Adult Fish Predation on Freshwater Limnetic Fish Larvae: A Mesocosm Experiment

We demonstrated that predation by adult bluegills Lepomis macrochirus and white crappies Pomoxis annularis may be a major source of mortality for freshwater larval fish. In 5.3‐m3 mesocosms, we compared the survival of larval bluegill prey (19 larvae/m3; mean standard length (SL), 8.6 mm; range, 3–22 mm) in the presence of adult bluegills, adult white crappies, or adult gizzard shad Dorosoma cepedianum with the survival of predator‐free controls. The duration of the experiments was 3.7 h for bluegills, 3.2 h for white crappies, 5.5 h for gizzard shad, and 4.3 h for the predator‐free controls. Larval fish survival in both the bluegill and white crappie predator treatments was lower than that of the predator‐free controls, whereas larval fish survival in the gizzard shad treatments did not differ from that of the controls. The instantaneous hourly mortality rate attributable to predation was 0.11 for bluegill (9.8 larvae consumed/predator each hour) and 0.05 for white crappie (4.8 larvae consumed/predator each hour). Larval fish mortality in bluegill treatments was related positively to water clarity. Furthermore, bluegills were size‐selective predators, with neutral selection for small larvae (12 mm SL). Despite a slight trend towards positive selection of small larvae, white crappies were not size‐selective predators. Our concurrent stomach content analysis of predators failed to accurately quantify predation rates or detect any consumption of small larvae, which partly explains the dearth of published evidence linking adult fish predators to larval fish prey in freshwater systems. Thus, predation on limnetic freshwater fish larvae by adult fish (both cannibalism and predation on other species) may be a major mortality source that varies with predator type, larval size, and water clarity. These findings suggest that in addition to competition, predation may greatly affect larval survival in freshwater fishes.