Invasive goldfish trigger a regime shift in experimental lake ecosystems of varying trophic state.

The ongoing pressure of biological invasions caused by the pet trade continues to restructure the earth's ecosystems. The release of pets and ornamentals has altered species interactions, ecosystem functions, spread disease and rewired food webs. We assessed the effects of a high-risk, yet understudied invasive species-that is, goldfish, Carassius auratus-on experimental freshwater ecosystems by integrating additive and substitutive experimental designs across two trophic states (i.e. oligotrophic vs. eutrophic). We hypothesized that goldfish would trigger a regime shift characteristic of more algae, reduced water quality and negative impacts on native species across multiple trophic levels. Under eutrophic conditions, goldfish induced a rapid shift in mesocosm state characterized by increased suspended solids and reduced water clarity; however, increases in phytoplankton and reductions in filamentous algae were largely driven by total fish density (native or invasive), consistent with strong niche overlap between the two species. Through consumption and habitat loss, goldfish caused reductions in snails, amphipods and zooplankton. Goldfish also reduced native fish condition, likely through exploitative competition. Our results indicate that goldfish are undesirable for both oligotrophic and eutrophic lakes, although some impacts will depend on trophic state. These results suggest that natural resource managers worldwide should consider management action to reduce or prevent goldfish invasions. Despite many viewing goldfish as signs of prosperity, luck and wealth, it is critically important to inform the public that their pets can grow into sizable pests that will harm freshwater ecosystems.