Direct and indirect effects of exotic bass and bluegill on exotic and native organisms in farm ponds

Exotic largemouth bass (Micropterus salmoides) and bluegill (Lepomis macrochirus) are thought to threaten native aquatic organisms worldwide, but few studies have demonstrated their community-wide impacts, including the interaction between these fish and other exotic organisms. We tested the hypothesis that bass and bluegill in Japanese farm ponds will reduce some native organisms (fish, shrimp, odonates) as well as exotic crayfish (Procambarus clarkii) via top-down effects, whereas other native organisms (chironomid larvae, oligochaetes, and macrophytes) will increase as a result of trophic cascades. To test this hypothesis, we conducted three types of field experiments. In the first experiment, we estimated predation pressure in ponds with and without bass and bluegills by using predator exclusion cages. This experiment revealed that predation on native odonates and exotic crayfish was greater in ponds with bass or bluegills, whereas predation on chironomids, oligochaetes, and macrophytes was lower in ponds with bass or bluegills. In the second experiment, we estimated the impact of bass and bluegills at the community level using four large mesocosms in a pond. Bass or bluegill were introduced into two mesocosms (treatment), but were absent in the other two mesocosms (control). We found that bass reduced native fish, exotic fish, shrimp, odonates, and exotic crayfish, while chironomids, oligochaetes, and macrophytes increased; however, introducing bluegill reduced only shrimp and odonates. In the third experiment, we established small mesocosms with and without exotic crayfish. This experiment showed that crayfish were responsible for a reduction of macrophytes. All three field experiments supported our hypothesis for bass effects, but not for most of the bluegill effects. The results provide important implications for strategies to eradicate exotic fish; when exotic crayfish are present, bass removal is likely to reduce macrophytes that perform important functions in freshwater ecosystems. To conserve macrophytes we propose that reduction of exotic crayfish should be considered when eliminating bass.