Adult fish predation on freshwater limnetic fish larvae: A mesocosm experiment

We demonstrated that predation by adult bluegills Lepomis macrochirus and w hite crappies Pomoxis annularis may be a major source of mortality for fres hwater larval fish. In 5.3-m(3)-mesocosms, we compared the survival of larv al bluegill prey (19 larvae/m(3); mean standard length [SL], 8.6 mm; range, 3-22 mm) in the presence of adult bluegills, adult white crappies, or adul t izzard shad Dorosoma cepedianum with the survival of predator-free contro ls. The duration of the experiments was 3.7 h for bluegills, 3.2 h for whit e crappies, 5.5 h for gizzard shad, and 4.3 h for the predator-free control s. Larval fish survival in both the bluegill and white crappie predator tre atments was lower than that of the predator-free controls, whereas larval f ish survival in the gizzard shad treatments did not differ from that of the controls. The instantaneous hourly mortality rate attributable to predatio n was 0.11 for bluegill (9.8 larvae consumed/predator each hour) and 0.05 f or white crappie (4.8 larvae consumed/predator each hour). Larval fish mort ality in bluegill treatments was related positively to water clarity. Furth ermore, bluegills were size-selective predators, with neutral selection for small larvae (<8 mm SL), positive selection for intermediate larvae (8-12 mm SL), and negative selection for large larvae (>12 mm SL). Despite a slig ht trend towards positive selection of small larvae, white crappies were no t size-selective predators. Our concurrent stomach content analysis of pred ators failed to accurately quantify predation rates or detect any consumpti on of small larvae, which partly explains the dearth of published evidence linking adult fish predators to larval fish prey in freshwater systems. Thu s, predation on limnetic freshwater fish larvae by adult fish (both canniba lism and predation on other species) may be a major mortality source that v aries with predator type, larval size, and water clarity. These findings su ggest that in addition to competition, predation may greatly affect larval survival in freshwater fishes.