Pets to pests: Goldfish found to be harmful to natural freshwater ecosystems

One of the most common "starter" pets for families, the goldfish, may not be so harmless when released into the wild.

Goldfish are widely distributed across the world, with an estimated several million sold each year.

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A new study done by researchers at the University of Toledo and University of Missouri offers some of the first rigorous experimental evidence that suggests goldfish can drastically change freshwater ecosystems when they are released or escape into the wild.

The study, published in the Journal of Animal Ecology on Tuesday, used a large-scale outdoor controlled model ecosystem to simulate realistic lake conditions and track the effects of goldfish introductions across different ecosystem types, the University of Toledo said. 

Goldfish were introduced into nutrient-poor and nutrient-rich waters and monitored over time to see the ecological consequences.

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Some of the results showed that the presence of goldfish, through direct consumption and destruction of habitat, led to a significant reduction in snails, amphipods and zooplankton, small invertebrates that are the basis of healthy aquatic food webs.

In nutrient-rich environments, it was found that goldfish triggered a rapid deterioration of water clarity and strong increase in suspended solids, signaling a primary shift in the ecosystem's state, the university said.

Native fish also suffered because of goldfish, having been out-competed for food and resources, reducing the body condition of native fish species.

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"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," Rick Reylea, co-author of the study and professor at the University of Missouri College of Agriculture, Food and Natural Resources, said.

The study found that no lake type is safe from goldfish impacts, proving harmful in both the nutrient-rich and nutrient-poor waters.

"No freshwater ecosystem appears immune," the university said.

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The study noted that a "regime shift," the scientific term used when an ecosystem crosses a threshold and rapidly reorganizes into a different, often degraded state, was documented in the goldfish-invaded ecosystems.

The university said regime shifts are very difficult and expensive to reverse.

The study called for pet owners to never release their pet goldfish into the wild.

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"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," the university said.

It also calls for natural resource managers to treat goldfish as an invasive species, and implement prevention, early detection and control strategies before goldfish populations become established in freshwater ecosystems.

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"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," the study’s lead investigator, Dr. William Hintz, associate professor in UToledo’s Department of Environmental Sciences and Lake Erie Center, said.