A new peer-reviewed study conducted by researchers at the University of Toledo and University of Missouri provides some of the first rigorous experimental evidence that goldfish — one of the world’s most popular pets — can dramatically change freshwater ecosystems when released or they escape into the wild.
The findings published today in the Journal of Animal Ecology, carry an urgent message for pet owners, resource managers and policymakers alike: goldfish might be great pets, but they are not harmless in the wild.
“It is critically important to inform the public that their pets can become pests that will harm freshwater ecosystems. The evidence is now clear — releasing a goldfish into the wild might be seen as an act of kindness, but it can turn into a major ecological threat,” said the study’s lead investigator, Dr. William Hintz, associate professor in UToledo’s Department of Environmental Sciences and Lake Erie Center.
Titled, “Invasive goldfish trigger a regime shift in experimental lake ecosystems of varying trophic state, " the study used large-scale, controlled outdoor mesocosm experiments to simulate realistic lake conditions and track the effects of goldfish introductions across different ecosystem types.
The researchers introduced goldfish ( Carassius auratus ) into experimental lake ecosystems representing two common environmental conditions — nutrient-poor (oligotrophic) and nutrient-rich (eutrophic) waters — and monitored the ecological consequences over time. The results were stark:
The study employed both additive and substitutive experimental designs — a rigorous scientific approach that allowed researchers to disentangle the specific effects of goldfish from those associated with total fish density. This revealed that while some changes to aquatic plant communities were tied to the overall number of fish present, the most damaging ecological outcomes were clearly attributable to goldfish themselves.
A “regime shift” — the scientific term for when an ecosystem crosses a threshold and rapidly reorganizes into a fundamentally different, often degraded state — was documented in goldfish-invaded systems. Once triggered, such shifts are notoriously difficult and expensive to reverse.
The pet trade moves exotic animals across the globe at an unprecedented scale, and goldfish rank among the most widely distributed ornamental fish on Earth. When released into rivers, lakes or ponds — whether intentionally or through flooding of ponds — they can quickly establish invasive populations.
“If goldfish are released into the wild, they rapidly grow into very large fish that stir up lake sediments, consume large numbers of prey and compete with native fish,” said Rick Reylea, professor in the University of Missouri College of Agriculture, Food and Natural Resources, director of Mizzou’s Johnny Morris Institute of Fisheries, Wetlands and Aquatic Systems and co-author of the study.
The authors urge natural resource managers worldwide to treat goldfish as a high-priority invasive species and implement prevention, early detection and control strategies before populations become established. They also call for stronger public education campaigns to ensure that pet owners understand the consequences of releasing aquatic animals.
For pet owners with unwanted goldfish, humane alternatives to release include returning fish to the pet store, rehoming them with other aquarium enthusiasts or contacting local wildlife authorities for guidance.
“ Invasive goldfish trigger a regime shift in experimental lake ecosystems of varying trophic state ,” was authored by Dr. William Hintz of The University of Toledo, Hannah Barrett and Dr. Rick Relyea of the University of Missouri.
The research was conducted using outdoor freshwater mesocosms designed to replicate realistic lake conditions. The research integrated additive and substitutive experimental designs across oligotrophic (nutrient-poor) and eutrophic (nutrient-rich) trophic states to assess the effects of goldfish (Carassius auratus) on water quality, phytoplankton, invertebrate communities, filamentous algae, and native fish condition.
The UToledo Lake Erie Center is a research and educational facility focused on environmental conditions and aquatic resources in Maumee Bay and western Lake Erie as a model for the Great Lakes and aquatic ecosystems worldwide.
Journal of Animal Ecology
Experimental study
Animals
Invasive goldfish trigger a regime shift in experimental lake ecosystems of varying trophic state
28-Apr-2026