoirs. We hypothesized that fewer YOY bluegill moving from the limnetic zone to the littoral zone in late spring probably slows or prevents the ontogenetic switch to piscivory by YOY largemouth bass (Micropterus salmoides), reducing their oversummer growth, overwinter survival, and hence recruitment to their second year. To determine whether shad and bluegill abundances indeed vary inversely in reservoirs, we quantified densities of YOY shad and bluegill in four reservoirs across several years (1987–1994; N = 22 reservoir-years). To assess how YOY bluegill abundance influenced the ontogenetic dietary switch to fish by largemouth bass, we conducted experiments with differing levels of realism and control (4-m2 littoral cages, 4.5-m2 outdoor pools, and 0.33-m2 laboratory aquaria).
In reservoirs, peak YOY bluegill density declined weakly in the limnetic zone but strongly in the littoral zone when peak limnetic YOY Dorosoma spp. exceeded 10 individuals/m3. In pools and aquaria, largemouth bass grew more rapidly at ≥3 than at zero bluegill per largemouth bass. Using known temperatures and largemouth bass growth in a bioenergetics model, we discovered that YOY largemouth bass in pools and aquaria ate ≤65% of their maximum daily consumption potential (in grams of wet mass) at ≥6 bluegill per largemouth bass. In cages, largemouth bass consumed only 40% of their maximum and grew less at bluegill abundances similar to those in pools and aquaria, probably because dense vegetation and depletion of bluegill inhibited predatory success. In reservoirs with abundant shad, reduced littoral bluegill density likely compromises first-year growth and recruitment of largemouth bass. However, variable abiotic and biotic factors may modify YOY bluegill abundance and hence invalidate our predictions for largemouth bass recruitment success. To better predict fish community structure and develop management actions for reservoir ecosystems, multi-scale experimentation should be combined with whole-system manipulations (e.g., via adaptive management) to bound these variable interactions.