Direct and Indirect Effects of Predation on a Fish Community: A Whole-Lake Experiment

We designed an experiment to test the relative importances of direct and indirect effects of piscivorous predation on an assemblage of small fishes in a piscivore-free lake, The direct effect is defined as consumption of prey fishes. Indirect effects include habitat changes associated with predator avoidance behavior, increases of emigration rates, and changes of composition and size structure of the prey community. After a year of premanipulation study, adult northern pike Esox lucius (a predator species) were introduced into the lake, Decrease of prey fish biomass due to increased emigration was at least as great as that due to direct consumption by the predators. Significant indirect effects included decreased abundance of dominant species, increases of some rare species, and decreases of mean size for species most vulnerable to predation. Indirect effects occurred rapidly and were most apparent during weeks immediately after northern pike were introduced.